What this question is asking is why there are so many more CMB photons than particles of matter (protons and neutrons are sometimes called "baryons") -- the ratio of densities of photons to baryons is about 1 billion. This number is somehow related to particle physics at the highest energies and the "baryogenesis" question.
Presumably early enough in the history of the Universe, all species were in complete equilibrium, meaning essentially equal densities of all particles with mass below the characteristic energy at that time. This would of course involve antiparticles as well as particles, i.e. there would be as many positrons (anti-electrons) as electrons, as many anti-protons as protons etc.
At some point the temperature dropped low enough that when particles annihilated there wasn't enough energy around to recreate the pairs. So particle + antiparticle gave photons. If there was an exact balance then there would be only photons today. But since the Universe is observed to be all matter, with about 1 proton per 109 photons, then there must have been an imbalance between matter and antimatter in the early universe. And so we know that the laws of particle physics at high enough energies must contain non-conservation of baryon number, at the level of 1 part in 109. So when the Universe cooled enough, all the matter and antimatter annihilated, leaving about a billion photons for every proton.
Exactly how this worked is an area of active research, with lots of good ideas, but no clear winner yet. The point is that the photon-to-baryon ration is an observable which tells us directly about properties of particle physics at extremely high energies. It's surprising that the mere observation that there's no anti-matter today, and that there are many more photons than baryons, is such a non-trivial observation!
Прислал francis4"AT"tcnj.edu 11/98
A very good question, and I must resist the temptation to answer "why not"!
As I mentioned in reply to the last question (and others earlier), particle-antiparticle pairs can be created out of pure energy (i.e. photons), and then annihilate again to produce photons. This happens if the average energy of the plasma is high enough that it's more than the rest mass of whatever particles you're thinking of creating. Once the energy drops below that level then you favour annihilation over creation, and you end up with a bunch of photons, and not many particles. That's what happens as the Universe expands and cools.
If the Universe was born with an exactly equal number of particles and antiparticles (which is the simplest assumption), then mostly they'd have annihilated, and you'd eventually have almost nothing left except radiation. In fact in an expanding Universe you don't get quite zero, since you reach a point where the density of particles and antiparticles is so low that they can't find each other, but in any case the trace amount left is almost entirely negligible. There are currently about a billion CMB photons for every particle (more technically every baryon, i.e. proton or neutron) in an average part of the Universe, whereas a particle-antiparticle symmetric Universe would have a ratio which is many orders of magnitude higher. So the real question, given the present density of particles is "why are there so few photons in the Universe?"
The reason is, that through some detail of particle physics, the early Universe preferred particles over antiparticles, ever so slightly. So when most of the anti-baryons annihilated, there were still some baryons left over. Given the ratio of photons to baryons of about a billion, it would appear that some high energy physics process favours baryons over antibaryons at the level of about one part in a billion. Exactly how that happened is an area of ongoing research in a field called "baryogenesis" or "baryosynthesis". The fact that the CMB gives a clue to some mysterious high energy physics process is a good example of why some people refer to the early Universe as the ultimate particle accelerator!
Прислал debouche"AT"kings.edu 1/00
The early Universe was much hotter and denser. At some point the average hydrogen atom must have been ionized by the hot radiation bath. This can be calculated quite precisely, knowing the current temperature of the CMB, together with the current density of matter and the expansion rate. The answer turns out to be a temperature of around 4000 Kelvin. The photons last interact with the matter at the slightly lower temperature (due to the small amount of plasma still remaining by then) of around 3000 Kelvin. This corresponds to a redshift of about 1000, or a time of about 300,000 years after the Big Bang.
If you want to know all the gory details of the process by which the Universe evolved from a plasma to being neutral (usually referred to as "recombination") then see my paper at this archive (but be warned it is long and boring!).
It is only very close to a black hole that things get strange. At large distances gravity behaves just the same way as it does for any other sort of massive object. So if the Sun suddenly became a black hole nothing very special would happen. The earth would carry on orbiting the Sun exactly as before, and even the orbits of Venus and Mercury would be unaffected. Of course the Sun would stop being luminous, which would have somewhat drastic consequences for all life on earth!
In fact the Sun will not turn into a black hole. Only very high mass stars (greater than about 8 times the mass of the Sun) are likely to become black holes when they reach the ends of their lives. When a star like the Sun runs out of fuel (in about 5 billion years) it becomes a red giant, throws off some of its outer layers, and then fades into obscurity as a white dwarf.
I understand how the &omega0 can be determined by the angular scale of first Doppler peak. What I don't immediately see is how we know the physical size of the horizon at decoupling, which is needed to figure out how the geodesics curve.
The point is that the relevant physical scale at the epoch of last scattering, is almost model-independent. This scale is the "sound horizon", meaning the distance that a sound wave can have travelled in the age of the Universe. It is this scale which is imprinted on the anisotropy power spectrum, through the physics of the gravity-driven acoustic oscillations in the early Universe. If the speed of sound is denoted by cs and the age of the Universe by t, then the sound-horizon is just cst. It turns out that the sound speed during this time is close to the sound speed for an ideal gas of relativistic particles, which is the speed of light divided by 3 (take my word for this for now!). The age of the Universe at the time of last scattering is also relatively model-independent, and hence the sound-horizon is pretty much a fixed physical quantity.
The angle that this length subtends on the sky will depend on how curved space-time is. It will look like a larger angle in a universe which is positively curved, so that light rays appear to converge, and a smaller angle in a negatively curved universe (i.e. an "open" universe, with less than the required density for closure). The main effect on this angle is this curvature of the Universe. Other effects (e.g. the ratio of normal matter to dark matter, whether there is a cosmological constant etc) are also measurable in principle, but don't make as noticeable a difference.
Of course the sign is just a convention. But you are right that the "+" sign corresponds to an effectively repulsive force, and at the same time gives a positive contribution to the overall density of the Universe.
The reason for this is really down to General Relativity, and I think it would be a lie to claim that there's a simple non-GR explanation. Einstein's field equations can be used to describe the whole Universe, and the solutions still work you can if you add an extra constant (&lambda ), which can be interpretted as the energy density in the vacuum. In cosmology the common form of Einstein equations are normally referred to as the Friedmann equations, and they describe how the scale of the Universe changes with time depending on the contents of the Universe. As well as the normal matter density parameter ( &omega ), you can also consider models with an additional &lambda term.
If you write down the Friedmann equations with just a &lambda term, and it has the usual positive sign, then exponential expansion results. This is just what happens during an inflationary epoch in the early Universe, when the expansion was driven by a large energy density in empty space (which appeared for some reason that particle physicists will understand one day!). So inflation is just a &lambda - dominated universe, and you can think of this extra energy of empty space corresponding to a "repulsive gravity". This repulsive term is what you get if you have &lambda with the usual positive sign.
Inflationary models generally predict that we should have (in appropriate units) &omega + &lambda = 1. This corresponds to flat, rather than curved, space for our Universe. The Universe is expanding, but the space within it has a flat rather than curved geometry.
Observations today suggest that we may live in a Universe with say &omega = 0.3. And many people prefer a model which is still flat, so that the contribution to the density from a cosmological constant term is about 0.7. The Universe you get is closed in the sense that the curvature is flat. But it's a Universe which is starting to get dominated by the &lambda term, i.e. if we live in such a Universe then we're just starting to inflate again! So if this is really our Universe, then it's flat space, but will expand forever, so that it's "open" in that sense.
Of course it's possible that the whole Universe is rotating (although let's not get into the issue of what it's rotating relative to!). But there are quite strict limits on how fast this rotation could be.
Like other things to do with the large-scale properties of the Universe, the best constraints come from the CMB. A rotating Universe would leave some recognizable pattern on the microwave sky - which we don't see. The general limit is about the size of the detected CMB fluctuations in dimensionless units. In essence this means that the rotational frequency has to be less than about a 100,000th of the expansion rate, or equivalently the rotation period has to be more than about 100,000 times the age of the Universe. That's a very slow rotation!
For specific models the limits are tighter still. The simplest rotating Universe is one which is entirely uniform on the largest scales, except for having rotation about some axis. It turns out that such a universe would have a spiral pattern on its microwave sky! The absence of such a pattern, even at a very faint level, in the data from the COBE satellite, allows very strict limits to be placed, which are as much as 10,000 times stronger. Such a universe (technically called a Bianchi VIIh cosmology) can only have rotated by a tiny fraction of a second of arc in the whole age of the Universe.
So the simple answer to your question is that we now know with a fair degree of certainty that our Universe isn't rotating.
I am not really an expert on either of these two topics, but let me give you brief answers.
For your first question, you figure out how old the Universe might be by trying to estimate the age of the oldest objects you can reliably find ages for. Locally we can use isotope ratios (radiological dating) to estimate that the solar system is around 4.6 billion years old. And then similar techniques can be used to date material throughout the disk of our whole Galaxy to be at least perhaps 10 billion. The oldest stars we know about appear to be those in some of the globular clusters in the outer regions of our Galaxy. The ages of globular clusters (agglomerations of thousands or even millions of stars) can be estimated by looking at which stars have already began to run out of nuclear fuel, and calibrating this using the properties of nearby stars and the physics of stellar evolution. The oldest globular clusters turn out to be perhaps 12 billion years, with some estimates being even higher. The end result is that we believe our Universe to be older than maybe 12 billion years, while there is no evidence that it needs to be much older than that.
Galaxy classification is just a matter of recognizing features in the make-up of a galaxy: shape, strength of spiral arms, presence of a central bar, general size, etc. Of course this is just pigeon-holing, and is only really useful if the classification scheme helps you understand how galaxies "work", or even better how they might have formed. This might be similar to observing different cars for instance - if you wanted to know how cars work, then certain properties might be useful (e.g. what sort of fuel they use, etc.), while other properties (e.g. recognizing the make of car) might not help with any fundamental understanding at all. Having said that, the main way of classifying galaxies has been useful because it appears to distinguish some fundamentally different sorts of object. Certainly ellipticals appear to be very different from spirals, and dwarf galaxies are different from giant galaxies.
For more details on this I suggest you search around with your favourite search engine on something like "galaxy classification". And for your first question look for something like "globular cluster ages". I found a whole bunch of useful information when I tried this myself, - and much more detail than I could give here.
It's interesting to get this query from you! Overdensity is one of the words I use fairly frequently, but which I have to keep in my personal spell-check dictionary!
Let me explain the picture here, so you have a clearer sense of the word. Standard models of the early universe have everything being very smooth, but with regions which are ever so slightly denser than average and regions which are ever so slightly less dense than average. Gravity acts to enhance the higher density regions, so they get more and more dense and eventually form into galaxies etc., with lower than average densities in-between.
This is such a fundamental part of the modern picture of structure formation in the Universe, and discussed so frequently in the scientific literature, that words are needed to describe these regions. Hence an "overdensity" is a region which contains more density than the average, with "underdensity" being a region with less density than the average (and here by "density" I mean the usual mass per unit volume). Since overdensities are more important than underdensities (in the sense that galaxies etc. from from them, rather than spaces between galaxies), then "overdensity" has also been appearing quite frequently in the popular science literature.
Where these overdensities come from in the first place is of course a matter of active research. And exactly whether every overdense region ends up forming a galaxy is also probably complicated. But certainly there had to be overdensities in the distant past, otherwise we wouldn't be here! And today, our galaxy is very much an overdensity, as is the Local Group, all clusters of galaxies, and on larger scales super-clusters are still overdense.
Прислал leadbeaters"AT"email.msn.com 9/98
"Flat" does indeed refer to the geometry of space. It means the same geometry that you (and Mr. Euclid) are used to thinking about. The angles in a triangle add up to 180o, the area of the surface of a sphere is 4 &pi r2, etc. However, it's not exactly the same as Euclidean space, since it's expanding! We know the Universe is expanding because more and more distant galaxies have higher and higher redshifts. This expansion is an empirical fact which is independent of the geometry of space (whether it's "flat", "spherical" or "hyperbolic"). "Flat" means that there is no curvature of space, while "spherical" means positive curvature (like the surface of a sphere, but in one extra dimension) and "hyperbolic" means negative curvature (like the surface of a saddle, but in one extra dimension).
