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Jan26-05, 06:33 PM
PF Gold
P: 667
Various Quotations:

One of the most persistently asked questions has been: How was the universe created? Many once believed that the universe had no beginning or end and was truly infinite. Through the inception of the Big Bang theory, however,no longer could the universe be considered infinite. The universe was forced to take on the properties of a finite phenomenon, possessing a history and a beginning.

About 15 billion years ago a tremendous explosion started the expansion of the universe. This explosion is known as the Big Bang. At the point of this event all of the matter and energy of space was contained at one point. What exisisted prior to this event is completely unknown and is a matter of pure speculation. This occurance was not a conventional explosion but rather an event filling all of space with all of the particles of the embryonic universe rushing away from each other. The Big Bang actually consisted of an explosion of space within itself unlike an explosion of a bomb were fragments are thrown outward. The galaxies were not all clumped together, but rather the Big Bang lay the foundations for the universe.

The origin of the Big Bang theory can be credited to Edwin Hubble. Hubble made the observation that the universe is continuously expanding. He discovered that a galaxys velocity is proportional to its distance. Galaxies that are twice as far from us move twice as fast. Another consequence is that the universe is expanding in every direction. This observation means that it has taken every galaxy the same amount of time to move from a common starting position to its current position. Just as the Big Bang provided for the foundation of the universe, Hubbles observations provided for the foundation of the Big Bang theory.

Since the Big Bang, the universe has been continuously expanding and, thus, there has been more and more distance between clusters of galaxies. This phenomenon of galaxies moving farther away from each other is known as the red shift. As light from distant galaxies approach earth there is an increase of space between earth and the galaxy, which leads to wavelengths being stretched.

What a cosmic can of worms. Turns out even the word "universe" is elusive, having three meanings (two of which depend on whether or not you hit the shift key). So we start with the basics.

What does the word "universe" mean?

The "observable universe," Sweitzer explained, "is the one astrophysicists generally talk about because it's the one open to empirical measurements. In fact it's the only one we can or ever will be able to talk with any certainty about."

He goes on to explain that "universe" (sans the word "observable") is a larger concept that scientists think "conforms to our laws of physics and all the assumptions that go with them." Comprehending this universe, Sweitzer said, "requires a leap of faith into unobservable realms."

Finally, there is "the Universe," which, by virtue of its capital "U," includes "absolutely everything, even possibilities of dimensions, modes and regions that obey laws of physics we don't know or maybe even can't know."

Is the universe finite or infinite?

"The observable universe is finite," Sweitzer said, which is to say that it had boundaries -- physical limits. Sort of. "It's a boundary to the events we can see directly, but not a boundary in the sense that New York State has a boundary."

And in an expanding universe, this boundary is constantly moving, as is everything within it. Cosmologists typically invoke a balloon with spots on its surface, representing galaxies, to explain the expanding universe. As the balloon is inflated, the spots grow farther apart. If you stood within one of these spots, you'd see all the others moving away from you, and the most distant spots would move appear to move the fastest.

Sweitzer goes on to say that the observable universe is probably part of a much larger universe, "which could be finite or infinite. Any global statements about the universe, such as overall extent, are speculative because they require extrapolating local mathematical theories and measurements beyond the observable universe."

So if the observable universe is finite, like the space occupied by a car or a house is finite, then there must be a brick wall or something up there, holding it all together. Right?

Does the observable universe have an edge?

No, said Livio.

Argh. First you guys tell us the universe is expanding. Then you say it is finite. Now you say it has no edge! We need a visual here.

Livio is up to the task. He dredges up the old expanding balloon as his prop. "An ant traveling on the surface of a balloon will never reach an edge," Livio explains. "In the worst case it will return to its starting point."

Before we discuss which of these three pictures describe our universe (if any) we must make a few disclaimers:

Because the universe has a finite age (~13.7 billion years) we can only see a finite distance out into space: ~13.7 billion light years. This is our so-called horizon. The Big Bang Model does not attempt to describe that region of space significantly beyond our horizon - space-time could well be quite different out there.

More specifically in response to the statement that:

But in the Friedmann-Robertson-Walker model of a flat or open universe, there would be no upper limit on d.
Because FRW pertains to the "observable universe", there would be a limit on d, because the observable universe is finite, although it does not have an edge. FRW does not model a universe of infinite dimension.

Most cosmologists agree that the observable universe is well approximated by an almost FLRW model, that is, a model which follows the FLRW metric apart from primordial density fluctuations. In a strictly FLRW model, there are no clusters of galaxies, stars or people, since these are objects much denser than a typical part of the universe.

However, for brevity, the almost FLRW model is often referred to simply as the FLRW model (or the FRW model).

And with regard to this statement:

I think you are fundamentally misunderstanding how the "conventional Big Bang scenario" (ie the Friedmann-Roberston-Walker model) actually works. There is no central position in space where the explosion originated, so the notion of radiation "outpacing everything else" doesn't make sense--rather, at all finite times matter and energy are evenly distributed throughout all of space, so there is no empty region where radiation hasn't reached yet.
While I would agree that there is no "center" where the explosion originated (or more usefully, due to the "stretching of space" we are all still in the center), the speed of radiation does define the distance of the big bang "horizon" which defines the "observable universe" which is the object described by FRW. Put another way, there is no "d" in the universe described by FRW at the present time with a length of 50 billion light years.

See also:

As I understand it, even in FRW with a flat topology, "stuff" only exists out to the "horizon" even though "stuff" will expand infinitely with the horizon.

Neither FRW, nor any mainstream Big Bang theory makes any statement about the "Universe" as opposed to the "universe".