(SNES) The Expanding Universe

[Albireo54@aol.com]

Hi Group,

I don't know if any of you frequent the Astronomy Discussion Page (a pretty 
good site) but this question and answer about the universe appeared the other 
day, and I thought it might be of general interest, given all the recent news 
of discoveries confirming certain theories of cosmology.

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Question:

If we're finding that the universe is flat, and can see large-scale structure 
amongst all the clusters and superclusters of galaxies...Could the universe 
have a very real centre/edge? 

Or does flat/closed have nothing to do with it? 

Thanks,
Kiwi
New Zealand
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Answer, from Dr. Brad Peterson, Ph.D., Ohio State University, Dept. of 
Astronomy and Astrophysics

The answer to this question is really kind of cosmology in a nutshell, so it 
may seem sort of superficial. I'd recommend one of the many fine popular 
books like Silk's "The Big Bang" if you're interested in more. 

Basically, in cosmology you start out with two basic apparently simple 
hypotheses: 

(1) The laws of physics are the same everywhere, and the same at all times. 
(2) Our place in the Universe is not special. 

The second one is tricky: it means that the Universe must be everywhere 
homogeneous (any large volume holds about the same amounts of different types 
of stuff) and it is isotropic (it looks the same in every direction). Again, 
the second of these is also tricky: if the Universe is isotropic EVERYWHERE, 
we can only conclude that the Universe has no edge. There are only two ways 
we can make a geometry that has no edge: either the Universe is infinite or 
it curves around into something like a sphere. Either geometry eliminates 
edges. 

Einstein's equations tell us that the Universe is a four-dimensional 
space-time. We can't visualize things in four dimensions, so we have to use 
analogies. The easiest one I know is to think of the Universe as a 
two-dimensional surface embedded in three dimensions. A flat Universe would 
be an infinite plane (remember, the Universe is confined to the 
two-dimensional surface, so light travels only in the plane: beings living in 
this plane have no concept of anything above or below it, they only know 
about the two dimensions in the plane. The flat Universe is the simplest of 
the infinite, or open, Universes. 

We could also embed a two-dimensional sphere in this three-dimensional space. 
If we lived on such a surface, it would be just like living on the surface of 
the Earth, but with no concept of "up" and "down." You could, in principle, 
travel in any direction and eventually return to your starting point (in the 
real Universe, you can't do this because the sphere is expanding too 
rapidly). 

So, the bottom line to this is simple: the Universe has no edge, and it has 
no center. When the Big Bang happened, it happened EVERYWHERE and it is space 
itself that is expanding. 

An attentive person hearing this story for the first time will say "hey, wait 
a minute. Everything you've said is based on your initial assumptions. What 
if they are wrong? Is there any evidence that your whole mathematical 
description has any basis in reality?" 

The answer to this is "yes, the model is in basic agreement with the 
observable facts." If you accept the mathematical model and start putting 
physics into it, you get some interesting predictions: 

(1) If you put mass in this Universe, gravity makes the Universe collapse, 
unless it is expanding. Einstein found a way around this, but nevertheless, 
we observe that the Universe is indeed expanding. 
(2) If the Universe is expanding, it used to be smaller (or at least more 
dense, if it is infinite). This means that billions of years ago, it was very 
dense and hot. We do indeed see the remnant of this hot phase, the cosmic 
microwave background, which was predicted theoretically before it was 
detected. 
(3) If the Universe started out in a very hot dense phase and then expanded 
and cooled, about 1/4 of the hydrogen in the Universe should have fused into 
helium during the first 3-4 minutes of the history of the Universe. Again, 
this is consistent with what's been observed. 

Again, this is a pretty quick overview of cosmology, but I hope that it 
partially answers your questions and gets you interested enough to read some 
of the excellent books on it that are now available. 

=============================================================

This discussion brings me back to our very human and limited sense of space 
and time. I have always envisioned that in a big bang scenario, where the 
entire known universe was created in one inconceivably powerful explosion, 
that that blast had to occur at some place - a fixed spot. That since the 
universe is expanding, with all the galaxies racing outward and away from one 
another, that they must have started their journey at a common location - way 
back when. Well, not so, if you follow current cosmological thinking. That 
primordial "place" wasn't a specific place at all - it was the entire 
universe at the time.

Normally, the more I wrestle with these concepts, the more confused I get. 
But bit by bit it is beginning to penetrate this skull of mine. Lots more 
reading to do, though!

And Les, what a great topic for more talks at the Nature Center, and later in 
the Sky Theater! Actually, it might all make more sense in the FDO dome, 
after a 14 hour day and 10 hour night of observing, cold, exhausted and 
hungry! I find that I can accept just about anything under those conditions.  
:o)

Doug Stewart
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