Whether the Universe will expand forever is a related, but different question. The answer depends on what sort of matter dominates the density of the Universe. Universes with the hyperbolic geometry are usually referred to as "open", since they have both infinite volume and will expand forever. Universes with the spherical geometry are referred to as "closed", since they have both finite volume and will recollapse. A flat geometry universe, if dominated by regular matter will expand more and more slowly, but never quite stop expanding and recollapse. However, if there is some contribution from the energy density of the vacuum (sometimes called a "cosmological constant"), as many cosmologists currently believe, then the overall geometry may be flat, but the repulsive effect of this weird form of matter will cause the Universe to expand faster and faster rather than slow down. It's also possible, for example, to make closed universes which expand forever - all you need is the right mix of exotic matter!
The answer to your question then is that it depends! But the current best-guess cosmological model (which doesn't of course mean it's right) has flat geometry, with a lot of common-or-garden dark matter, but an even bigger contribution from the cosmological constant, causing the Universe to be on the verge of expanding more and more rapidly forever. The best evidence for flat geometry comes in fact from the power spectrum of CMB anisotropies, as described in the answers to many questions on this page!
Прислал DAUISE"AT"aol.com 6/99
There are answers to a couple of similar questions near the top of this page. But this is a little different, so let me try to explain.
Things which are self-gravitating are not expanding (e.g. galaxies, you and me, meter sticks, etc.). The blob of stuff that formed a galaxy used to be expanding, but a little slower than the average, then eventually it stopped expanding and started to collapse, then complicated physics took over (gas and radiation processes, electromagnetic forces, etc.). Once a galaxy has "formed" it no longer cares about whether it's in an expanding Universe or not.
So it's much better to think about the space expanding between galaxies. Objects like meter sticks, and anything else within a galaxy are so much more dense than the average parts of space, that they are unaffected by the expansion of the Universe as a whole. This means that you can in principle measure the expanding space between galaxies by using a common or garden meter stick (I wouldn't recommend trying it though, since I'm afraid that the average human lifespan isn't well-matched to the task!).
Прислал Plasticplate-eng"AT"worldnet.att.net 10/99
I try to avoid talking about religion here, for fear of offending people. So let me try to answer from the purely cosmological point of view.
Basically, I don't understand what is being stated. The implication is that there is time dilation involved, but that makes little sense (there are big differences between an expanding Universe and the physics of special relativity). If we could see something happen at a redshift of around 1012, then a 6 day long event would appear to us to be stretched out by that factor of a thousand billion and hence be around 15 billion years - but there's no way of observing anything happening at that distant time, which isn't a particularly special epoch. And even if there was, then an observer at that epoch wouldn't be seeing this time dilation. So I can't figure out what is meant here!
Moreover, there is no such thing as "the other side of the Big Bang", since the whole point of the model is that you can extrapolate back arbitrarily close to t=0, when time itself (somehow!) began. But certainly there was a point in the evolution of the Universe when the age was 6 days. However that was long after (speaking relatively of course!) the rather well understood era of synthesis of the light elements. And at that time the light travel distance is only about 6 light days, so there's no way that you could affect things further apart than 6 light days - or in other words the causal distance was 6 light days, which is pretty small (by astronomical standards anyway!). You might argue that it's possibile to break causality and affect the whole Universe. But if you can break causality then you can do anything you like, whenever you like. Or even before!
Прислал mb10117120"AT"isbank.com.tr 2/00
Our motion through the sea of CMB photons shows up as a "dipole" pattern on the sky, with one side of the sky having a redshift (and so appearing a little cooler than average) and the other side having a blueshift (and so appearing a little warmer than average). There is obviously a frame in which we would not observe a CMB dipole at all, and in a sense this defines an "aboloute rest frame" for the Universe.
This seems to worry some people, since Einstein (and all relativists since then) have taught that there is no such thing as an absolute reference frame. But in fact you have to listen a little more carefully to what is stated. The statement of the principle of relativity is that there are no special frames for the performing of any physics experiment. In other words, the laws of physics are expected to be the same in all frames. The fact that one frame has a different observed CMB dipole than another frame is really inconsequential - the performance of any physics experiment (other than merely trying to measure the dipole!) would be exactly the same in all the frames.
So yes, there is in a sense an absolute rest frame. And through observation of the CMB dipole we can measure our velocity with respect to that frame.
Прислал skelley"AT"fc.fcps.k12.va.us 2/00
This is an area in which I should make it clear I am not an expert!
Certainly there are some data, at least at relatively low energies (by early Universe standards at least). There are some indications that the strong interaction coupling decreases at high energies, to reach that of the electro-weak forces at some very large energy (more than 10 orders of magnitude higher than are probed in today's biggest particle accelerators). This is the strongest empirical reason to believe in the idea of Grand Unification of the forces at extremely high energies, which must have described the Universe at extremely early times.
If you want more detailed information about particle physics, then I suggest you start by looking at material and resources on the web-page of the Particle Data Group.
Прислал zylon"AT"netzero.net 02/00
This is certainly possible. I might say that it's even my favourite model for the Universe! That's not so much for empirical or philosophical reasons, but just because it seems to me to be more amusing!
Although I've explained similar things before, let me do so again. There is a certain "critical density" for the Universe, which is determined by how fast the Universe is expanding. Specifically we can write &rhocritical=3 H02/(8 &pi G), where H0 is the Hubble constant, or expansion rate of the Universe. It 's conventional to define a new parameter &omega as the average density of the Universe in units of this critical density, i.e. &omega = &rho / &rhocritical. The spatially and temporal desciption of the Universe then depends on the values of &omega , interpretted in two slightly different ways.
Whether the Universe will expand forever or not, is a balance between kinetic and potential energies. If the Universe is expanding fast enough compared to all the normal gravitating matter within the Universe, then it will expand forever. If there is enough gravitating matter to halt the collapse, then eventually the Universe will re-collapse. This is determined by whether &omegamatter > 1.
Whether the Universe is spatially closed or not depends on the overall curvature of space. This in turn depends on the total mass-energy density of the Universe. This can include exotic forms of energy, like the energy density of empty space, which there is some reason to believe might be greater than zero (otherwise known as a postitive cosmological constant). In otherwords the Universe can be spatially closed, i.e. have finite volume, if &omegatotal > 1. In fact with a positive cosmological constant, or other exotic forms of matter which behave similarly, the future Universe eventually becomes dominated by this exotic "dark energy" and can in principle expand forever (even if &omegamatter > 1!).
Cuurently favoured cosmological models have &omegamatter around 0.3-0.4 and &omegatotal close to 1 (derived from the macth to the characteristic angular scale in CMB anisotropies), with the balance made up in some form of dark energy. It is certainly possible to have a Universe where the dark energy slightly overdoes it, and the Universe is spatially closed but will still expand forever.
Прислал jeu198"AT"soton.ac.uk 3/00
Although I'm not an expert in this area, I've answered a few questions on black holes before. So let me give a short reply and then point to a better source of black hole information.
The answer is that black holes are made of whatever collapsed to create the black hole. Exactly what is going on inside the event horizon right now may depend on how long ago stuff fell in, and what form that matter had. It might all be in a singularity at the hole's centre, or it might be more complicated than that. Such speculation is moot however, since you can't tell anything about what's going on from outside the event horizon anyway!
For more answers to common questions about black holes see the Black Hole FAQ pages by Ted Bunn and by Robert Nemiroff.
Прислал rgonzalez"AT"nwic.edu 4/00
The Universe today appears to have essentially no anti-matter. You can make anti-particles in accelerators of sufficiently high energy, but they tend to annihilate again with particles pretty quickly. There's also a small fraction of anti-particles in cosmic rays that are raining down on the Earth - these are consistent with the idea that high energy particles smash into heavy nuclei in the interstellar medium, making some anti-particles as well as other nuclear debris. So we know that there are some anti-particles around, but in insignificant amounts.
There has long been speculation (particularly in the pages of science fiction more than in physics journals!) that there may be anti-planets or anti-galaxies in the Universe. In fact the gas that exists in galaxies and even between galaxies would annihilate with any anti-galaxies that existed. And so if there were anti-objects on astronomical scales, then the sky would be glowing with the gamma rays created during the annihilation. The lack of such a gamma-ray signature makes it pretty certain that there's little if any anti-matter in the Universe.
This leads to the obvious question of how the Universe got to be so asymmetric with respect to matter and anti-matter. It's easy to form matter/anti-matter particle pairs from pure energy, but it's essentially always done in equal amounts. Nevertheless, since the Universe is now dominated by matter, then for some reason the physics of the early universe must have preferred matter over anti-matter. This is known as baryosynthesis, and is related to the properties of particles at very high energies, including what is called CP violation (that particles are preferred over anti-particles, by very small amounts in accelerator experiments). There are many ideas for models which carry out adequate baryosynthesis, but no obvious winners yet.
Nevertheless, it's interesting to know that the mere fact that there's not much anti-matter around potentially tells us something profound about high energy particle physics!
Прислал eng"AT"computron-corp.com 4/00
You are entirely correct! It's wrong to say that the Universe was smaller in the past (whether near the time of the Big Bang or any other time).
In an expanding Universe what is meant is that the distances between observers in the Universe is getting larger with time. In a sense this means that the Universe is getting bigger - but if the Universe is truly infinite in size, then in the past it was still infinite, only it was smaller! Infinity can be a tricky concept. This is particularly true if the Universe began with a Big Bang singularity, in which all of infinite space started in a single point! If you know of a good way to picture that, then let me know!
Прислал LABELE"AT"aol.com 4/00
I have to confess to not entirely understanding this question, but I'll have a go at answering it anyway!
The speed of light is (as far as we know) a fixed physical constant. The expansion of the Universe is firstly an empirical fact (discovered in the 1920s from the Doppler shifts of galaxy spectra), and secondly is understood through simple models of the Universe (in fact static models are unstable in General Relativity). The rate of expansion is measured as a speed per unit distance. So it has the dimensions of an inverse time. Speed has the dimensions of length divided by time. And so these two quantities cannot be directly compared, since they are entirely different things.
If you interpret the expansion of the Universe as a speed of recession for two given objects, then that speed depends on how far apart the objects are. For objects near the edge of the observable Universe, the speed of recession approaches the speed of light. This is entirely a consequence of what is meant by the "observable Universe". It is really a definition, since something which is so far away from us that it is effectively moving away faster than the speed of light is unobservable!
Прислал shantanu"AT"bu.edu 4/00
Yes there is.
Прислал rlsbk"AT"gly.uga.edu 5/00
I think you're right that there may be a problem with some people using "Universe" in two senses. But personally I try to stick to one use! This is partly why I like to capitalize the word Universe - to emphasize that I mean the entirety of stuff. In other words, one shouldn't think of the Universe as being a collection of things embedded in some larger object, for then that larger object should really be the thing that you label "Universe".
So first of all you should erase any ideas that you can talk about the Universe being "in" something else! This may help clear up the conceptual misunderstanding.
Secondly, the other problem people can have is with a mental image of the Universe expanding outwards. This obviously has no meaning unless there's something the Universe is in, and so we're back to the first problem. To solve this, many cosmologists try to avoid using the word "explosion" to describe the Big Bang. "Explosion" (as indeed the inappropriate term Big Bang itself) conjures up an image of something sitting there localised in space, which then explodes outwards.
Instead think of the Big Bang as being the model in which the Universe is getting less dense and cooler with time, and which can be extrapolated backwards to arbitrarily early times (with t=0 perhaps excluded). Expansion of space should be thought of as the distance between objects getting larger, rather than the volume of space growing as it expands into something.
And always remember that this is hard stuff to get your mind around!
