Is the Universe Truly Infinite and Ever-Expanding?

[hellfire]

The Universe started as a singularity of infinite density, but not necesary of zero volume.

I fail to understand how the universe could have been spatially infinite at the beginning of time. For every cosmological model the volume contained within every boundary at the big bang is zero because for a zero scale factor a(t=0) = 0 the volume element is zero (or all distances are zero in the singularity). A different situation arises as soon as one considers any time slightly greater than zero, avoiding the singularity.

 

[EL]

I fail to understand how the universe could have been spatially infinite at the beginning of time. For every cosmological model the volume contained within every boundary at the big bang is zero because for a zero scale factor a(t=0) = 0 the volume element is zero (or all distances are zero in the singularity). A different situation arises as soon as one considers any time slightly greater than zero, avoiding the singularity.

I agree with that using the term "spatially infinite" at t=0 may not really have any meaning, since as you say the scale factor is zero. However, what I wanted to point out is that it need not have started as what we intuitively think of as a point either. I.e. the reasoning Danger is using, that it has to be of finite size, does not hold.
The point I wanted to make is that even though the Universe started as a singularity, it may be infinite at any t>0. Wheter one call it infinite or not at t=0 is more of a personal taste.
Anyway, we do not really know how to describe the universe as time approaches zero, since we don't know about what laws will hold.

 

[SpaceTiger]

You lost me on that one. How could it have been infinite when it formed? If it started at zero volume/infinite density, and is now at medium volume/medium density, then it had to have passed through small volume/high density on the way.

I think Ned Wright addresses this in a fairly straightforward manner:

http://www.astro.ucla.edu/~wright/infpoint.html"

 

[EL]

I think Ned Wright addresses this in a fairly straightforward manner:
http://www.astro.ucla.edu/~wright/infpoint.html"

Yes! And the question adressed is what confuses many people, since it is simply not the whole thruth. The Universe (i.e. the whole Universe) doesn't have to be concentrated into one point at the Big Bang.

 

[Danger]

Thanks, ST. It's not all that straight-forward to me, though, because I'm missing most of the educational background to follow it properly. I'll certainly read as much as I can when I have more time, but the math is really going to mess me up.

 

[SpaceTiger]

Thanks, ST. It's not all that straight-forward to me, though, because I'm missing most of the educational background to follow it properly. I'll certainly read as much as I can when I have more time, but the math is really going to mess me up.

Are you sure you're looking at the right thing? The link only contains two paragraphs and a diagram.

 

[mars2]

Sorry, I can't find "involuted" in my dictionary...what does it mean?

My English is poor.So ...The threads I read is more than I reply .When I meet lion in threads ,I often use this on line http://cb.kingsoft.com/".
"involuted" mean "hard to learn" 、"complex".

I want to improve my English .^_^
The universe is so strange .I think if I fly in one direction,and many years later I can back ~~~ kidding^^

 

[EL]

My English is poor

Well, in this case it was my English which was poor! Thanks for the clarification.

 

[Danger]

Are you sure you're looking at the right thing?

Oops!
I was reading the linked tutorials at the bottom of the page. Unfortunately, I still can't quite grasp the concept. It was always my understanding that the entire universe erupted from the BB. This makes it look as if BB happened inside a pre-existing universe. What the hey am I missing here?

 

[Dmstifik8ion]

Infinity does not exist in nature (out there in reality land). It is a creation of the human mind, (an abstract in mathematics, like zero), to help us to comprehend the relationship between nothing and everything else. Things, (that are inclusive in the set of existents) seem infinite when we are no longer able to attach any concepts or ideas to what something is (its nature). Learning about, understanding and defining the universe will lead us to an appreciation of its finite quality.
This is my humble attempt to clarify something I know very little about. My hope is that it contributes less confusion to the subject than it eliminates. Please feel free to clarify this 'clarification' on a really big subject.

 

[Dmstifik8ion]

When we have achieved the ability to wrap our minds around the universe we will then, thereby, have encircled it finite quality.

 

[EL]

Oops!
It was always my understanding that the entire universe erupted from the BB.

You're correct in that!

This makes it look as if BB happened inside a pre-existing universe.

I cannot really see that. I think you may still be stuck to the idea that the BB started as what we intuitively think of as a "point". Anyway, it's kind of hard for our minds to grasp how the BB really looks like. What matters in this case is that GR allows for the existence of an infinite universe with a singularity as starting point.

 

[Danger]

Yeah, I'm stuck on the point thing. That's the only way that I've ever seen it described. I'll try to get caught up on the rest of it. Thanks for your patience.

