Question on the STM continuum

[Drotar]

What evidences are there that our universe is isotropic? What evidence proves or disproves that is does or does not have a center, or an edge, or a boundary?

 

[lucaspa]

What evidences are there that our universe is isotropic? What evidence proves or disproves that is does or does not have a center, or an edge, or a boundary?

Science recently had a special section on spacetime.
http://www.sciencemag.org/cgi/content/summary/296/5572/1417 Science May 14, 2002

You can also try at:
http://image.gsfc.nasa.gov/poetry/ask/acosmexp.html Questions about Big Bang
http://image.gsfc.nasa.gov/poetry/ask/askmag.html#list General Ask the Space Scientist

Most of this resides in Special and General Relativity. Surveys of the universe at large scales provide the evidence that the universe is isotropic. A logical consequence of Big Bang (for which there is overwhelming evidence) is that the universe does not have a "center".

The edge and boundary are different. Since space is expanding, there is a limit to how much of the universe we can see. At large distances from us, space is expanding faster than the speed of light, so there is a limit to the observable universe.

Logic would have it that there is an "edge" or "boundary" to spacetime. That is, that spacetime is finite. The concept is a little fuzzy since there is nothing beyond spacetime. So on a practical matter, you can't ever leave spacetime because there is nothing to go into. So you can't ever find the "edge". If you can't find the edge or boundary, is there one?

 

[ReUsAbLePhEoNiX]

Most of this resides in Special and General Relativity. Surveys of the universe at large scales provide the evidence that the universe is isotropic. A logical consequence of Big Bang (for which there is overwhelming evidence) is that the universe does not have a "center".

But it seems logical that the universe would have a center if galaxies are rushing away from each other. Unless you mean that the entire Universe IS the center?

 

[Jet Black]

But it seems logical that the universe would have a center if galaxies are rushing away from each other. Unless you mean that the entire Universe IS the center?

this is where the expanding balloon analogy is helpful. if you blow up a balloon, all points on the surface of the balloon are moving away from one another (forget about the volume of the balloon for the purposes of the analogy) since the surface area of the balloon is increasing. The same is true for the universe, however in this case the volume of the universe is increasing, and it is the expansion of that volume that makes the galaxies move away from one another.

 

[Jet Black]

The edge and boundary are different. Since space is expanding, there is a limit to how much of the universe we can see. At large distances from us, space is expanding faster than the speed of light, so there is a limit to the observable universe.

was there a name for this sort of space, where it is all broken up into bubbles of non-interacting spaces because of the expansion... I thought it was de sitter spaces but I looked those up and it doesn't look like it...

 

[fungle]

Could the micro wave analysis give an idea of the limits of the universe as well as a timeframe.?

 

[Jet Black]

Could the micro wave analysis give an idea of the limits of the universe as well as a timeframe.?

not really, or at least not in any way I can think of. the problem is, that at a certain point in time, the universe was opaque, then the light that was released when the universe became transparent became the CMB. we can only ever see as far (back) as the point where the universe becomes opaque. basically it is a problem of the finite nature of the universe, we can only see as far away in light years as the universe is years old.

 

[Jet Black]

also I think it has been theoretically shown using a big bang model, that if the universe is closed in on itself (like a circle) you could never travel right round the universe (even at the speed of light) and end up back where you are. Either the universe is expanding too quickly to allow it, or it will collapse before you get back to your starting point. THat is only a rough memory though.

 

[fungle]

Microwave analysis gives us a time relatively close to the big bang and with a knowledge of the velocity of the seperation of the galaxies couldn't this give us an idea of the approximate size of the universe and the centre.Obviously if we are close to the extremity of the universe, this would be very difficult to find the centre.
Isotropic and anisotropic are terms that I have used with mineral determination so I don't know whether I am on the right track.