Прислал DLZC"AT"aol.com 5/00
Objects aren't generally expanding relative to their neighbours, because the local force of gravity dominates compared with the expansion. So you're not expanding, the Earth isn't expanding, the Milky Way Galaxy isn't expanding, etc. The Local Group of galaxies are whizzing around in each others' gravitational pull. And this is largely true for the whole Local Supercluster too. It's only on larger scales that everything is moving apart.
As to the question of whether we are at the centre, consider this. It isn't obvious until you've worked it out for yourself, but in a uniformly expanding medium everything is moving away from everything else. To convince yourself of this you can draw dots on a piece of paper, then photocopy to a bigger scale (say a factor of 2). That will stretch all distances by the same factor. Now pick a dot in the first picture and measure the change in distance for a bunch of the other dots. You'll find that something that was 1cm away got 1cm further away, something that was 2cm away got 2cm further, something that was 3cm away got 3cm further, etc. This is precisely the Hubble expansion law: change in distance in a certain interval of time (otherwise known as speed!) is proportional to distance. The trick is now to choose another dot and repeat the exercise. You'll find that exactly the same thing is true. In other words if the Universe is undergoing uniform expansion, then every observer sees distant things expanding away according to Hubble's Speed Distance law.
So in a sense we are at the centre of the Universe, but then so is everyone else!
Прислал DLZC"AT"aol.com 5/00
Firstly it's by no means certain that there was a singularity. That's just the simplest concept for the beginning. But there are plenty of ideas for more complicated things, which are even harder to picture!
Secondly, if indeed there was a singularity, then it would contain the whole of the Universe. So, by definition, there would be no getting out of it, because that would mean leaving the Universe. The whole Universe would be contained in that single point - it would just stop being a singularity when it started to expand!
This is true for distant galaxies. But not for objects which are "gravitationally bound" (i.e. not flying apart relative to each other in the expanding Universe). The Local Group of galaxies, for example, is being pulled around by the mutual gravitational forces of all the members. That region of the Universe is expanding away from distant regions, but is not itself expanding. In fact the Milky Way galaxy is currently moving towards our nearest neighbouring big galaxy, Andromeda. We are also bound to many other galaxies within the Local Supercluster. And so we should expect at least those objects to stay in our skies for a very very long time!
Прислал gazic_miles"AT"si.com 6/00
Perhaps the best up-to-date textbook for someone with some physics background to understand modern cosmology is John Peacock's Cosmological Physics, which is published by Cambridge University Press, and a section of which is available here. I probably wouldn't recommend this to anyone who has never taken a university level Physics course, however! For a less equation-laden introduction I tend to recommend one of the two general books by Martin Rees - Before the Beginning: Our Universe and Others and Just Six Numbers, which are available at the usual on-line book sources.
Since both of these gentlemen were once my teachers, then my opinion may be biased (but I think I've been fortunate to have some excellent teachers!).
Прислал John.Campbell"AT"siinet.trw.com 6/00
General Relativity seems to be a good theory for describing gravity at all scales and strengths which we have experimentally probed. That theory has a set of equations (known as Enistein's Field Equations) which allow you to relate the distribution of matter and energy to the curvature of space, for any particular arrangement. One set of solutions to those equations involves considering a point mass in an otherwise empty and static Universe. This solution (technically called the Schwarzschild metic) is the approximate situation which describes black holes in the Universe. There is a "horizon" separating the inside of the hole from the outside, so that once you fall in you can't communicate again with the outside.
There is an entirely different set of solutions to Einstein's equations (technically called the Friedmann-Robertson-Walker metric) which describe the curvature of the whole of space for universes which are the same density at every point (homogeneous) and have no preferred directions (isotropic). These are believed to be good models for describing the structure of the entire Universe in which we live. Much of current cosmological research is directed towards figuring out precisely which one of this set of models best describes our Universe.
The answer to your question is that the description of a black hole and the description of the entire Universe are two very different things. In the cosmological solutions there are typically no horizons (although in principle you could contrive for there to be one if you really wanted), and so in that sense we don't live in a black hole. Moreover the Universe is expanding, and, as far as we know, infinite in extent. On the other hand the black hole solution describes a locally very curved region, embedded in a space which is flat at distances sufficiently far from the black hole. And this is really a very different picture.
Прислал zeynel"AT"axiom-one.net 8/00
This is a good point - it is important to keep in the back of one's mind that scientific inferences are always based on a set of assumptions and that there's an additional difficulty in cosmology because we only have one Universe to observe.
The Cosmological Principle is the name given to the idea that we don't occupy a special place in the Universe. This is generally assumed when considering various observations that we make. However, like other assumptions in Science, it is testable. We could discover that in fact we are in a special place, and that the Universe looks very different when viewed by observers at different locations. Then we would have to give up the Cosmological Principle and replace it with something else. However, as far as we can tell the Universe is pretty much the same everywhere - on sufficiently large scales at any rate.
We have mapped out the distribution of galaxies around our own Milky Way galaxy, and there is nothing particularly special about where we reside. We live in the Local Group, which is a pretty wimpy bunch of galaxies. This lies somewhere out towards the edge of the Local Supercluster, which is one of a few tens of similar large scale objects spread throughout the obervable Universe. On still larger scales the Universe is quite homogeneous, with no indication that there are any particularly special directions out there. Failing any good evidence to the contrary, the best approach is to assume the simplest thing, which is that we occupy a perfectly average place.
There are other assumptions we often forget we make, which are also testable in principle. One of them is that the laws of physics are the same everywhere in the Universe. It would be possible to find that this is not the case - however, there is no such evidence at the present time, and so again it is simplest to adopt the working assumption that you can apply the same physical laws everywhere. A last assumption we make is that the Universe is understandable at all! Whether that turns out to be true remains to be seen. But it is quite astounding that Human beings, which encrust this small rock called Earth, have the ability to construct even the sketchiest of models for the whole Universe!
Прислал feltzmike"AT"netscape.net 9/00
This isn't exactly a question, but let me comment anyway!
The connection between Riemannian geometry and the curvature of the Universe is certainly a good one - it was in fact one of the things on Einstein's mind when he came up with General Relativity. The Friedmann models are solutions to Einstein's field equations for uniform and isotropic universes. So Friedmann models are not only consistent with Riemannian geoemtry, they are entirely based on Riemann's ideas!
The only extra ingredient in the Friedmann models is the dynamics, i.e. that the evolution of the Universe is related to its contents. In our particular case we live in an expanding universe, and the rate of expansion depends on the forces due to matter, radiation, dark energy etc. that fill the Universe. So Friedmann models are simply expanding Riemannian geometries.
It now appears that our Universe is close to flat (although expanding). Whether it is a little bit "open" or "closed", in the Riemannian sense, is still to be established though.
Прислал jwinsor"AT"kelvin.physics.mun.ca 10/00
The Hubble expansion law is v=H0d, where "v" is the recession velocity of a galaxy, "d" is its distance, and "H0" is Hubble's constant. The way to find ths constant is to measure the speed and distance for a bunch of galaxies and fit the best value of H0.
Measuring speeds is the easy part, since the spectra of galaxies are redshifted, and measuring the shift in the spectral lines gives the speed directly. Estimating distances is the hard part. A range of different techniques is used, mainly involving comparing properties of distant objects with nearby objects whose distances have already been estimated with other methods. Generally astronomers are ecstatic if they get distances accurate to say 20%!
The end result is that the Hubble constant has its order of magnitude very well determined, but its precise value still elludes us. Normally it is quoted in the units of km/s per Mpc (Mega parsec, roughly equal to 3 × 1026 metres), because velocities are usually measured in km/s and cosmological distances in Mpc. The values found are typically in the range 60-80 km/s/Mpc.
As to the age of the Universe, that's easy in principle, but in detail depends on the precise model. The easy part is to imagine running the Universe backwards in time. If it had been expanding at a constant rate, then all points in the Universe would come together a time 1/H0 ago. So this is the estimate of the age of the Universe (if you think about the units of H0 you'll notice that it's really just 1/Time in peculiar units). The complicated part is that the Universe hasn't been expanding at a constant rate, and so the true age depends on the past history. Typically the Universe gets decelerated by the mass within it, and so the 1/H0 estimate of the age is too high. But in models with so-called Dark Energy (also known as the cosmological constant or vacuum energy), the Universe has recently started to accelerate, and then 1/H0 is closer to the correct age.
Прислал cmcdonald"AT"regenstrief.org 10/00
From observations of the Universe it is possible to determine the average density. In other words the mass per unit volume (or the mass-energy equivalent per unit volume). Currently the mass-energy census of the Universe identifies at least 5 separate components: ordinary matter (baryons); massive neutrinos (a known, but ellusive particle, which may have a small mass); cold dark matter (some as yet unidentified particle); photons (mainly the CMB); and Dark Energy (which may dominate the census, even although it doesn't behave like matter at all!). It appears that the Universe has a "flat" geometry, so that &omega =1, and estimates for the contributions from each of the 5 components are 5%, 0.3%, 30%, 0.01% and 65%, respectively.
Coverting &omega into density requires having an estimate of how fast the Universe is expanding, i.e. the Hubble constant (since that goes into the definition of &omega , as described in another answer). Using a typical value for H0 the overall density of the Universe turns out to be about 10-26kg/m3. This corresponds to about 1011 times the mass of the Sun in every cubic Megaparsec of volume. This value is uncertain both because the value of &omega isn't precisely known, and also because the value of H0 isn't precisely known either. But it's certainly the correct order of magnitude. You'd also get a proportionately lower number if you wanted only the density in baryons, for example.
The total mass of the Universe, on the other hand, is not a very clear concept. The Universe is likely to be either infinite in volume, or so very large that it can be considered infinite for all practical purposes. That means that the total mass of the Universe is also infinite. The thing which can be well-defined though, is the mass within the observable part of the Universe. In other words we can ask: how much mass is contained within the volume that we can have observed since the Big Bang? The radius of the observable Universe is about 10,000 Megaparsec (or about 3 × 1026 metres). Using the above estimate for the total density this gives a total mass in the observable Universe of about 1054 kilogrammes. That's the best answer I can give for the mass of the Universe!
Прислал ForthEJ"AT"Cardiff.ac.uk 10/00
This is an excellent question!
There are several answers to this question, depending on the level of detail, or what you are prepared to accept as given. Let me give you 2. You can find more discussion in many standard undergraduate astronomy texts, in the cosmology section.
The first answer is that locally energy is always conserved, and that the rate of change of total energy density (in matter plus radiation plus whatever other kinds of fields might exist) is balanced by the work being done (per unit volume) by the pressure. So as long as you consider the total energy density (actually you need to use the correct relativistic quantity, which means adding in the contribution of pressure to the energy density), you find that at each point is is balanced by the work done by the pressure in expanding the volume.
However, you still have the problem of whether the global energy is conserved. CMB photons have their number per unit volume (or number density) reduced as the Universe expands, and the energy in each photon also gets reduced by the expansion. So the energy density in the CMB decreases faster than just the number density. In other words the energy density is falling off faster than the volume is increasing. So the total energy in CMB photons appears to be decreasing with time! The ultimate resolution to this problem is that the issue of global energy conservation in an infinite universe (or even a closed universe for that matter) is a thorny one within general relativity. If this bothers you, then rest assured that for all practical purposes the resolution of the problem is surely entirely irrelevant!
Прислал cmcdonald"AT"regenstrief.org 10/00
If the Universe has "flat" or "open" geometry, then formally it has infinite volume, and therefore infinite mass. Something which is better defined is the mass within the "Observable Universe", which means the part of space from which we can have received light in the history of the Universe so far. This gets bigger every day!
The approximate answer is that the radius of the observable Universe is currently estimated to be about 300 Ym (that's "yotta-metres", or 1024m, which is the largest SI prefix!). So you take that number, cube it, and multiply by 4 &pi /3, to get the volume of the whole sphere.
