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Star 7.5B L.Y. away expodes

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sol invictus said:
The part of the universe that is currently observable (i.e. from which light has reached the earth) has a finite size, and that's often referred to as the size of the universe.

Strictly speaking we do not know what's beyond the part we can see, but the most favored theory is that its infinite.
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Thank you sol invictus and aggle-rithm.

Half-way across the visible universe...
 
Is the most favoured theory really that it's infinite? I find that slightly strange. Given that the most favoured theory of the age of the universe = finite, I would have thought that would apply to the size also?
 
dogjones said:
Is the most favoured theory really that it's infinite? I find that slightly strange. Given that the most favoured theory of the age of the universe = finite, I would have thought that would apply to the size also?
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The part of the universe we see is expanding, and is also homogeneous and isotropic. That means every point is equivalent to every other point, and every direction looks like every other direction (that's on average, not precisely, but it's true to about 1 part in 100,000 when you average over large scales). If you ask, what are the expanding solutions to Einstein's equations for gravity which are homogeneous and isotropic, you find precisely three.

Of those three solutions, two are spatially infinite and one is finite (their spatial geometries are planar, hyperbolic, or spherical, and they are called flat, open, and closed universes). The data tell us that the universe is very close to planar (flat), and that if it's not planar its radius of curvature (which is the size of the sphere in the closed case) must be very, very large - larger than the observable universe. That's possible, but the simplest option is to simply assume it's flat until or unless data contradicts that.
 
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Wicked explanation, thanks Sol!

Although I still find this paradoxical. If the universe was a singularity before the big bang, was it an infinite singularity?
 
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dogjones said:
If the universe was a singularity before the big bang, was it an infinite singularity?
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Yes, in the sense that we will have to wait an infinite amount of time to see light from the whole thing.

The confusion here is that whether something is infinite, finite, or a point can depend on how you choose to measure it. Our everyday intuition about volumes and time is just completely wrong at a singularity.

One way to define distance to a point is by how long you have to wait before light from that point reaches you. In that case, the big bang singularity in a flat or open universe is infinite. However the same definition says the horizon of a black hole is infinitely far from any point outside, and that's not very nice.

Another way to define it to take some group of stars today - say a galaxy - and trace it back in time, and ask how much volume those stars (or the particles which formed them) occupy when you get to the bang. The answer is zero - everything in the group collapses down to one point - which might lead you to say the singularity is a point.

Really you just need to write things down mathematically, and then you can figure out the answer to any definite question like that.
 
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sol invictus said:
Yes, in the sense that we will have to wait an infinite amount of time to see light from the whole thing.

The confusion here is that whether something is infinite, finite, or a point can depend on how you choose to measure it. Our everyday intuition about volumes and time is just completely wrong at a singularity.

One way to define distance to a point is by how long you have to wait before light from that point reaches you. In that case, the big bang singularity in a flat or open universe is infinite. However the same definition says the horizon of a black hole is infinitely far from any point outside, and that's not very nice.

Another way to define it to take some group of stars today - say a galaxy - and trace it back in time, and ask how much volume those stars (or the particles which formed them) occupy when you get to the bang. The answer is zero - everything in the group collapses down to one point - which might lead you to say the singularity is a point.

Really you just need to write things down mathematically, and then you can figure out the answer to any definite question like that.
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A physicist and I were talking the other day, and he pointed to the nature of the singularity at the beginning of the universe as a demonstration that God is needed. Do you know if physics/astronomy has a good enough understanding of the beginning of the universe to claim that it came about naturally? (I would prefer to avoid "where did God come from" discussions here, and stick to what science knows)
 
m_huber said:
Do you know if physics/astronomy has a good enough understanding of the beginning of the universe to claim that it came about naturally? (I would prefer to avoid "where did God come from" discussions here, and stick to what science knows)
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We don't know how to resolve singularities like the big bang. Probably the best way to describe what that means precisely is as follows: we know that the laws of physics we teach to school children cannot be precisely correct because they are not internally self-consistent (I can give more details on that if you ask). This means there must be corrections to them which (presumably) render them consistent, but we do not know what those corrections are. However we do know that whatever they are, they must become very important at the big bang (or in general at any gravitational singularity). Therefore, until we know what they are, we can't say what happened at times very close to the bang, and so we cannot say much at all about what, if anything, came before.

However it's perfectly possible that we will discover a more complete set of physical laws which can handle such singularities and will tell us how to resolve them. In fact there are many proposals for such theories and many people working on them, although none are very compelling so far. So unless by "god" you simply mean ignorance or the limits of current himan knowledge, I see no reason why god needs to be invoked here. To paraphrase the Marquis de Laplace, I have no need for such a hypothesis.
 
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