Will the Universe one day become smaller?

[nico nico nii]

hi, I'm new here.

just a random thought.

everything in the universe keeps going further away from each other as time goes, which means everything was once at one place until the big bang happened. as far as i know, the universe may not be infinite; there HAS to be a limit to its expansion.

if the universe is not infinite and at one day the universe reaches that limit, then two things could happen: either the universe stops expanding or it will come back as one.

the question is: is it possible for the universe to come back as one and how can it possibly happen?

 

[Buzz Bloom]

Hi noco:

Welcome to the PF.

everything was once at one place

You have a common misunderstanding. The universe was never "at one place" with there being some other place that was not part of the universe.
I strongly recommend you read the following thread.

https://www.physicsforums.com/threads/effort-to-get-us-all-on-the-same-page-balloon-analogy.261161​

As I understand the various discussions on this topic, the consensus among experts is that it is possible for universe to someday stop expanding and start to shrink. There a consensus about the "standard model" which provides a best fit to all the data astronomers have collected so far, which is a lot. The standard model says the universe will expand forever, and most experts accept that this is by far the most likely future. However, there are some other controversial concepts about how the universe works that gives a very remote possibility the universe might someday shrink, and as of now these concepts have not yet been completely debunked.

Regards,
Buzz

 

[newjerseyrunner]

It would appear from observational evidence that not only will it expand forever, but it'll get faster and faster. There are two large scale forces in the universe: gravity and dark energy. The further things get apart, the less gravity plays a role and the more dark energy comes to dominate (it's already dominate and has been for more than a billion years.)

 

[Drakkith]

While fairly technical and complex, the wikipedia article on the shape of the universe might prove useful: https://en.wikipedia.org/wiki/Shape_of_the_universe

 

[AgentCachat]

The common interpretation of the Big Bang is that, extrapolating backwards from present data, the universe would be infinitely small, with infinite density. It would be meaningless to talk about where the primordial universe was located.
But can we justify extrapolating backwards just because a graph ends at zero volume? Seems to me quantum theory would result in a smallest volume just as there is a Planck time and Planck length. So the density of the universe, while high beyond comprehension, would still be finite.

 

[weirdoguy]

extrapolating backwards from present data, the universe would be infinitely small

That is not true if Universe is infinite. If it is, it was infinite even 'during' Big Bang.

Seems to me quantum theory would result in a smallest volume just as there is a Planck time and Planck length.

Quantum theory says nothing about smallest length or time. Planck time and length are just combinations of fundamental constants that gives value with appropriate unit.

 

[Drakkith]

But can we justify extrapolating backwards just because a graph ends at zero volume?

Extrapolating backwards doesn't lead to zero volume. As you extrapolate backwards, the density of the universe increases, but the size of it remains the same.

 

[newjerseyrunner]

The VISIBLE universe approaches zero as you rewind backwards towards the Big Bang. That's not due to the size of the universe, that has to do with the fact that causality has a speed limit.

 

[AgentCachat]

Extrapolating backwards doesn't lead to zero volume. As you extrapolate backwards, the density of the universe increases, but the size of it remains the same.

Oh, I didn't say it leads to zero volume. But plenty of physicists/cosmologists have. Regarding your last statement, this implies mass increases.

 

[Grinkle]

But plenty of physicists/cosmologists have.

Possibly you mis-read or you are taking some license in who you acknowledge as a physicist or cosmologist. Can you provide any references for such statements?

 

[Drakkith]

Regarding your last statement, this implies mass increases.

It implies that mass per unit of volume (density) increases, yes.

 

[phinds]

Oh, I didn't say it leads to zero volume. But plenty of physicists/cosmologists have.

I SERIOUSLY doubt that. Citations?

Regarding your last statement, this implies mass increases.

No, it does not. It implies the same mass takes up a smaller volume

EDIT: I see Drakkith is back from his nap and beat me to it.

 

[Jacob Mybiz]

I began seriously studying cosmology in the early 90s and your question relates very much to the then popular debate between endless expansion and eventual death(in the sense of star formation and visible cosmology) of the universe versus the Big Crunch theory(Essentially that our universe has been expanding and contracting forever and at some point this would reoccur). Since that time data from a ton of different sources have from most Cosmologist's perspectives favored almost unanimously the endless expansion paradigm. I have always favored the opposite paradigm for one major theoretical reason and also because I believe we have far to little understanding of the forces at work to be swayed by data implying either idea is accurate or in fact neither. My fundamental objection to the idea of endless expansion and the universe essentially becoming inert is it implies creation, essentially that the big bang was the birth of all that is, which of course is a logical paradox not unlike a single creator...if it created everything what created it. Something cannot come from nothing. I am open to theories involving endless expansion that aren't based on a linear progreasion of the universe, but to me without a much more complex theory multiverse etc. one has to favor the idea of cyclical expansion and contraction. The other again without a vastly more complex overlay simply violates fundamental scientific law. Just my take but I think we need to accept in this field how much we truly don't yet know rather than assume small bits of evidence which appear to confirm an idea that encompasses the entire life of a universe is both unscientific and not necessarily productive.