Then you have to decide what density you want to use. Are you only interested in luminous matter, or do you want to include all the baryons (regular stuff made of protons and neutrons)? Or do you want all the particle dark matter too? And what about the dark energy? If you include everything, then the average density in the Universe today is about 10-26kg/m3. And the mass in the observable Universe can be estimated accordingly.
There are several uncertainties and approximations here though - so don't expect your answer to be much better than an order of magnitude estimate!
Прислал yuriohara"AT"hotmail.com 12/00
This sounds like an attempt to get me to do your homework! While, I regularly give out free information for general questions on cosmology, there is a fee for doing homework!
Let me just try to offer a hint, which might help others who read this. The issue is units. If I told you that I was travelling for 30 hours at a speed of 3 miles per fortnight, and asked you to calculate how far I would get, then you wouldn't just divide 30 by 3 to get 10, right?
Прислал thom.warmerdam"AT"mail.verizon.net 1/01
Your image of the Big Bang is far from primitive, but I'm afraid it is still incorrect. There is a very common misconception that the Big Bang is an "explosion" that happened at a point in space. This leads to some problems in understanding some things in cosmology, like the CMB photons. Let me try to paint you a correct picture.
First of all, no one knows exactly what happened to set the Universe expanding (although there is no shortage of ideas). What we mean by the Big Bang is that a model in which the Universe has been expanding, and used to be hotter and denser, provides a very good fit to a wide range of pieces of empirical evidence.
But let's imagine, for the sake of argument, that the Universe really did start with a singularity, which started expanding. The problem is that this singularity contains the whole Universe - so it's infinitely small and infinitely big at the same time! This is something which is very hard to picture. It turns out to be most helpful to think about a very large Universe which started expanding everywhere at once (and ignore the singularity aspect, since singularities by definition are places where your intuition tends to be in trouble!). So in the very early Universe everything was much closer together, but the volume of the Universe could still be infinite (if that's the sort of Universe we live in). Every point in the Universe expanded from every other point, with no special place being the "centre" of the "explosion".
I think if the word "explosion" was banned from all articles written on cosmology, then it would do much to alleviate this misconception!
Прислал ulbusi"AT"tiscalinet.it 2/01
It's an interesting fact that in a Universe dominated by ordinary matter, and with enough density to have a "closed" geometry, you get get exactly once round the Universe in the time between the Big Bang and the Big Crunch! (Think of this as a hard-to-picture 3 dimensional analogy of going round the 2 dimensional surface of a sphere).
With the possible existence of "Dark Energy" or the "cosmological constant", it turns out that it is possible to traverse a closed Universe - and there never is a Big Crunch in fact (since the Universe continues to expand forever, driven by the energy density of the vacuum).
Best guesses for the sort of Universe in which we live make it very close to having "flat" geometry. In that case the Universe is really infinite in volume, and so you can't go round it (since it's radius is effectively infinite in size!).
It's still empirically possible, of course, for the Universe to be a little closed. But it is close enough to flat, that the possibility of things going all the way round the Universe remains basically a mathematical quirk, with little or no observational consequences.
Прислал sorceresscherish"AT"hotmail.com 4/01
Some more people wanting me to do their homework for them! All of those questions can easily be researched by surfing various web-sites. You could start by looking for ones that I link to on these pages. It's much better to try to find out for yourself through reading, rather than just asking someone!
But let me not be entirely unhelpful! I'll tackle your Cosmological Principle question.
That term was probably coined by a mathematical cosmologist called E.A. Milne in the 1930s. It's the idea that we don't live at a special place in the Universe. More specifically it is used, together with the observed isotropy of the Universe, to imply that the Universe must be statistically homeogenous (i.e. the same at all places, averaged over sufficiently large scales). In other words the Universe will look roughly the same, no matter where you are.
Прислал Glytrin"AT"aol.com 4/01
This question could mean at least a couple of different things, so to be safe let me answer both!
The normal stuff in the Universe (technically called "baryons") is, by number of particles, about 90% hydrogen, 10% helium and a fraction of a per cent heavier elements. If you ask what fraction by mass, then becuase helium weighs about four times as much as hydrogen you get about 3/4 of the mass of the baryons is in hydrogen, with about 1/4 in Helium, and only a trace amount of heavier stuff. So most of the normal matter in the Universe is simple hydrogen. The Earth has way more heavy elements than the average part of the Universe, because of gravitational and chemical settling in the formation of planets. So we're not made out of very common elements.
But various pieces of evidence point to the fact that normal matter (baryons) makes up only a small fraction of all the matter in the Universe. About 90% of the matter is in fact not baryons at all. It's not hydrogen, or helium or any other element. It's not made of protons and neutrons and electrons, but something else entirely. It has to interact very weakly with normal matter, exerting a gravitational force, but nothing much else. So we call this stuff "Cold Dark Matter", although we don't really know what it is!
Recent evidence also suggests that there's another kind of "stuff" in the Universe which isn't even matter at all! This stuff is called "Dark Energy", and we have even less idea of what it's nature truly is. But there appears to be about twice as much of this Dark Energy as there is Dark Matter.
So the Universe has only a small amount of baryons (only a tiny fraction of which consists of heavy elements necessary for life), and then even the Dark Matter isn't the dominant form of stuff. Why the Universe has chosen to be this way is one of the greatest mysteries known to modern science.
Прислал wjw"AT"bellsouth.net 5/01
One way to think about this is the following. The Hubble expansion law, discovered about 75 years ago, is that the velocity of recession is proportional to the distance. So what kind of "explosion" does that represent? In fact, if you think about it for a while (draw a bunch of dots at two difference times if it helps), this is precisely what you get in uniform expansion. If every distance gets multiplied by some constant at each time "frame", then more distant things are moving proportionally further away from you. But uniform expansion doesn't require a centre -- the opposite really. In uniform expansion everything is moving away from everything else, and so there's no special centre. It might look like things are moving away from you, but if speed is proportional to distance, then every other observer would see things moving away from them too.
So the observation of the Hubble expansion really fits with a picture in which everything is expanding from everything else. There's no centre, and need to think of it as an explosion localised in space.
Прислал jmorgan"AT"logantele.com 5/01
You are right! Infinity is a very tricky business.
One way to see this is to try to understand that if you have an infinite amount of something, and then you add a little bit, then you have exactly as much as you started with - an infinite amount! In otherwords if you add a finite number to infinity you still have infinity. And even if you multiply infinity by something you still get infinity. This is true even for the highest number you can think of (and the only exception is if you multiply it by zero!).
Thinking about it ths way, if the Universe is genuinely infinite and it's expanding, then everything used to be closer together, but that doesn't mean it was smaller! It has been infinite from the beginning. At the very beginning, if there was a singularity (and to tell you the truth, most cosmologists suspect you can't push things back that far) then everything was at one point. So the Universe would have been both infinite in volume, and have no volume at the same time. This is the problem of multiplying zero by infinity - which one wins?
Let's face it, at the moment no one has a clue how to picture the very beginning of the Universe, infinite or not!
Прислал nico.benschop"AT"philips.com 9/01
Photons travelling large distances through the expanding Universe do lose energy, since that's what the redshift is. The wavelength of the photons gets shifted towards lower energies. There have been various ideas about how maybe the light from quasars loses energy from some other process, so that quasars don;t havw to be at their cosmologically assumed distances. But such ideas, like "tired light" haven't fitted the data well for decades, as I've discussed on this page before.
The reasons to believe in an expanding universe are basically two-fold. Firstly you observe bigger redshifts for apparently more distant objects, and the simplest explanation is just that they're moving away from you at a speed which increases with the distance. This implies a framework in which the Universe as a whole is expanding uniformly in all directions. Which brings us to the second part - that you figure out what that means for observations of other astrophysical phenomena and see whether there's some contradiction or something that doesn't fit. The fact there is that the picture is entirely self-consistent. Almost uncountable amounts of observational data involving a huge number of different kinds of study (distant galaxies, clusters of galaxies, gravitational lensing, distant supernovae, the CMB anisotropies, quasar absorption clouds, abundance of the light elements, blackbody spectrum of the CMB, etc., etc.) make sense within the context of the expanding Universe picture.
This basic picture fits so well that we can start trying to determine more details than just this framework. The focus right now is on trying to figure out precisely what sort of expanding universe we live in.
Прислал frankhamersley"AT"hotmail.com 11/01
There are more full answers to essentally this same questions elsewhere on this very page!
For now let me just say that points 1. and 3. are correct, that 4. is true for everything in the Universe (by definition) including us obviously, but that point 2. is not a correct statement. There have always been regions of the Universe farther away than light has been able to travel in the age up to that point. That applies today and equally well to early times in cosmic history. The CMB photons that we see today came from such parts of the Universe.
Прислал pnkumar"AT"csnpl.ren.nic.in 11/01
I think you are ascribing way too much mystery to the dark matter. Think of it simply as a form of matter that's not very luminous - "unshiny stuff" I like to call it! There's no reason to believe that it doesn't conform to the normal laws of physics (like E=m.c2). Indeed we only know about the existence of this form of matter through its gravitational influence. So we know that it feels (and creates) the force of gravity just like any other kind of matter. But it doesn't seem to have any electromagnetic interactions (otherwise it would be way easier to detect). In that regard it's like the neutrino, and several other particles, which we know a lot about. But those already known particles can't be the dark matter, since they have the wrong masses.
Most cosmologists think that the dominant form of dark matter is simply some as yet undiscovered kind of particle. Or at least "undiscovered" in the sense that we don't know any of its properties, even although we know it's there. All we do know about it is roughly how much of it there is, that it experiences the gravitational force, and that it's not very shiny!
Прислал a.kay"AT"fisheries.ubc.ca 12/01
You could start by reading the answers to some of the relevant questions on these pages.
Another place to try is the CDMS (Cryogenic Dark Matter Search) experiment education page. And there are many more resources on the web.
Прислал rwpcuus"AT"yahoo.com 12/01
Let me say that I quite often get emails from people suggesting alternative theories for the formation, evolution or structure of the Universe. This one contained many more interesting ideas than most.
One of the first statements is wrong - since the CMB anisotropies give very clear evidence that there were slight overdensities in the early Universe, of just the right amplitude for gravity to have grown all the structure we see. But the ideas of cosmic centres around which structure gew are very reminiscent of cosmic-string seeded ideas which were popular with some cosmologists in the 1980s, but ultimately proved not to explain more recent CMB anisotropy results. And the last idea involving cosmic expnasion through some kind of ejection is not too dissimilar from "kinetic relativity" of the 1930s.
But if you want frank advice from me, it has to be this. Understanding, in detail, the formation of structure in the Universe, is not an easy matter. If you want to make any sort of valuable contribution to this area, then there is no substitute for an awful lot of reading and study. Go through some good popular books on the subject, and then work your way up to the more technical stuff. Once you are able to understand the average paper published in the cosmological literature, and you still disagree with the conventional picture, let me know your detailed objections. Sorry if this sounds dismissive, but look at it this way. I wouldn't seriously cosnider myself qualified to build a highway bridge, or perform surgery, without several years of formal study. Put it another way. I don't even take half the stuff I read ion the scientific literature terrribly seriously, and it's all written by people with Ph.Ds. from famous universities!
Прислал rreeves"AT"winfirst.com 12/01
Let me be frank here, and state that this stuff is hard to understand from the viewpoint of everyday common-or-garden experience!
To fully appreciate what's going on, you may need a deeper understanding of General Relativity than I'm likely to be able to impart to you in a few sentences! But nevertheless, let me try a brief answer.
The first point is that pressure of very fast objects can act as a source of gravity, just like mass does. Typical radiation, like the CMB, actually exerts twice as much gravitational force per unit volume than Newton might have guessed, because of this fact.
The second point is that empty space, "the vacuum", is something we know much less about than we'd like. Once physicists understood that particlce-antiparticle pairs can be blinking in and out of existence all the time, then it was no longer obvious that the energy density of the vacuum was zero. It turns out that the vacuum has to have a pressure which is negative and equal in magnitude to its energy density (that's a necessary consequence of the structure of space-time, effectively). iSo if the energy density is zero, then everything seems OK, since the pressure is then zero too.