 

[Jet Black]

well there is a problem with the "center" because there is no such thing. imagine a sphere.... where on the surface is the "start" of the sphere? it is completely arbitrary. one has to imagine the universe like this, unless it has a clear boundary, which we don't know (and a boundary would have problems since there would be edge effecs, the boundary would be measurable and have properties etc, making it a part of the universe ad infinitum. from this we get either an infinitely large universe, or a finite, but unbounded one. incidentally, with regards to the CMB we are at the center (of sorts) but the center of all the light which started travelling towards our current location some 13.7 billion years ago!

the next issue is on the velocity and separation of galaxies; remember we can only see as far as the age of the universe, so imagine drawing a circle on a sheet of paper of arbitrary size. now imagine you are inside that circle, there is no way to measure the size of the paper without leaving the circle, and the universe is much like this. The velocity and density of galaxies do not give us any real clue, since on a large enough scale the universe appears to be isotropic, even though it is locally clumpy, and the velocities are just dependant on distance and a few constants. whatever direction we look in, we see the same roughly isotropic distribution of matter, not like the stars of the milky way, where we see more in one direction than the other.

 

[fungle]

If using the term isotropic then would there be an equal distribution of elements in all directions.

 

[Jet Black]

I just mean isotropic in terms of mass, and I say isotropic on a large scale, which it is.

 

[fungle]

Thankyou, so many meanings of the same word. So mass distribution is not equal throughtout the universe and therefore not isotropic.

 

[Jet Black]

Thankyou, so many meanings of the same word. So mass distribution is not equal throughtout the universe and therefore not isotropic.

not locally. remember that when you are talking about cosmology, you are talking about millions of light years, so while it is clumped into local groups and clusters and even superclusters, there is no gradient across the entirity of visible space. The reason for this clumping is gravity, but there is no "overall" gravity pulling everything in one direction or something like that. (as there would be if there was a "center" to the universe)

The best comparison I can give is imagine big sheet of paper, on which you sprinkle iron filings in an isotropic way. now you chuck a bunch of tiny magnets on it, and these pull the iron filings towards them. locally the iron filings are clumpy, but looking at the big picture across the whole sheet, the distribution is still isotropic - there is no directional preference, as there would be if there was a big magnet to one side of the sheet.

I think a better definition in this case for isotropic would be "the same in all directions"

 

[fungle]

Well explained. I could understand it.

 

[Jet Black]

Well explained. I could understand it.

thanks sometimes I am terrible at explaining things, sorry about that!

 

[MartinM]

Logic would have it that there is an "edge" or "boundary" to spacetime. That is, that spacetime is finite

Then logic is wrong. There's nothing whatsoever that requires spacetime to be finite.

 

[MartinM]

was there a name for this sort of space, where it is all broken up into bubbles of non-interacting spaces because of the expansion... I thought it was de sitter spaces but I looked those up and it doesn't look like it...

De Sitter spaces expand exponentially, making them the simplest inflationary models. What you're describing sounds like Andre Linde's chaotic inflation. We start off with an infinite, inhomogenous Universe. Since the inflationary potential is different at different points, various patches of this Universe will behave differently. Some will inflate insanely fast, others will collapse into black holes. A few will inflate, then settle down and thermalise - just like ours. The effects of inflation mean that we never have to worry about what exactly lies outside our own patch - it's way outside our horizon in any case.

Yet another nail in the coffin of the Cosmological argument, hmm?

 

[MartinM]

Could the micro wave analysis give an idea of the limits of the universe as well as a timeframe.?

Yes. Analysis of the CMB yields information on the mass and vacuum energy densities which govern the geometry of the Universe. The best such data, from WMAP, suggests that the Universe is flat, or very nearly so. Assuming trivial topology, a flat Universe is infinite in spatial extent. Non-trivial topologies would also leave their mark on the CMB. There was a story in Nature recently about just such a possibility.

 

[ReUsAbLePhEoNiX]

not locally. remember that when you are talking about cosmology, you are talking about millions of light years, so while it is clumped into local groups and clusters and even superclusters, there is no gradient across the entirity of visible space. The reason for this clumping is gravity, but there is no "overall" gravity pulling everything in one direction or something like that. (as there would be if there was a "center" to the universe)

The best comparison I can give is imagine big sheet of paper, on which you sprinkle iron filings in an isotropic way. now you chuck a bunch of tiny magnets on it, and these pull the iron filings towards them. locally the iron filings are clumpy, but looking at the big picture across the whole sheet, the distribution is still isotropic - there is no directional preference, as there would be if there was a big magnet to one side of the sheet.

I think a better definition in this case for isotropic would be "the same in all directions"

excellent explaination.

I cant get past the concept that the Hubble Constant would show a center where the Big Bang started. Is the universe "stretching" as compared to "expanding" from a singular point? I must be misunderstanding some crucial concept here.