 

[Drakkith]

. My fundamental objection to the idea of endless expansion and the universe essentially becoming inert is it implies creation,

I don't agree. It certainly indicates that the universe was in a hot, dense state 13.7 billion years ago or so, but it doesn't imply creation in my opinion. We have very little idea of how the laws of physics worked at the density and temperature scales of the universe at this time. The universe may have existed in this state for a trillion years or a single heartbeat for all we know.

Just my take but I think we need to accept in this field how much we truly don't yet know rather than assume small bits of evidence which appear to confirm an idea that encompasses the entire life of a universe is both unscientific and not necessarily productive.

Cosmologists are well aware of the limitations of their knowledge. But they have to make models which conform to the available evidence. They can't simply make a more complex model or refuse to include something based on non-scientific grounds (i.e. "X appears to be not logical therefore we shouldn't include X as an option and certainly shouldn't say X is the best explanation we have right now"). That would be unproductive.

 

[Jacob Mybiz]

I don't agree. It certainly indicates that the universe was in a hot, dense state 13.7 billion years ago or so, but it doesn't imply creation in my opinion. We have very little idea of how the laws of physics worked at the density and temperature scales of the universe at this time. The universe may have existed in this state for a trillion years or a single heartbeat for all we know.
Cosmologists are well aware of the limitations of their knowledge. But they have to make models which conform to the available evidence. They can't simply make a more complex model or refuse to include something based on non-scientific grounds (i.e. "X appears to be not logical therefore we shouldn't include X as an option and certainly shouldn't say X is the best explanation we have right now"). That would be unproductive.

I think you state that quite eloquently and I agree that Cosmologists are aware of the limitations of the predective capability of their research. My essential point, we can't in fact refer according to essentially all theories of the early universe, the superdense state which existed immediately preceding the big bang in regards to time. They are predicated on spacetime being a result of the big bang what came before is simply inobservable if it had a duration it couldn't be described using any of the forms of measurement we use in science. However physics is awash in terminology which in fact implies it was the beginning not an intermediate state. When we discuss the end of the universe it mandates the existence of a beginning. It seems vastly more rational to have the default assumption be, it has always existed in one form or another. I understand that may seem overly reductionist but I think we consistently approach this issue from a default assumption and have for over a century.

 

[Drakkith]

My essential point, we can't in fact refer according to essentially all theories of the early universe, the superdense state which existed immediately preceding the big bang in regards to time. They are predicated on spacetime being a result of the big bang what came before is simply inobservable if it had a duration it couldn't be described using any of the forms of measurement we use in science. However physics is awash in terminology which in fact implies it was the beginning not an intermediate state.

Yes, most terminology refers to this period as the "beginning" of the universe. That's because, if the universe existed prior to this time, it was either in another state wildly different from what we're familiar with or it went through a similar, but transient, state on its transition from a previous time to the current time. It would be the beginning of the universe as we know it. If it didn't exist prior to this time then it was a true beginning.

In any case, both this possible transient state and the state prior to that would be, as you mentioned, not observable at this time. All we know is that we can look back and see a beginning to the universe as we know it. Whether this is a true beginning or merely one part of an endless cycle is unknown at this time. Is there something wrong with that being the actual default view? If you've only ever read "pop-sci" articles and books I wouldn't be surprised if you thought that all cosmologists think of the big bang as a true beginning to the universe. But that is not accurate.

When we discuss the end of the universe it mandates the existence of a beginning.

Any end to the universe is at least as speculative as any beginning. We have no idea if there is a true end or not.

It seems vastly more rational to have the default assumption be, it has always existed in one form or another.

Personally I think it entirely irrational to say that either is inherently more rational than the other. Why should the rationality of the laws of the universe be judged based on our limited experiences?

I understand that may seem overly reductionist but I think we consistently approach this issue from a default assumption and have for over a century.

Again, I don't agree. The consensus of mainstream cosmology has changed drastically over the last 100 years. Several notable scientists tried to develop models in which the universe has no beginning, including Einstein and Fred Hoyle. But no such model has been able to explain all of the observations nearly as well as the big bang theory. I'm sorry but the assertion that we've had the same approach for over a century doesn't appear to hold water when I look back at the history of cosmology.

