# Is the universe finite or infinite?

Does the universe has boundaries?, is it finite?

mathman
Universe does not have a boundary. Finite or infinite is an open question.

Universe does not have a boundary. Finite or infinite is an open question.

But there is no such thing as infinity in physics !!!

haruspex
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But there is no such thing as infinity in physics !!!

Probably, but it's hard to be sure. Certainly the visible universe is finite and bounded. According to the inflationary theory of the big bang, space expanded faster than the speed of light for a while, so there are regions from which the light will never reach us.

But more interesting is what mathman said about the finite case. The universe can be finite in volume yet have no boundary. It depends on the topology. Imagine space as a cube, but in which each point of the left side is the same place as the point straight across from it on the right side. Similarly top/bottom and back/front. If you wander off through one face you just reappear out of the opposite face. And you wouldn't notice anything special, because where you choose the faces to be is arbitrary. Every point is as good as any other. (Topologically this would be a 3D torus, I think.)

phinds
Gold Member
But there is no such thing as infinity in physics !!!

Oh? Can you prove that? Do you have an accredited references to back up such a categorical statement?

I believe there is a simple solution to this problem and it is based on behavior all through the Universe so I do not believe it is an inappropriate stretch to apply it to the entire Universe: we observe phenomena in the Universe having critical points in their dynamics. Breaching such a point often causes the dynamics of the phenomenon to change qualitatively and by doing so, the rules change. For example asking what does swimming mean beyond the critical point of freezing? What happens to a hydrogen atom beyond the critical point of fussion? In a small section of the ground it looks flat, even my whole yard. But it's not always flat, beyond the horizon the rules change and asking whether the earth is infinitely flat or we just fall off is simply not following the new rules of a spherical earth in a gravity field.

Therefore, in regards to a "size" of the universe, I do not think it is unreasonable to suggest at some large "size", a critical point is reached, the rules change, the concept "volume" loses meaning, and asking for a "size" of the Universe beyond that point is simply not following the new rules.

Oh? Can you prove that? Do you have an accredited references to back up such a categorical statement?

I think the big bang "Theory" can proves it, however I still don't know what do we mean by "universe", is it the space that we know, or it also inclues beyond space "Vacume !!!". What do you call the "No Space/No Time" before the big bang???!!!

I believe there is a simple solution to this problem and it is based on behavior all through the Universe so I do not believe it is an inappropriate stretch to apply it to the entire Universe: we observe phenomena in the Universe having critical points in their dynamics. Breaching such a point often causes the dynamics of the phenomenon to change qualitatively and by doing so, the rules change. For example asking what does swimming mean beyond the critical point of freezing? What happens to a hydrogen atom beyond the critical point of fussion? In a small section of the ground it looks flat, even my whole yard. But it's not always flat, beyond the horizon the rules change and asking whether the earth is infinitely flat or we just fall off is simply not following the new rules of a spherical earth in a gravity field.

Therefore, in regards to a "size" of the universe, I do not think it is unreasonable to suggest at some large "size", a critical point is reached, the rules change, the concept "volume" loses meaning, and asking for a "size" of the Universe beyond that point is simply not following the new rules.

Very logical explanation....Thanks.

Astronuc
Staff Emeritus
"Space," it says, "is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space, listen..." - Douglas Noel Adams, Chapter 8, The Hitchhiker's Guide to the Galaxy, Earth Edition, 1979

Paraphrased - "The Universe is Big. Really big. It may seem like a long way to the corner chemist, but compared to the Universe, that's peanuts."
http://astro.gmu.edu/classes/a10695/notes/l01/l01.html [Broken]

The Hitchhiker's Guide to the Galaxy offers this definition of the word "infinite".

Infinite: Bigger than the biggest thing ever and then some. Much bigger than that in fact, really amazingly immense, a totally stunning size, real "wow, that's big," time. Infinity is just so big that, by comparison, bigness itself looks really titchy. Gigantic multiplied by colossal multiplied by staggeringly huge is the sort of concept we're trying to get across here.
http://www.acc.umu.se/~ola/hitchhik.htm

The challenge for one is not to let the vastness of the universe boggle one's mind.

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phinds
Gold Member
I think the big bang "Theory" can proves it

I did not ask what you think, I asked what you can support by science.

, however I still don't know what do we mean by "universe", is it the space that we know, or it also inclues beyond space "Vacume !!!".