But if there's even a tiny little bit of non-zero energy density in space which is devoid of matter or radiation, then there's also a negative pressure there. It's dwarfed by normal forces over small length sclaes, but it builds up over cosmological scales, so that it can dominate the dynamics of the Universe as a whole. Going back to the first point above, it turns out that a vacuum (with pressure equal in magnitude to its energy density, but negative) leads to a gravitational repulsion. So the Universe behaves very differently if there's a non-zero vacuum energy - or what Einstein called the "cosmological constant" is non-zero.
And at the moment that's precisely the kind of universe we think we live in!
Прислал loedvdh"AT"vgernet.net 2/02
This is a very good question!
Inflation is an idea which has several appealing consequences, and so is quite popular with cosmology theorists. However, there are also some shortcomings. The main one is that it is really just a good idea, looking for a specific theory! In other words, no one knows precisely what kind of inflationary model may describe what happened in our Universe.
So the simple answer to your question is "nobody knows"! It's clearly a requirement that the epoch of extremely rapid expansion has to stop at some point. So all reasonable inflationary models have to build in a way of stopping this inflation, as well as generating all the matter in the Universe from the fields that existed during inflation, and then turning into a run-of-the-mill expanding Universe.
The hope is that one day a single inflationary theory will come to the fore, through a combination of empirical evidence and theoretical work perhaps. Then everything will be clear!
Прислал gte878n"AT"prism.gatech.edu 3/02
In fact some nearby galaxies do have blueshifts! That's because they happen to be coming towards us. Every galaxy has its own local or "peculiar" motion. For ditant galaxies the expension speed dwarfs these "peculiar" motions, and so you can ignore them for most practical purposes. But for nearby galaxies like Andromeda (M31), their motion towards us is bigger than the motion which could be casued by the expansion of the space between us, simply because they are so close.
However, these local blueshifts tell us nothing very fundamental about the nature of the Universe.
Howerer, if we were to observe very distant galaxies blueshifted, then that would indicate something was quite wrong with our picture (e.g. that Universe had been contracting in the past). So far we've never seen anything with a large blueshift which appears to be very distant. All distant objects have a redshift, with that redshift increasing with distance away - hence our picture of the uniformly expanding Universe.
Interestingly, if we lived in a universe which stopped expanding and started contracting again (and let me state clearly that we do not currently think that it's likely we live in such a Universe), then there would come a time in the future when all the galaxies would become blueshifted. They'd rush faster and fast towards us until we all disappeared in a Big Crunch!
Прислал ArtCatsDOTNet"AT"aol.com 4/02
We know from surveys of regions of the Universe, that objects are not distributed in such a geometrically simple way. There are objects all over the place, and they are clustered together over a wide range of scales. There are groups of galaxies clumped into clusters, which lie at the intersections of "filaments", and with low contrast "voids" separated by "walls", etc. The distribution looks a bit "foamy", but the "voids" are far from empty, just under-dense compared with the clusters.
Cosmologists have developed many statistical tools to help quantify and understand this clustering. One tool which is sometimes useful is to use the so-called "Voronoi tesselation", which is a little like what you describe. Although galaxies don't lie along the sides of such polyghedra, the picture can still be used to help describe some of the average properties of the distribution. If you are interested in this topic I suggest you search for things on the web relating to Voronoi foams and cosmology.
Прислал katharos1.scotland"AT"tinyworld.co.uk 4/02
It's easiest to think about this in 2 dimensions (where your brain can easily visualize things), and then extend that to 3 dimensions by analogy.
In 2D, there are 3 possible ways that things can be curved. Either the space is flat, or it's like the surface of a sphere (which is called "positively curved"), or like the inside of a trupmet or the surface of a saddle (curved one way in one direction, but the opposite way at right angles, which is called "negatively curved").
In flat space you know that the angles of a triangle add up to 180 degrees. And that if you start with a straight line, there's only one parallel line which goes through a specified point. But if space is positively curved then the angles of a triangle add up to more than 180 degrees (think about drawing a triangle on the surface of a sphere), and there are no parallel lines! In negatively curved space, the angles add up to less than 180 degrees and there's an infinite number of parallel lines!
In 3D space, you have to imagine things curved in some hypothetical 4th dimension, which obviously you can't see. But you can still do experiments to figure out what kind of space you live in. For example you can try constructing a huge triangle and then measure the angles. It turns out that you can perform just such an experiments using the characteristic angular size of variations on the microwave sky. The answer seems to be that the geometry is very close to flat, just like Euclid described thousands of years ago!
It's the material contents of the Universe that cause the curvature. A purely empty universe would have negative curvature. So in order for space to have nearly flat geometry, there needs to be enough stuff to make the curvature close to zero. Since there isn't enough normal matter around, or even Dark Matter, then that means there has to be another component of stuff filling the Universe, and contributing to the curvature. This mysterious stuff is sometimes referred to as Dark Energy. And we think it makes up about two thirds of the total mass-energy content of the Universe.
Прислал sa3910"AT"mail.eclipse.co.uk 05/02
The simplest view of the earliest history of the Universe is that it expanded from an intial singularity. That means that the whole Universe used to have infinite density. Or in other words, all of infinite space used to be in one place!
Although I said this was the simplest picture of the beginning of the Universe, I didn't claim that it was either (a) considered to be the most likely scenario or (b) easy to get your head round!
Although I cannot speak for all cosmologists, I suspect that few of them believe that the beginnings of the Universe were quite so straightforward. Once things get close enough together you need a quantum theory to describe the whole Universe, and such a theory does not currently exist. One of the most promising avenues is something that would come out of string theory - but really no one has much of a clue right now.
If I personally had to put money on anything it would be a picture something along the following lines. At the very earliest times in the history of the Universe things were very simple, but in a way which we currently do not understand, with notions like time and space being replaced with something else. Out of that a small part of the Universe inflated to enormous size very rapidly for a while, and ended up expanding in the "normal" way. We live in a fairly normal part of that expanding region. And hopefully there are observational signatures which will allow us to piece together some information about the underlying concepts which made all this happen. In fact, I would
Прислал rkail"AT"swissonline.ch 8/02
The basic idea is that inflation (or whatever) gives you a scale-invariant initial power spectrum. Then the magic of the expanding Universe and the domination by dark matter does the rest, and makes a spectrum of matter perturbations which closely matches observations. The "cold dark matter" model gets this right without trying, which has always been its major success. Then by tuning to get it exactly right you can constrain some of the parameters of the model in detail.
The basic idea is that P(k)k turns over as the Universe goes from radiation to matter domination because of the different growth rates of perturbations inside the "horizon" in a radiation vs matter dominated universe. The calculation can be found in any standard text on cosmology, e.g. the ones by Peebles, Peacock or Padmanabhan.
As for your other question, there is no unique relationship between the matter power spectrum and the CMB power spectrum. They each depend differently on the underlying parameters of the model. So if you know one, you don't know the other. But that's a good thing, since if you measure both, then you've learned a lot more!
Прислал colin.penney"AT"uk.goldschmidt.com 06/02
I've had basically this same question before, so you should search above for my inadequate answers!).
I'm assuming that they just mocked something up for the TV documentary, and I can imagine a couple of different things that you could imagine doing that you could very loosely call "the sound of the Cosmos" or some such.
If anyone knows of something which exists on a web-page already, I'd be interested to hear it.
Прислал Stuart_Casey"AT"aporter.com 10/02
From the rest of this question, it seems that there is some confusion about distances, times and expansion. So perhaps it will help if I explain as clearly as I can.
The Universe is expanding. So objects further and further from us have their light more and more redshifted.
For very distant objects, we use this redshift as a stand-in for distance. So when some newspaper report says "the light left this galaxy 11 billion years ago" or "the quasar is 10 billion light years away", in fact those numbers come from turning the measured redshift into a distance (or light travel time) given a favourite model for exactly how the Universe has been expanding.
So whenever such numbers are given, they have to be consistent with the expanding picture, since they were derived by assuming it in the first place!
The other thing to remember is that we see the objects as they were when the light left them. So an object 11 billion light years away is seen as it was 11 billion years ago. The furthest objects we can see are obviously the light travel time in the age of the Universe (reckoned to be something like 14 billion light years).
When the light left the distant object, we were in fact much closer together than we are now. But there's no direct way to tell how distant an object is "now". So in fact all astronomical information is constrained to come from times very nearly equal to the light travel time between us.
So it would really be impossible for the distance (in light years) of an object to be inconsistent with the age assumed for the Universe, since those are really just the same thing. (On the other hand it is possible to estimate the age of some objects, globular clusters for example, and thereby determine a lower limit for the age of the Universe - and so far there's no indication of anything older than the models are comfortable with.)
Прислал Bravo_Lawrence"AT"windalco.com 10/02
This is a good question, and gets at the heart of what Science is.
The answer is that one should only accept that something is a good explanation if there is good evidence. The Big Bang picture rests on very good evidence, specifically: the expansion of the Universe; the thermal nature of the Cosmic Microwave Background; the abundance of the light elements; the evolution of the properties of galaxies over time; and the successes of structure formation models within this picture. If the edifice of the Big Bang wasn't so strong (and hadn't been gaining in strength since introduced about 40 years ago) then there would be no reason to take it seriously.
Прислал Bravo_Lawrence"AT"windalco.com 10/02
There are two fundamental problem here. Firstly, the "Big Bang" is a model for how the Universe has been expanding and cooling from early times, which says nothing about what happened in the first instant. Secondly, the usual picture within the context of curved space-time (i.e. the theory of garvity we call General Relativity) has the whole Universe expanding, rather than an object within the Universe. It is probably helpful to try to dispell all notions of an "explosion" from your mental image of the earliest times in the history of the Universe. It is (let's be honest here!) quite hard to picture an infinite expanding Universe which was once all close together! But that's something like the right picture.
Прислал procheck"AT"procheck.cc 11/02
The picture you describe sounds like the boundary of the "observable Universe", which is an imaginary sphere around us, expanding at the speed of light. We don't know about anything outside this region, since the Universe has a finite age, and information can't travel faster than the speed of light. But every year we see another light year's worth of the Universe.
The parts of that imaginary sphere which are at opposite sides of the sky (for us) are moving apart from each other at faster than the speed of light. But that's not actual "stuff" moving, but just the mathematical description of the edge of our observable region.
However, in the "inflationary universe" picture, there was a period of rapid expansion in the early history of the Universe, in which the expansion was faster than the speed of light. That allows for an initially small region to have blown up to be much bigger than what appears to be the size of the observable Universe today. One of the things this helps to explain is why regions on opposite sides of our observed sky can have the same CMB temperature, even although they appear never to have been in "causal contact" (i.e. contact at slower than light speed) with each other.
So certainly it's possible to have regions moving apart faster than the speed of light. And certainly it's possible to have the structure of spacetime outside our observable region have an influence on the sort of universe that we live in.
Прислал jsnyder527"AT"hotmail.com 2/03
I assume this means what's the chance that they'll go away?
Remember that the CMB is in a sense the "ether", since we can in fact tell what speed we're going through the sea of CMB photons, by measuring the CMB dipole - so the ether came back!
But to return to the question, the evidence for dark matter is extremely robust at this point. It is very clear that over a wide range of (large) scales there is more gravitating matter than accounted for by the luminous stuff. And it is also clear that this matter must be mainly non-baryonic (i.e. made of something other than protons and neutrons). The "missing mass" problem goes back about 50 years, and has only become clearer with time.
The evidence for Dark Energy has been amassing since about the mid-1980s. There are now several separate lines of evidence (including from CMB anisotropies) indicating that most of the "stuff" in the Universe isn't even matter, but made of something with negative pressure. Exactly what this is remains to be seen, but there are some promising methods of studying its effects in more detail in the near future. The chances that it will go away altogether seems quite remote. But it's true to say that the evidence is not nearly as secure as it is for dark matter.