 

[Jacob Mybiz]

Completely agree, although I think that abbreviation of The Observable Universe or Universe as we know it to the Universe is the primary issue. What modern cosmologists think compared to what the prevailing views were when much of this terminology became standard is in my mind a crucial distinction, they are not the same, yet the terminology remains remarkably unchanged.

 

[Buzz Bloom]

When we discuss the end of the universe it mandates the existence of a beginning.

Hi Jacob:

I do not understand why you call the never ending expansion an "end of the universe". If the expansion never ends, then there is no end. In my mind a "big crunch" future would be more like an end, except for the uncertainty of what happens during the Planck period of the crunch.

Regards,
Buzz

 

[Drakkith]

Completely agree, although I think that abbreviation of The Observable Universe or Universe as we know it to the Universe is the primary issue. What modern cosmologists think compared to what the prevailing views were when much of this terminology became standard is in my mind a crucial distinction, they are not the same, yet the terminology remains remarkably unchanged.

Perhaps, but that happens everywhere, not just in cosmology. It's very natural to shorten and abbreviate things and to simplify explanations when talking to people who aren't familiar with your field.

 

[AgentCachat]

Possibly you mis-read or you are taking some license in who you acknowledge as a physicist or cosmologist. Can you provide any references for such statements?

Oh, Alan Guth. Hawking and Penrose. If you want publications and page numbers, It would take more time than I'm willing to spend. I take it these three meet your specifications?

 

[AgentCachat]

It implies that mass per unit of volume (density) increases, yes.

So you are saying density increases, but volume stays the same? d=m/v

 

[Grinkle]

If you want publications and page numbers, It would take more time than I'm willing to spend.

I see. In any case, yes, I am asking for something specific enough that I can check and assess whether I can agree with you.

 

[phinds]

Oh, Alan Guth. Hawking and Penrose. If you want publications and page numbers, It would take more time than I'm willing to spend. I take it these three meet your specifications?

They would if you provided actual citations, but sans citations we have to assume that you have misunderstood them if you think that they said that at one time in the past the entire universe was contained in a dimensionless point. If you can look up even one of them I think you'll see that you have misunderstood them. I HAVE heard reputable physicists say such nonsense in pop-science venues where their contract seems to call for specific dumbing down to the point of being wrong, but they would never say such a thing in an actual physics text.

EDIT: Ah, wait ... if you are saying that they have said that the MATH would imply zero volume if extrapolated backwards, then sure. That why it's called a "singularity" because it's not believed to have any physical reality. And of course even that only works if the universe is finite in extent which we do not know it was. If it is infinite now, then it was infinite then.

 

[Drakkith]

So you are saying density increases, but volume stays the same? d=m/v

My understanding: Divide the universe into unit volumes of space at t=now, let's say 1 cubic light-year each. As we roll back time we will find that the average density of these 1 cubic light-year volumes increases as t approaches t=0. But the volume of each cube remains the same.

 

[phinds]

My understanding: Divide the universe into unit volumes of space at t=now, let's say 1 cubic light-year each. As we roll back time we will find that the average density of these 1 cubic light-year volumes increases as t approaches t=0. But the volume of each cube remains the same.

But only mathematically does the density reach infinity as a limit. That's not believed to be physical. It's the singularity.

 

[Grinkle]

As we roll back time we will find that the average density of these 1 cubic light-year volumes increases as t approaches t=0

And, as far as I understand it, the number of such volumes does not transition from infinite to finite or vice-versa.​

 

[Drakkith]

And, as far as I understand it, the number of such volumes does not transition from infinite to finite or vice-versa.

Sure. If you have an infinite universe then the number of cubes would be infinite as well, and all of them would be increasing in density as you wind the clock back.

 

[Chronos]

We infer the universe was denser in the past based on theory and observational evidence. We have no observational evidence regarding the future and theory without observational support, while entertaining, is not meaningful.

 

[jbriggs444]

We infer the universe was denser in the past based on theory and observational evidence. We have no observational evidence regarding the future and theory without observational support, while entertaining, is not meaningful.

Despite the lack of observational support for tomorrow, we stock our refrigerators today.

 

[AgentCachat]

My understanding: Divide the universe into unit volumes of space at t=now, let's say 1 cubic light-year each. As we roll back time we will find that the average density of these 1 cubic light-year volumes increases as t approaches t=0. But the volume of each cube remains the same.

OK