So you are making a definitive statement about something you can't even define. Now that's REALLY scientific.

What do you call the "No Space/No Time" before the big bang???!!!

I don't call it anything since there is no evidence that there was such a thing and it is not part of the big bang theory

But there is no such thing as infinity in physics !!!

Infinity is a really very very large number no one can reach. Its a concept used in math and physics. Nobody knows the real size of the universe. It is really really large. Like mathman said finite or infinite is a open question.

Drakkith
Staff Emeritus
One cannot probe anything tangible is infinite in size, extent, or count. It would take an unending amount of time. However the concept of infinity does exist.

the big bang does not prove that there is no boundary to the universe. the expansion of space isnt the physical expansion of acquiring more territory that we experience here on earth. the big bang occurred everywhere at once.

and there can be an infinity. if you follow these two patterns you will notice that they will go on forever
1 2 3 4 5 6 7...
2 4 6 8 10 12...
divide them and you still get a number yet they are both infinitely large.

to add to the idea of no space or time before the big bang, well you pretty much said it. there was no time. kind of a hard thought for human logic but time started at t=0 so yes, time does have a beginning.

plus... at the end of the day, the big bang is still a theory =P

haruspex
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the big bang does not prove that there is no boundary to the universe.
I can't find a post claiming it does. OTOH, does anyone think it does have a boundary?

and there can be an infinity. if you follow these two patterns you will notice that they will go on forever
1 2 3 4 5 6 7...
2 4 6 8 10 12...
divide them and you still get a number yet they are both infinitely large.
But those are not infinities in the physical world, just in mathematical theory.

Chronos
Gold Member
Er, maybe. That's my answer and I'm sticking to it.

I did not ask what you think, I asked what you can support by science.So you are making a definitive statement about something you can't even define. Now that's REALLY scientific.I don't call it anything since there is no evidence that there was such a thing and it is not part of the big bang theory

No offense, but thats not helping at all.

Acc. to me, since light is the fastest thing, then you can define an 'energy & mass' boundary, beyond which only those things lie which existed before the big bang (if anything did). But you can't define a boundary, neither to space, nor to the universe.

to add to the idea of no space or time before the big bang, well you pretty much said it. there was no time. kind of a hard thought for human logic but time started at t=0 so yes, time does have a beginning.

There is a logic behind the beginning of time. Any events which occurred before the big bang does not affect us or the universe today. There is no need to assign these useless and unknown events with a time. Thats why we have BIG BANG occurred at t=0;

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Ken G
Gold Member
I think there is even some confusion here about the kinds of questions that science is able to answer. We actually don't use science to answer questions like "is the universe finite or infinite", we just use it to address questions like "is the universe finite." The answer to that question is, "we have no scientific evidence that the universe is finite." That's it, that's all we can use science to say. This is not evidence that the universe is infinite, such a thing is not likely to even be possible to obtain. Since absence of evidence is not evidence of absence, we simply cannot use science to say anything at all about whether or not the universe is finite in size (in comoving coordinates, etc.), we can only say we have no evidence that it is finite. Why must we always try to use science for more than it is intended or appropriate?

haruspex
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The answer to that question is, "we have no scientific evidence that the universe is finite." That's it, that's all we can use science to say. /QUOTE]

I think we can go a little further than that. Many/most people would assume that if space has no boundaries then it must be infinite in volume. Science can be used to produce models, consistent with physics as far as is known, in which space can be finite yet unbounded. There may even be implications of such models which could be tested.

The universe is finite but with no boundary. It is similar to a bubble or sphere

Ken G
Gold Member
I think we can go a little further than that. Many/most people would assume that if space has no boundaries then it must be infinite in volume.
It's not terribly relevant, but that doesn't follow. Space could curve back on itself, have finite volume, and still have no boundaries. But the real issue here is, despite looking very hard (and quite possibly as hard as we can ever look), we have no evidence that it does have boundaries, nor do we have any evidence that it does not have boundaries, nor do we have any evidence that it curls back on itself, nor do we have any evidence that it does not curl back on itself. All we know is, what we see looks flat, and we have no idea how long it stays looking flat. That's it, that's the scientific evidence in its entirety.
Science can be used to produce models, consistent with physics as far as is known, in which space can be finite yet unbounded. There may even be implications of such models which could be tested.
But making and testing models has nothing to do with answering the OP question. The models we make are intended as idealizations, and the standard idealization is that of a flat infinite universe. That model works quite well. Is that evidence that the universe really is flat and infinite? Of course not. If I am digging a foundation for my house, I'm certainly going to use a model that the surface of the Earth is flat and infinite (in that I will certainly not model any curvature of the Earth), and it will work great for digging my foundation, but I'm never going to conclude that any of this is evidence that the Earth really is flat and infinite.