 

[CJames]

I'm pretty sure that COBE and WMAP both suggest, though don't prove, that the universe is flat and therefore most likely infinite. That is, it started out infinite and will always be infinite. At the beginning of the universe, the universe was infinitely dense and infinitely vast. Now the density has dropped but it is still infinite. That is the prevailing theory at this time, although it is possible to have a finite universe that "wraps around" like a pac-man screen without curving, fulfilling the expectation that the universe is indeed flat.

 

[hellfire]

I'm pretty sure that COBE and WMAP both suggest, though don't prove, that the universe is flat and therefore most likely infinite. That is, it started out infinite and will always be infinite. At the beginning of the universe, the universe was infinitely dense and infinitely vast. Now the density has dropped but it is still infinite. That is the prevailing theory at this time, although it is possible to have a finite universe that "wraps around" like a pac-man screen without curving, fulfilling the expectation that the universe is indeed flat.

We may have also an universe that is nearly flat. This would be consistent with inflation, that drives the energy density towards the critical energy density but does not necessarily lead to \Omega = 1. If \Omega \simeq 1, the radius of curvature of the universe is much larger than the Hubble radius. We would measure a flat universe, but it might be, however, curved. A value \Omega = 1 seams to be a very fine tuning.

 

[alfredblase]

There is something I don't understand here. How can physicists and astronomers be talking about anything but the observable universe? I mean, you can conjecture about what is outside the observable universe but never prove your conjectures... right? The most you can say is we cannot, in anyway, describe the universe outside our observable universe; as far we can measure the observable universe is flat and by definition finite.

 

[EL]

There is something I don't understand here. How can physicists and astronomers be talking about anything but the observable universe? I mean, you can conjecture about what is outside the observable universe but never prove your conjectures... right? The most you can say is we cannot, in anyway, describe the universe outside our observable universe; as far we can measure the observable universe is flat and by definition finite.

Often when scientists talk about "the universe" they mean just the observable universe, for instance when one here someone estimating "the number of particles in the universe", or "the total mass of the universe".

As you say, it may sound meaningless to talk about something which we cannot observe. However, we can see from the CMB that parts of the universe which are now causaly disconnected must have been connected at some earlier time. The solution to this problem is probably inflation.
Hence, the parts of the universe which is at the time not observable for us, may have been observable before inflation, and hence it makes sense, at least in certian cases, to discuss the "whole" universe.

 

[alfredblase]

Are you saying that before inflation the "whole" universe was causally connected, or just that more of the universe was causally connected? In either case it only makes sense to talk about outside our currently observable universe when referring to a period of time before inflation right? If that is true are you saying that by knowing how at least some of the currently unobservable universe was before inflation, that we can predict how it is today? I tend to disagree, simply because predictions that cannot be proven or disproven are not to be accepted as truth in physics.

 

[EL]

Are you saying that before inflation the "whole" universe was causally connected, or just that more of the universe was causally connected?

That must depend on the geometry of the universe.

In either case it only makes sense to talk about outside our currently observable universe when referring to a period of time before inflation right? If that is true are you saying that by knowing how at least some of the currently unobservable universe was before inflation, that we can predict how it is today? I tend to disagree, simply because predictions that cannot be proven or disproven are not to be accepted as truth in physics.

I agree with you that it is actually not very scientifically correct to talk about something which cannot be proven by observations. But the point is that if we agree on sticking to GR and standard cosmology, the theory will model a whole universe, not just some observable part. Actually I think that Dangers statement that the universe must be finite included the "whole" universe as modeled by standard cosmology. But as you say, by strictly defining the universe as the observable one, it of course is finite by definition.

 

[hellfire]

The homogeneity we see today in the CMB should had been impossible to reach without inflation, because at the time of the formation of the cosmic background (400.000 years after the big-bang), a causal connection was given only in patches of 1 angular degree in the CMB sky-map we observe today. However, the CMB map is extremely homogeneous throughout the whole sky. To explain this inflation comes into play.

According to inflation there was a causally connected partch that inflated immediately after the big-bang to a size that is equal or larger than the observable universe today. Therefore the causal connection of in our inflated bubble was given before the formation of the cosmic background.

There may be deviations from homogeneity and flatness at larger scales. However, it would be a great coincidence if the inflated bubble we are located in would be exactly equal to the size of the observable universe today. The size of the current observable universe has no especial meaning in the theory of inflation and it is an essential feature of inflation to involve superhorizon physics during previous epochs of the history of the universe.

