Gramm wrote:
Since nothing can exist outside of the Universe, then how can the Universe have a boundary in any conventional sense?

Gramm wrote:

yours is more of a metaphysical answer than one based on observable fact.
Gramm

Sepiraph wrote:

I ask Google (the "smartest person" I know ) and this is what I got:

"Does space have a boundary because it has a finite age since the Big Bang? "

But of course, if you REALLY want to know, you need to learn General Relativity.

Is the Universe finite or infinite?

An interview with Joseph Silk : 2 May 2001.

ESA: Is the Universe finite or infinite?

Joseph Silk: We don't know.

The expanding Universe theory says that the Universe could expand forever [that corresponds to a 'flat' Universe]. And that is probably the model of the Universe that we feel closest to now. But it could also be finite, because it could be that the Universe has a very large volume now, but finite, and that that volume will increase, so only in the infinite future will it actually be infinite.


ESA: It sounds like a game of words, is it?

Joseph Silk: No. We do not know whether the Universe is finite or not. To give you an example, imagine the geometry of the Universe in two dimensions as a plane. It is flat, and a plane is normally infinite. But you can take a sheet of paper [an 'infinite' sheet of paper] and you can roll it up and make a cylinder, and you can roll the cylinder again and make a torus [like the shape of a doughnut]. The surface of the torus is also spatially flat, but it is finite. So you have two possibilities for a flat Universe: one infinite, like a plane, and one finite, like a torus, which is also flat.


ESA: ‘Flat' seems to have a different meaning to non-scientists. By 'flat' we understand to be like a table, which has width. Does the Universe have width?

Joseph Silk: Flat is just a two-dimensional analogy. What we mean is that the Universe is 'Euclidean', meaning that parallel lines always run parallel, and that the angles of a triangle add up to 180o. Now, the two-dimensional equivalent to that is a plane, an infinite sheet of paper. On the surface of that plane you can draw parallel lines that will never meet. A curved geometry would be a sphere. If you draw parallel lines on a sphere, these lines will meet at a certain point, and if you draw a triangle its angles add up more than 180o. So the surface of the sphere is not flat. It's a finite space but it's not flat, while the surface of a torus is a flat space.


ESA: Planck will measure the Cosmic Microwave Background (CMB), which carries information on the geometry of the Universe. Will we be able to find out if the Universe is finite or not?

Joseph Silk: Even if with our Cosmic Microwave Background data we can prove that the Universe is flat, we still won't know whether it's finite or infinite.


ESA: Then how are we going to know whether the Universe is infinite?

Joseph Silk: With great difficulty! We may never know it. If the Universe is finite, that means that in a two-dimensional geometry it would be like a torus. Now, think about a torus. In such a Universe, light travelling on the surface of a torus can take two paths: it can go around the sides but it can also go in a straight line. This means that if the Universe is like a torus, light can have different ways to get to the same point. You can have a long way and a short way. And that would not be true on a plane. But a torus means that space is more complicated. It would mean that when you measure the CMB you will see strange patterns on the sky, because the light from far away would not have come to us in quite a straight line because of the topology of the Universe. So the hope would be, eventually, to look for those strange patterns on the sky.


ESA: Will Planck be able to see those patterns?

Joseph Silk: In principle, yes. If the Universe is like a torus you can see something. If the Universe were finite it would be 100 times larger than the horizon, which is the distance the light has travelled since the Big Bang. That would correspond to the size of the 'doughnut' of the torus. We could in principle be able to measure that with Planck. On the other hand, if the Universe was truly infinite then we would see no signal at all from this peculiar thing. What we could really say in that case is that the Universe is larger than a certain size. But if it was finite it could be measurable.


ESA: What would be the size of the Universe if it was finite?

Joseph Silk: It could be as large as 100 times the horizon. That means that the Universe would be as much as a 100 thousand million parsecs, about 300 thousand million light years, if we could measure the topology.


ESA: We seem to agree that the Big Bang started with an 'inflation', a short period of high-speed expansion. But what happened before that?

Joseph Silk: Maybe long before inflation there was a Universe that was collapsing near a singularity, which then inflated again, so there was already a history before the Big Bang. Some people think there was a 'pre-Big Bang'. One possibility is that this pre-Big Bang, if there was such a place, would have made lots of entropy (the amount of disorder in the Universe). And the Universe we live in does have huge amounts of entropy. That's one theory. But we have no understanding of how to change from collapsing to expanding. There's no physical way to explain that transition. Some people believe that they have explanations the pre-Big Bang, so it's a respectable theory.

swstephe wrote:
is mind-boggling that the Universe "ends" for us at 18 billion light years, (or around there). But a galaxy 9 billion light-years from us can observe 9 billion light-years beyond that. It seems intuitive that matter would expand from some point, and a natural bias to assume that we are at that center, but that center is only relative.