Прислал Peter.R.Menge"AT"saint-gobain.com 3/03
The answer to this question is "no!" Despite how bizarre the dark energy appears to be, it does not in fact violate conservation of energy. The ultimate reason for this is that it is entirely consistent with General Relativity, which effectively has energy conservation built into it.
One way to understand what is going on is to remember that you have to do "work" on something which is under pressure if you want to shrink its volume (work done = p dV). The vacuum has negative pressure, and therefore it does work on its surroundings when it expands. This precisely balances the increase in energy in the volume, and hence locally energy is conserved. However, the global amount of energy is another thing entirely!
Прислал ianw"AT"netspace.net.au 3/03
You are right that one has to be very careful in talking about times and distances in cosmology! Rest assured that when real cosmological calculations are done, that all the effects of expansion and curvature are taken into account. What this means is that the answers often depend on the precise cosmological model being used (i.e. the parameters that describe our Universe).
Fortunately it looks like the Universe has close to flat geometry, so there's no real need to worry about large-scale curvature effects. Space is just like Euclid imagined it, except of course that it's expanding. The expansion of space means that you have to take care to define exactly what you mean by lengths. Cosmologists often deal with "comoving" lengths, which means lengths which have had the expansion of the Universe divided out. Or in other words, what the length would be as measured at the present day. For objects which are separating along with the expansion, true physical distances were smaller than comoving distances in the past, and will be larger in the future.
By using comoving distances for a specific cosmological model it is possible to be quite precise about the meaning of distances in cosmology. However, when one reads statements in popular accounts, like "the quasar is 10 billion light years away", one is never entirely sure what is meant!
Прислал erckjcbsen"AT"att.net 3/03
The original data (Hubble's) for showing that the Universe was expanding came from much, much closer than that. All galaxies other than the closest few are redshifted. So you "only" need to go out to a few 10s of millions of light years in order to test that the Universe is expanding. Relative to the age of the Universe (more than 10 billion years), this is a relatively local scale. Hence we can be pretty confident that we know how the Universe is expanding today. Then when we look to much greater distances (and hence probe earlier times) we can find out about how the Universe was expanding in the past. This is how we have discovered that the Universe has been accelerating over the last few billion years.
Прислал jsnyder527"AT"hotmail.com 3/03
Let me try to give you my frank opinion here. There is no doubt that there is a lot of "unshiny stuff" in the Universe. The question is just how much is there, and the answer, traced by gravity, is that there's really quite a lot. This evidence is very direct, and has been there since the 1940s, getting stronger by the decade. There have been attempts to invoke alternative theories of gravity to explain the results without the need for dark matter, but they have been quite unsuccessful. And what's wrong with "unshiny matter"? Why should a whole new theory for gravity be any more appealing?
The somewhat surprising thing is that there's good evidence (from the abundance of the light elements) that there's too much matter to be bayons (i.e. normal stuff). So it's presumably some kind of particle that interacts weakly, so it's only easy to detect through its gravity. This may seem a bit uncomfortable, but I think if we can swallow that neutrinos exist and are passing through you at the rate of maybe 1015 per second, then some other particle can't be that hard to accept! We now know that neutrinos have mass, and if they were a bit heavier they could be the dark matter. So we just need another particle streaming through the Universe that's heavier than a neutrino. I think this is actually not particularly difficult to believe.
What's much more surprising is the Dark Energy, which seems to dominate the total budget of the Universe. There's no obvious reason why that should exist at a level just a factor of 2 or 3 times more than the matter. It might just be the energy density of the vacuum, but if so we're quite surprised at its value. Or of course it might be something else entirely, that we just don't understand yet. The evidence has been growing steadily since (I would say) the late 1980s. The combination of the supernovae results and the CMB results on top of the weaker indications from before, mean that we have to take the idea of Dark Energy very seriously. Ugly or not!
Прислал christalynn50"AT"hotmail.com 4/03
You may have seen the recent article in Scientific American by Max Tegmark. There's more discussion on parallel universes at his web-site: here.
The basic point is that the concept of different "observable universes" is very solid, but the different regions can't be very different, since they are all part of the msame underlying model and certainly obey the same laws of physics. On the other hand the idea of actual "multiple universes" is much more speculative. Scientists disagree on whether there is any use talking about such concepts, and if there is, then should it be kept for the coffee room? In other words is it really science? Can one imagine experimental evidence for or against such a picture?
Everyone agrees that it's fun to speculate about such ideas! And since there no general agreement on the "right" thing to think, then you're free to imagine whatever you like!
Прислал therman1"AT"mindspring.com 4/03
My own estimate would be a number larger than that, but the point is that the calculation is finite. Therefore if the Universe is genuinely infinite, then there would indeed be effectively copies of everything somewhere!
However, speculating about what goes on beyond the observable Universe is really just speculation! We don't know much of anything on that subject. So I'm prefectly free to imagine that there's something more complicated about the Universe which makes it finite but unbounded in some way. So I don't necessarily have to hurt my head by thinking about things which involve infinity if I don't want to!
Прислал shamsmonaas"AT"yahoo.com 7/03
In the picture which most cosmologists have there was nothing before the Big Bang, because time itself didn't really exist. But there are also more speculative ideas in which, before the Big Bang, there were previous phases in the history of the Universe. The main thing to realise is that we know a lot about the Universe today, and by studying things like the CMB we can push back our understanding to very early times - but it becomes increasingly shaky to talk about things as you approach t=0. So right now there are no defnite answers to your question!
Прислал Chloalip"AT"btopenworld.com 8/03
The approximate answer is that this happened when the average kinetic energies of particles was about equal to the rest mass equivalent of an electron or positron. So in particle physics units that's about an MeV. You get the temperature just by converting this to Kelvin, which gives you around 1010K. The time depends somewhat on the model, but is around a few seconds.
Прислал gregl"AT"debut.to 9/03
There's nothing very peculiar going on here, just the expansion of the Universe!
The best estimates for the age of the Universe are about 14 billion years. Distances in cosmology are complicated by the fact that the Universe is exapnding, and so you have to be careful to define precisely what you mean. The "size of the observable Universe" can be thought of as the answer to a question like: "if I started at time zero moving at the speed of light in a straight line, then how far would I have gotten by now?" The answer is a bit more than the age of the Universe times the speed of light, because the space that you're moving in has been stretching in the time it takes you to complete the journey. The precise answer depends on how this stretching changes with time. The current best picture is that the Universe was decelerating until relatively recently (on some cosmic scale at least!) when it started to accelerate again. Putting in the best estimates for this variation of the expansion over the last 14 billion years gives a factor of around 3.5. Then if you want the diameter rather than the radius of the observable Universe, you get a number close to 100 billion light years.
Прислал tmgulland"AT"hotmail.com 10/03
Red-shift means that all the wavelengths are stretched to be longer, not that all the light becomes red. Galaxies have energy over a wide range of wavelengths, so if the rest-frame blue gets shifted to be red, then the rest-frame ultra-violet gets shifted to be blue, etc. The shade that a galaxy would appear depends on details of the shape of its spectrum, as well as how red-shifted it is.
If spectra well featureless, then we wouldn't be able to tell that things were Doppler shifted. The way that we tell is by looking for spectral lines, i.e. specific patterns of absorption or emission at wavelengths determined by particular atoms or molecules in the object. So a high redshift galaxy might have the "Lyman alpha" line of hydrogen appearing not around 120 nanometres but at, say, 500 nanometres (and known lines at all other wavelengths would be similarly stretched). By recognizing characteristic features in the spectra of nearby galaxies and seeing that the same patterns occur at longer and longer wavelengths in more and more distant galaxies, we can conclude that the Universe is expanding.
But in a sense the entire sky is quite red. The most distant source of light that we can observe is the CMB. That was emitted at optical-type wavelengths, but has been redshifted all the way to microwaves!
Прислал ws"AT"pop.socialstudies.com 10/03
This is an interesting question. I am tempted to answer "it wouldn't look like anything, because the Universe wouldn't exist if tachyons did"! However, let me try to say a little more than that.
Tachyone are hypothetical particles with imaginary mass which move faster than the speed of light. When people have tried to study their consequences, things get pretty silly pretty quickly! Hence most physicists regard them as being of only theroretical interest at best, although it is probably also true to say that there is no fundamental proof of their non-existence.
But this hasn't stopped them entiring the popular consciousness through being a useful plot contrivance used to get round problems of the vastness of space in science fiction. And they have also entered "New Age" literature, as you can read about in the excellent Skeptical Dictionary. You can also read more about tachyons in general at many places on the web (e.g. here.
The jist of your idea, of the Universe being created out of the "wavefront" of tachyons made at the Big Bang has some similarities with the "kinematic relativity" idea of E.A. Milne, who was one of the fathers of modern cosmology. You can also read more about Milne and his ideas on the web. Although he was attempting to develop a cosmology which didn't rely on General Relativity, he developed a bunch of the key ideas in our current cosmological picture. So, who knows, maybe something useful might come out of thinking about even stranger ideas, like faster than light particles at the Big Bang!
Прислал tmgulland"AT"hotmail.com 11/03
Firstly, there's no "centre of the universe"! Everyone can think of themselves as the centre if they like though. They're all equivelent.
Secondly, the amount of absorption through a galaxy cluster is something like a per cent. And the hot regions of galaxy clusters certainly don't cover the whole sky, but only a very small fraction. So the total amount of absorption (actually scattering of the photons to higher energies) through clusters is somewhere around one part in a million.
Прислал gterry"AT"au1.ibm.com 12/03
Infinities are things which make your head hurt if you think about them too long!
But certainly there's no mathematical problem imagining something which is infinite and expanding. If you take an infinite thing and multiply it by a constant, then it's still infinite. To a mathematician the new thing is precisely as big as the old thing. However, to a physicist, if you take an infinite Universe and move everything further apart, then it's bigger!
Прислал gterry"AT"au1.ibm.com 12/03
Although you need to be careful about how you define distances and times in an expanding Universe, so that you use a meaningful concept of speed. This is all taken care of within general relativity, which is the framework which underlies all modern cosmological models.
Прислал pareynolds"AT"sasktel.net 01/04
It is certainly the case, that since light has a finite speed, that it takes time from the light to reach us from distant objects. For example, an object 1 billion light years away is seen as it was 1 billion years ago. Hence, even if we could build a rocket which can travel near the speed of light, it would take us another billion years to reach the object and by then it would be 2 billion years older than it appeared to be when we first saw it! (this ignores the fact that the Universe is expanding during the time that the light and our spaceship make their journeys, but that just makes it a little harder, and certainly doesn't change the basic picture).
You can bet that if we really could do such a thing, we'd find objects to be very different than how they appear now. As we make the journey towards them we'll intercept photons that left later and later, of course. And when we're about half way there, we'll see the object as it was when we left our own Galaxy!
Прислал vda"AT"port.imtp.ilyichevsk.odessa.ua 01/04
First of all, this explanation shouldn't be taken too literally! Secondly, the correct calculation is carried out within General Relativity, and hence Einstein would have been perfectly happy with it (although note that this isn't special relativity). And thirdly, there's nothing wrong with things moving apart at faster than the speed of light in an expanding Universe - it all depends on the coordinates you choose. Nothing can move relative to a neighbouring objects at v>c. And no signals can propagate between two points faster than it would take light to travel the distance. The point about inflation is that things expand apart from each other so fast that they overtake light signals passing between them. So a small "causal" patch (over which there has been enough time for light to propagate) gets blown up to gigantic size. And hence our whole observable Universe was in very good causal contact at much earlier times than you would have thought.