 

[alfredblase]

ok I think I understand know. Thanks. mmm ...I understand that you guys are all crackpots! hehehe xDD (joke)

But seriously, ok so you are modelling the universe from the time of the Big Bang (more of which, if not all of which, was causally connected before inflation than is the case now); and this model universe atleast contains our observable universe. Fair enuff, still I reckon you are asking questions that you will never be able to answer... =)

 

[SpaceTiger]

Oops!
I was reading the linked tutorials at the bottom of the page. Unfortunately, I still can't quite grasp the concept. It was always my understanding that the entire universe erupted from the BB. This makes it look as if BB happened inside a pre-existing universe. What the hey am I missing here?

Actually, I'm now thinking that link was somewhat deceptive. What he's trying to say, I think, is that these models describe a universe that is always infinite, from the present day all the way back to the big bang. At the "moment" of the big bang, the universe would still have an infinite spatial volume, but it would also be at an infinite density. As EL said, it just becomes a matter of definition at that point. However, there would be no question about the volume of our observable universe at t=0. Since it occupies a finite volume at the present time, it must occupy zero volume when compressed to infinite density. Saying that the entire universe's volume suddenly drops to zero at t=0 would create a discontinuity, so it makes more sense to keep calling it infinite.

I wouldn't worry yourself so much with these questions, however. Although we have no good physical reason to say that the universe isn't infinite, we do have good physical reason to think that the Big Bang isn't actually a singularity. Unfortunately, we'll need a theory of quantum gravity to describe how the universe behaves before the Planck time, so a more detailed answer will have to wait for new physics.

 

[SpaceTiger]

Infinity does not exist in nature (out there in reality land). It is a creation of the human mind, (an abstract in mathematics, like zero), to help us to comprehend the relationship between nothing and everything else. Things, (that are inclusive in the set of existents) seem infinite when we are no longer able to attach any concepts or ideas to what something is (its nature). Learning about, understanding and defining the universe will lead us to an appreciation of its finite quality.

I think a lot of physicists would agree with you, but I'm afraid it's not something you could ever prove. For now, we'll be responsible scientists and just say what the data are or are not consistent with.

 

[Danger]

Thanks, Mr. Tiger. I couldn't quite follow what EL was proposing (my fault, not his), but you have once again managed to explain things in a manner that is clear to someone without the requisite background in the subject. Your impending guruship is well deserved.

 

[EL]

I couldn't quite follow what EL was proposing (my fault, not his)

Sorry for making a mess out of it.

but you have once again managed to explain things in a manner that is clear to someone without the requisite background in the subject. Your impending guruship is well deserved.

I think we all can agree on that! (https://www.physicsforums.com/showthread.php?t=102199 )

 

[DragonPetter]

If the universe were infinite, wouldn't that violate laws of thermodynamics?

If two particles are infinitely apart from each other, the potential energy between them would be infinite.

If two particles infinitely away attract to each other at a constant acceleration, wouldn't they eventually accelerate to the limit of C, where either that limit has to be invalid? or if they are infinitely away, the force equations must end up at 0 force interaction between them to begin with? Wouldn't that then cause conflicts about locality??

I'm very unskilled in physics, and don't have the knowledge to answer my questions or show the flaw in my reasoning :(

 

[hypernation]

It puzzles everyone. We just have to get over it.

The problem is that there isn't really any 'outside'.

I came here to log in purely to commend your answer. The notion of "outside" is conceptual. When space ends, there's no "outside."

And if time ends, there's (presumably) no ability for motion towards the outside. (Can anyone reflect on this second point?)

 

[seto6]

think universe as a balloon as its expanding as you can see it will expand for every.

 

[Nihar Shah]

I think universe is out of the boundry of finite and infinite.
It is neither finite nor infinite
In other words it is both finite and infinite

It is also the possibility that there can be more than 1 Universe.
In Sanatan Vedic Dharma, it is written that there are 14 Universes and the rules and laws of Physics are quite different from each other.

For example,
In our universe, if north-north pole goes away from each other but they can come together from each other in other universe.

 

[Chronos]

CMB, as well as other studies, indicate omega is so close to exactly 1.0, it is too close to call - meaning the universe is virtually dead flat. A dead flat universe, under current space-time models, is spatially infinite. Now just what 'spatially infinite' means is a horse of different color. It is, however, safe to say it means the observable universe does not curve back on itself - i.e., you will never see the back of your head no matter how powerful your telescope.