Ken G
Gold Member
The universe is finite but with no boundary. It is similar to a bubble or sphere
There is zero evidence that this is the case, and there is also zero evidence that this is not the case. And humanity should be prepared for the possibility that this situation will never change for us, as it seems quite likely at present. Even if efforts to detect a tiny spatial curvature do eventually succeed, it won't require that the universe maintains that same curvature everywhere, that will simply be an idealization of the model, like any other idealization of any other model. It will never be testable as fact, we pretty much already know this.

haruspex
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... that doesn't follow. Space could curve back on itself, have finite volume, and still have no boundaries.
Yes, that's the point I was making.
nor do we have any evidence that it curls back on itself,
I'm far from expert in this area, but my understanding is that General Relativity posits a "flat" spacetime, but a finite, curved space, with a compensating curvature in time.
If so, and if you accept evidence for GR, that is surely evidence for a finite universe.
I would also have thought that an infinite universe was inconsistent with the Big Bang model, and there is much evidence for that.
You seem to be demanding much more direct evidence.
But making and testing models has nothing to do with answering the OP question.
I disagree. The question was "Does it have boundaries? Is it finite?" Establishing that 'finite without boundaries' cannot be ruled out is a partial answer.

Oh? Can you prove that? Do you have an accredited references to back up such a categorical statement?

lolz

Ken G
Gold Member
I'm far from expert in this area, but my understanding is that General Relativity posits a "flat" spacetime, but a finite, curved space, with a compensating curvature in time.
No, GR says that in comoving-frame coordinates (which is what is generally used in cosmology to talk about what the universe as a whole is doing), all the observed curvature due to gravity is in the time dimension (associated with cosmological redshifts), none is in the spatial dimension. We say the universe is "spatially flat" in this sense. The observations cannot rule out some small spatial curvature, but they can rule out the idea that the universe curves back on itself over the range that we can observe or in some way test our inferences about-- and of course we have no idea what it does beyond that range. Even if we do detect some small spatial curvature, it would not require that this curvature is maintained beyond what we can observe-- the cosmological principle applies to explanations of what we actually observe, it is not a philosophical claim about what we cannot observe.
I would also have thought that an infinite universe was inconsistent with the Big Bang model, and there is much evidence for that.
No, the Big Bang model includes neither finiteness nor spatial curvature at present. So the model is one of an infinite universe. However, the model need not make any claims that this is actually true, it just means we have no reason to model finiteness of the universe.
I disagree. The question was "Does it have boundaries? Is it finite?" Establishing that 'finite without boundaries' cannot be ruled out is a partial answer.
I said it could not be ruled out. I also said the alternative could not be ruled out. In fact there is no evidence at all either way. When there is no evidence in favor of a proposition, nor evidence against it, it doesn't leave you with a whole lot more to say, which is the point.

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Even if we do detect some small spatial curvature, it would not require that this curvature is maintained beyond what we can observe-- the cosmological principle applies to explanations of what we actually observe, it is not a philosophical claim about what we cannot observe.

But CMB power spectrums can and do constrain large scale anisotropy. We can directly measure what's in our bubble, we can infer things for some distance outward.

Also, if we do detect small scale curvature, this is going to very strongly constrain the details of inflation and we can use that to infer a lot of stuff.

No, the Big Bang model includes neither finiteness nor spatial curvature at present. So the model is one of an infinite universe.

This is incorrect. LCDM doesn't require finiteness, but it doesn't exclude it. Also whether the current model allows for a finite universe is an observational equation that changes from moment to moment. Before the discovery of dark energy, the amount of dark matter in the universe was clearly insufficient to close the universe so there was a period of a few years in which the prefered model was infinite and negatively curved.

Then we have dark energy and everything changed.

When there is no evidence in favor of a proposition, nor evidence against it, it doesn't leave you with a whole lot more to say, which is the point.

At that point you step back and figure out what you need to find out to constrain what you know. If you don't know, the next question is what do you need to do to find out.