Прислал aaronernestoortizlopez"AT"yahoo.com 03/04
It all depends on the cosmological model. Changing the Hubble constant (i.e. the present day expansion rate), changing the amount of dark matter and changing the amount of dark energy, will make the Universe expand differently with time. So objects at the same redshift will be at different distances in models with different values for these cosmological parameters. The equations describing this are indeed derived from Einstein's General Relativity, but in fact they're not too tricky to deal with. However, in general you have to perform integrals over redshift to estimate cosmological distances, and so you can't usually write down the redshift-distance relation in a simple expression. But if you tell me your favourite values for the cosmological parameters, then I could give you a plot of distance versus redshift. However, you also need to be careful about what you mean by distance (there are several possible definitions) in a Universe which is expanding and possibly curved!
Прислал labeleh"AT"msn.com 03/04
Yes, space curvature is governed by gravity. But it's a little more complicated than you suggest. You need to take into account pressure as well as energy density, and so different kinds of stuff contribute differently to the dynamics and the curvature.
In fact a completely empty Universe has open ("hyperbolic") curvature, despite what you might have thought! And a Universe with a total density equal to the critical density has flat geometry. This certainly doesn't mean that there's no gravitational force between things, just that space overall isn't curved.
Прислал joekuhn"AT"comcast.net 03/04
The wavelengths of the light emitted by distant galaxies are stretched, i.e. shifted redwards in the spectrum. We call this redshift. This is similar to the familiar Doppler effect caused by moving objects. A car moving away from you fast will have a noticeably lower pitch than a stationary car, and a car moving towards you will have a higher pitch. You get the same effect for light waves as for sound waves, but the effect only becomes big when you're moving fast relative to the speed of light, rather than the speed of sound.
In cosmology we think of this redshift not as a velocity effect, but simply as a stretching of lengths. When the photons left a high redshift galaxy all objects in the Universe were closer together than they are now, since the Universe is expanding. As the photons travelled towards us from this distant object they are stretched along with the expanding space. And so we observe all the wavelengths in the spectrum of a galaxy to be shifted redward.
Прислал mmakover"AT"optonline.net 03/04
There is no clear answer to why the Universe is made of matter rather than half matter and half anti-matter. But it surely has to do with an asymmetry between matter and anti-matter particles at very high energies. This is something which can be probed in particle accelerators, and so we may learn something about this from experiments currently underway (such as the BaBar experiment at the Stanford Linear Accelerator Center. The way the Universe works made a definite choice between matter and anti-matter at early times, and the trick is to figure out the details of the correct theory which made the imbalance just the right size.
Hypothetically speaking, if the Universe had come out with more anti-matter than matter, then almost everything would have been no different. Anti-atoms behave just like atoms, with electromagnetic and gravitational forces operating on them in exactly the same way.
This means of course that in principle you could be made of anti-matter, and you'd never know!
Прислал howard"AT"thrinberry-frog.com 03/04
The "Big Bang" isn't a localised place, but the whole Universe.
Different observers will measure different rates for time, depending on their frame of reference. This include the speed they're moving at and the gravitational field they're in, both relative to another observer. Observers in very strong gravitational fields experience strange things like strong "time dilation" compared with other observers. You get this near a black hole, since that's a localised region of high gravitational field. But you don't get this for the "Big Bang", since there's nowhere special for you to have a higher gravitational field relative to any other observer - because the Big Bang is everywhere!
The definition of "time" that we use in cosmology is actually quite precisely defined (but once you've understood that such a thing can be defined, you can forget about it!). You imagine "fundamental observers" all over the Universe, who are not moving with respect to each other except through the uniform expansion of the Universe (i.e. everyone moving awy from everyone else, so that all length scales are multiplied by one universal function of time only, with no spatial dependence). Then you imagine all of these observers sharing information and coming to an agreement about how to synchronise their watches. This "cosmic time" is well defined and is the obvious time coordinate to choose in a uniformaly expanding model of the Universe. That's what we mean in cosmology when we say "formation of the light elements happened in the first 3 minutes" or "we see the CMB anisotropies as an imprint of conditions in the Universe when it was about 400,000 years old".
Прислал grojo"AT"elpn.com 04/04
The question of the future history of the Universe is a fascinating (though useless) one to think about.
In the Dark Energy dominated Universe that we appear to live in, the distant galaxies will move away from us at ever increasing rates, until eventually they disappear altogether! We end up with an "island Universe" around us, consisting only of the galaxies to which we are currently gravitationally bound, with the rest of the observable Universe getting emptier and emptier, while the stars in those galaxies slowly run out of energy.
It's true that it might be hard to convince people of the Big Bang picture if we lived in an apparently finite volume universe!
Прислал grojo"AT"elpn.com 04/04
No the Earth is not shrinking (I can think of several flippant comments here, but will resists the temptation!).
All the galaxies are receding from each other and not just from us. So the idea that our rulers are shrinking doesn't work.
Прислал zero.in"AT"verizon.net 05/04
It seems that you are a bit confused by a number of potentially related ideas. I agree that there is a great deal of confusing terminology!
Space-time may well be "quantised" at some fundamental level. And that may have a dramatic effect on our ideas of the very first instant of the Universe. But it really has no influence on the generation and subsequent behaviour of the CMB, which is really a low energy phenomenon.
As for the Dark Matter, it's probably composed of some particle which we've yet to understand in detail. But really no one has much more than a hunch about what exactly the dark matter is (other then knowing that it affects things through gravity, but little else), just as no one really has much more than a hunch about the basic structure of space-time and the origin of the entire Universe! The amazing thing though, is that we can imagine learning the answers to just these sorts of mysteries by studying the large-scale Universe (and particularly the CMB) in more detail.
Прислал johnk"AT"surreal.com 07/04
So that's where you're going wrong! Who says no two objects can't move apart faster than the speed of light? In a well defined sense, this is precisely what is happening for objects separated by more than the size of the observable Universe.
Прислал bullbull104"AT"hotmail.com 07/04
Gosh, I don't really know how to answer that!
The biggest unsolved problems in cosmology are basically: (1) where did the parameters that describe the Universe come from?; and (2) how exactly did structure form? Those are the "why" and "how" questions, which are always the hardest! You can split those into many more detailed questions, like "why is about 70 per cent of the Universe dark energy", or "when did stars first ionize the Universe and end the cosmological dark ages?" - but mostly such questions are sub-sets of those 2 basic issues.
Inflation is a very promising idea which solves some problems with the standard Big Bang picture, which people didn't realise were problems in the first place (the so-called "horizon" and "flatness" problems)! It also gives you a reason for why the Universe is expanding today other than "because it was expanding yesterday". But more importantly it gives a mechanism for generating density perturbations in a generic way. But there is no detailed "model" of inflation and no direct connection with high energy physics (string theory for example) at the moment. However, there is some hope that with better data (from CMB polarization for example) it might be possible to test whether inflation is right, and what flavour of inflation describes our Universe.
Прислал johnk"AT"surreal.com 07/04
That would certainly be interesting! If we really saw something funny on large scales on the CMB sky, then it would likely indicate something odd about the Universe as a whole. So far the CMB is extremely isotropic (except for the effects of our local velocity) - and this is the best piece of evidence we have that the Universe is pretty much the same everywhere. We can also use the CMB to show that the Universe isn't rotating very much, and that the size of the Universe is a bunch bigger than the distance out to which we see the CMB photons.
It's always possible, of course, that we might one day see some sign of a finite Universe imprinted on the CMB. But for now (and the next millenium or so) things look pretty much as you'd expect from the hot Big Bang picture in which the hot phase happened everywhere at once in a very uniform and extremely large universe!
Прислал johnk"AT"surreal.com 07/04
It might be true that it would be hard for life to form in the early Universe, since there was so much radiation around. But here we're talking about the very early Universe, and at those times there wasn't any structure in the Universe either!
Before about 300,000 years after the Big Bang, the CMB made the Universe hot enough that all the hydrogen was ionized. So the temperature was like the surface of a star, and so presumably prety hostile to life! But the variations in density hadn't collapsed enough to form galaxies, planets, etc.
When the Earth was forming (about 4.6 billion years ago) the Universe was already about 9 billion years old. In the best-fitting cosmological models the redshift at that time was about a half. That means that the temperature of the CMB was only about 1.5 times what it is today, i.e. about 4 Kelvin. So the CMB was as negligible (biologically speaking) back then as it is today.
Прислал Alexander.Zlotnik"AT"cgi.com 08/04
Well, this all depends what you mean by "proven fact" - but let's not get into a discussion of the nature of proof!
The brief answer is that there is now a fairly heavy burden of evidence pointing to something like a cosmological constant, i.e. a non-zero energy density for the vacuum which is significant on large scales. The evidence is perhaps not quite conclusive yet, although this is probably coloured by the feeling that "extraordinary claims require extraordinary evidence". And by the fact that we have no clear theoretical explanation for why the vacuum should behave in this way. If the vacuum energy evolves with time, then that may be a clue as to its origin. We call such general forms of energy "dark energy" or "quintesssence".
Despite its theoretical ugliness, the evidence has been growing since around 1990, and is now strong enough that a randomly selected jury of cosmologists would undoubtedly reach a majority verdict in favour of "dark energy". However, it's still unclear whether it's proven "beyond reasonable doubt".
An accelerating Universe implies dark energy fairly directly. And features in the CMB anisotropy power spectrum also imply that the Universe has a large contribution to its overall energy density from something which is not matter. These pieces of evidence, together with some other supporting data, leave us with the only real possibility being that about 2/3 of the Universe's energy density is in a form with negative pressure, behaving like empty space with non-zero energy density. There are other possibilities that have been explored for explaining the data, but these are either unworkable or even more preposterous sounding!
Прислал gblouin"AT"rice.edu 09/04
It's always nice to get questions that aren't about microwave ovens!
You're entirely right that in general the curvature of space will change as the Universe evolves. In fact the exactly flat case is unstable, in the sense that the Universe tends to diverge away from being flat, and adopts an increasingly curved geometry. So if it's a little bit less than flat, then it will get more and more negatively curved, i.e. saddle-shaped, while if it's a little bit spherically then it will become more and more positively curved.
The current constraints say that the Universe is within a few per cent of being flat, and we don't know precisely how close to flat it is. But whatever that turns out to be, it's still a little bit mysterious that it's even close to flat today, since that would imply it had to be incredibly close to flat at very early times. This is sometimes called the "Flatness Problem", and the most reasonable resolution of this problem is an idea called "inflation", where at a very early time the Universe was driven to be very close to flat. Whether inflation really happened, and whether we can learn anything about its details, is one of the major motivations for planning future CMB experiments.
Прислал dreeves"AT"brandeis.edu 09/04
I think those books are wrong! Or at least there's no reason to accept that as the right answer!
The correct value for the volume of the Universe before the Big Bang is "undetermined". I mean that partly we just don't know what the state of the Universe was like back then. But also that it might be fundamentally an unanswerable question, like "what's further North than the North Pole?" And also that in most cosmologists pictures of the very early Universe there's something quantum mechanical (read "fuzzy"!) going on, which makes quantities like time and volume tricky concepts!
Прислал rkohler"AT"mail.sdsu.edu 11/04
It sounds like you already have partial answers from Ned Wright and Eric Sandquist. I'm not sure there's much I can add.
The general principles are fairly clear, and you can look up the relevant equations and quantities in many books or web-pages. However, perhaps you are actually asking for the detailed answer to your question of where we're moving through the Universe? I can't answer that for 2 reasons: firstly because it depends precisely on what things you want to include (which I'll come back to in a moment); and secondly because I frankly have better things to do with my time!
One nice thing about the CMB is that it gives us an "absolute rest frame" in which to talk about velocities. If you really want to calculate how fast and in what direction you are moving, you would need to include at least the following list: the Earth's spin, which depends on your position on the Earth and the direction of which depends on the time of day; the Earth's motion around the Sun, which depends on the time of year; the motion of the Sun with respect to neighbouring stars; the motion of the "local standard of rest" with respect to the Mily Way Galaxy; the motion of the Milky Way with respect to the centre of the Local Group; and the motion of the Local Group relative to the CMB. The problem is that the different terms are known with different levels of precision, and that since velocity is relative, you're free to decide that you want, for example, the velocity of A relative to C, when I might have expected you to want it relative to B.
Another problem is that you need to decide ahead of time how accurate you want your answer to be. That's because there are lots of subtle effects that I didn't list above, which may or may not be important, depending on your required accuracy. For example I can imagine corrections because of the non-circular Earth's orbit, because the Sun is also wobbling around, because of precession of the Earth's orbit, various accelerations or epicycles in the orbit of the Sun around the Galaxy, perturbations to the motion of nearby galaxies and clusters, etc. etc.
So if you really want to work this out, you need to specify exactly what question you want to answer, stick to a given level of accuracy, and then get out your astronomical almanac and calculator!
Прислал email@example.com 01/05
I'm afraid that your picture is quite wrong! But don't worry, because the mistake you make is very common among people trying to understand the expanding Universe. I hope that there are some answers on this page that may help you.
But the answer to your question is that an infinite universe and a finite universe are in fact different. The way to think about a finite universe is in terms of the curvature of space. Imagine that we live on the surface of a "3-sphere" (i.e. like being on the surface of a normal "2-sphere", except with one extra dimension, which we can't really picture, although we can imagine how it works by analogy). This means our 3-dimensional Universe is curved in some other dimension that we don't live in (and is really just a mathematical convenience rather than being real in any case!). So if you go off in a straight line in any direction, you will eventually return to where you started (just like being on the surface of the Earth, except this works in all 3 dimensions). So this kind of Universe has a finite volume, but has no edge.
That's the sort of finite volume universe that we might live in, according to our best ideas for how gravity and geometry are related (i.e. the theory called General Relativity). The nice thing about it is that it contains everything in a finite amount of space (hence satisfying your idea that the Universe should be finite on some kind of aesthetic grounds), and at the same time you don't have to think about edges or what's outside it, because it's everything!
Прислал jcoldrey"AT"bigpond.net.au 01/05
Although you might expect that the "Big Bang" occurred in one place, in fact it happened everywhere! This might at first seem like a complicated idea, but once you get it, I believe that you'll realise that it is in fact an incredibly simple and appealing notion!
Try to picture an infinite Universe with the space itself expanding, and with no centre, just everything expanding from everything else. We're all at the position of the Big Bang!
Прислал tomwsmith88"AT"hotmail.com 02/05
I've had way stranger questions than that, never fear!
In the standard picture, one imagines that all 4 of the fundamental forces are combined into some kind of "quantum gravity" at around the "Planck time", which is about 10-43 seconds, or perhaps a little later in some ideas related to string theory! Then gravity separates from the other 3 forces, and a bit later (say at 10-35 seconds) the strong nuclear force becomes distinct from the other 2, which are then the "electroweak force", but separate much later into the weak nuclear and electromagnetic forces.
So the forces are always there, although in some more symmetric form in the earlier and earlier Universe. And these forces are included in detailed calculations of the early Universe, for example in the formation of the light elements etc.
No one of course really has a clue what happened in that very first tiny fraction of a second as we approach the Planck epoch. The "quantum gravity period" may in fact be the beginning of everything, and so the first 10-43 seconds may not even exist! But now I'm starting to get into answering a question that you didn't ask!
Прислал hswyers"AT"spa.com 06/02
Although we speak about the Universe accelerating, this actually means that Hubble's constant approaches a constant!
What is meant by acceleration is that the rate of change of the rate of change of scale factor (which determines large-scale distances) is positive, rather than negative (which would be deceleration). But Hubble's constant is the rate of change of scale factor divided by the scale factor. And in an exponentially expanding model this becomes a constant.
So the Hubble constant will get a little smaller than it is today, but only by a small factor (a few tens of per cent).
Прислал hswyers"AT"spa.com 06/02
This sounds pretty similar to inflation to me!
Прислал Abk2005"AT"aol.com 08/02
Two lots of outer space!
Прислал ma"AT"star.ucl.ac.uk 02/05
I confess that I really don't understand what model you have in mind here!
However, the one comment I'd make is is that the crucial thing is ensuring that you generate the CMB while being in very good thermal equilibrium. This is typically quite hard to arrange in "local" or "recent" ideas for producing the photons. It is of course one of the triumphs of the standard hot Big Bang picture that if you assume the Universe was once as hot as the surface of a typical star and has been expanding since then, you naturally get a blackbody spectrum.
Прислал tjax"AT"comcast.net 03/05
Thanks for helping spell things out for other readers of this page!
The answer to your first question is that if 2 objects started off that far apart at some very early time, then they'd be ginormously far apart today, well outside today's observable Universe. To work out the actual distance you'd have to be more explicit about exactly when they were 1 light year apart (since at exactly t=0 nothing is any distance apart from anything else!), and you have to be quite clear about your definition of distance (e.g. when is it measured?).
For the metre stick, it depends whether it's expanding or not. If it's an imaginary metre stick, expanding along with the rest of the Universe, then the answer is basically the same as for the previous question (depending exactly when it was defined as 1 metre of course). But if it's a self-gravitating "metre stick" (like a galaxy, for example), then it doesn't care about the fact that the rest of the Universe is expanding, and it stays 1 metre.
Прислал Keijo.Musto"AT"bluescopesteel.com 03/05
The best answer is "3", the expansion of the Universe. You can think of the wavelengths getting stretched as the space they're travelling through expands.
However, you can also think of this as a Doppler shift caused by the recession velocity. This is really the same thing, and at least when the velocities are small, it's easy to interpret the cosmological redshift either way. However, when the velocities get large (i.e. at large enough distances that the recession speed isn't very small compared with the speed of light) you need to be very careful in interpretting cosmological redshift in terms of a recession speed (i.e. you start to realise that your interpretation might be different depending whose frame you're in, when you measure the speed and the distance, etc.). So except for relatively nearby objects (up to 100 Mpc say), it's best to stick with thinking of cosmological redshifts as being caused by "3" in your list, and not "1".
Effects "2" and "4" are also real effects which change the signals you might get from a distant object, but not in the same way as the cosmological redshift. Effect "2" is the effect that clocks will appear to run slow in an object at high cosmological redshift. So in a sense this is just the same thing as "3", but for frequencies rather than wavelengths. We certainly can see this effect in the timescales of some physical processes in distant objects of various sorts (supernova light curves, for example). Effect "4", the gravitational redshift, depends on how massive and compact the source is. For typical stars or galaxies the gravitational "potential well" isn't deep enough to make a very noticeable redshift - you have to be something like the surface of a neutron star in order for the redshift to be genuinely large. So for all galaxies observed in the distant Universe this gravitational redshift is quite negligible compared with the cosmological expansion effect.
Прислал Keijo.Musto"AT"bluescopesteel.com 03/05
No. The Universe is not a "localised" object, but everything! It used to be denser everywhere, and has been expanding, making the photons stretch (lose energy) with time. But this cosmological redshift is caused by the expansion, rather than being a gravitational redshift effect.
Прислал goran.krstic"AT"trow.com 03/05
In the standard cosmological picture (the "Friedmann-Lemaitre" models) the Universe can be expanding, static or contracting. Einstein looked for a static solution, which is actually quite contrived and unstable, while it is more natural for the Universe to be expanding and contracting. But then in the 1920s Hubble discovered that the Universe is expanding, and hence all models of the dynamical behaviour of the Universe since then have focussed on having expansion today. Of course, it is possible that in the past or the future the Universe could have been contracting. But current observations generally rule these models out. Our Universe now appears to have accelerated expansion, and if that carries on forever, then obviously it will never contract (although there are some speculative models in which the Universe does eventually stop accelerating and start to contract).
It may be that you have quite a different picture in mind, and so I do not know for sure whether your ideas will be compatible with what we know about the Universe - but I'm afraid it seems pretty unlikely. A contracting Universe would manifest itself with a negative Hubble law, i.e. galaxies would be blueshifted. In fact if we lived near the epoch when the Universe was starting to contract, then we might see nearby galaxies blueshifted, while more distant galaxies (seen earlier in time because of the finite speed of light) were still redshifted. This would be pretty cool! But unfortunately it's nothing like the emprically observed Universe, which is consistent with uniform expansion, isotrpic in all directions, and currently speeding up.
Прислал helmuthansen"AT"t-online.de 09/05
An awful lot has been written about Mach's principle, and much of it by people who are smarter than me and have thought more deeply about the nature of interia. However, it's not at all obvious that much of what has been written is terribly useful! People can't even agree on precisely what is meant by Mach's Principle, and there's even disagreement about whether Mach ever stated it!
Because of this, I'd prefer not to get into a discussion of this topic - sorry if that sounds wimpy!
But rest assured there are plenty of other places to read such discussions. When I googled "Mach's Principle" I got 38,200 hits. So presumably some of those are worth reading! I find that wikipedia is often a good place to start.
Прислал sc02492"AT"yahoo.com 09/05
There are several different levels of answer to your question, depending on how much you've been thinking about this already - so choose your favourite from these 3!
At the simplest level, a linear velocity-distance law is the same thing as uniform expansion. This is an important concept to get straight if you're going to understand thge way the Universe expands. One way to sort this out for yourself is to draw a 2-dimensional arrangement of dots (representing galaxies in the Universe), then make a larger scale drawing (or enlargement on a photocopier or scanner). Choose one dot to represent you and measure the change in the distance between you and a bunch of other galaxies. You should see that the change in the distance is proportional to the distance, e.g. if you expanded everything by a factor of 2, then a galaxy which used to be 1 unit away is now 2 units away, so has moved 1 unit in the time it took the Universe to expand that much, whereas a galaxy that was originally 3 units away is now 6 units away and so has moved 3 units further away. So uniform expansion (i.e. all distances getting multiplied by the same factor) is Hubble's Law.
The next thing you might realise is that measuring the Hubble Law for distant galaxies gets complicated because of the light travel time. You're measuring the speed of a distant galaxy when the light left it, which is an earlier time than for a more nearby galaxy. So in fact the Hubble Law is only really an approximation for relatively nearby galaxies, where you don't have to worry too much about the effects of observing at different times.
Then you realise you've heard that the Universe is accelerating, rather than exapnding at a constant speed. This means that the rate at which the overall scale factor of the Universe is increasing, is itself increasing. So in fact you don't expect the Hubble constant to be constant at all! And since the Hubble constant is actually defined to be the rate of change of the scale factor divided by the scale factor, it turns out that "H" was larger in the past and is getting smaller (and will probably approach a constant once the "dark energy" takes over). In fact using observations of distant supernovae we can see back to the time when the Universe was decelerating and switched to accelerating as the "dark energy" started to dominate.
Прислал fbaer"AT"c2i2.com 10/05
The curvature of the Universe is the curvature of space, i.e. in principle the geometry may not be flat. The easiest way to picture non-flat geometry is by analogy with a sphere, except in one higher dimension. So you think of going outwards in a straight line as being motion on the surface of the sphere. And as far as beings who live on the sphere's surface are concerned, they could go all the way round the Universe while always going in a straight line. Now you imagine that we live on a "3-sphere", rather than the more traditional "2-sphere". Mathematically speaking the "3-sphere" is curved when you embed it in a 4-dimensional space. But that extra dimension is just a mathematical convenience, which doesn't exist in the same way that the real 3-dimensional world exists.
But, you don't really need to worr about this, since the CMB anisotropies tell us that the Universe is quite close to being flat!
Прислал firstname.lastname@example.org 10/05
Ah, someone else hoping that I'll do their homework for them!
I think you'll find the answers to most of those questions on this very web-page! And there are zillions of other resources on the internet where you can also read about cosmology and answer those questions.
Except for "provide an everyday example" of the "cosmic wave background" - no idea about that one!
Douglas Scott dscottATastro.ubc.ca Последняя редакция: 15 Сентября 2005
..:: Перевел с английского В.Г. Мисовец