Possible Endings for the Expanding Universe: Exploring Alternative Theories

In summary: I think that's a fair point. Probability is a tool that can be used to form hypotheses and test them, but it cannot be used to determine certainty.
  • #1
durant35
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Are there any plausible models based on current evidence about the universe which ends in some way and doesn't last forever?

I am aware that the accelerated expansion leads to eventual heat death for eternity, but are there any other plausible ideas what can happen?
 
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  • #2
It doesn't seem likely since that would contradict the currently understood cosmological model, which as you state is accelerated expansion forever. The "Big Crunch" scenario has been discarded as a viable model.
 
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  • #3
Based on a few unproven [and likely unprovable] assumptions - all matter in the universe may eventually evaporate [don't hold your breathe, it will take an incredibly long time]. The universe will then resemble the state that existed prior to the big bang and could fluctuate itself back into existence.
 
  • #4
Chronos said:
Based on a few unproven [and likely unprovable] assumptions - all matter in the universe may eventually evaporate [don't hold your breathe, it will take an incredibly long time]. The universe will then resemble the state that existed prior to the big bang and could fluctuate itself back into existence.

Interesting.

Wouldn't that open the prospect of Boltzmann brains (when the universe resembles the pre-big bang state)?
 
  • #5
Chronos said:
Based on a few unproven [and likely unprovable] assumptions - all matter in the universe may eventually evaporate [don't hold your breathe, it will take an incredibly long time]. The universe will then resemble the state that existed prior to the big bang and could fluctuate itself back into existence.
Wouldn't it be more appropriate to say "and could then change state ... " ? I mean, it would BE in existence, it just wouldn't have anything much IN it.
 
  • #6
Observational evidence does not support the case for Boltzmann brains as anything more than a mathematical curiosity. In theory, a tossed die has a non zero probability of coming to rest perfectly balanced on the apex of one corner, but, you could toss dice continuously for billions of years and never observe such an outcome.
 
  • #7
Chronos said:
Observational evidence does not support the case for Boltzmann brains as anything more than a mathematical curiosity. In theory, a tossed die has a non zero probability of coming to rest perfectly balanced on the apex of one corner, but, you could toss dice continuously for billions of years and never observe such an outcome.

I agree with you and your example is quite good.

The only problem is the concept of infinity, since BB-s and similar low likelihood events (like tossing a coin million times heads in a row) will eventually come to existence just because of infinity. Or perhaps I am wrong?
 
  • #8
Division by zero [or multiplication by infinity] turns probabilities into gibberish. We routinely renormalize predictive models to fit observed outcomes. There is a non zero probability you could quantum tunnel right through the walls of a bank vault, but, you might plausibly attribute the crime to a Boltzmann brain. While all things possible can happen, not all things that can happen do happen - nor are they compelled to do so.
 
  • #9
Chronos said:
Division by zero [or multiplication by infinity] turns probabilities into gibberish. We routinely renormalize predictive models to fit observed outcomes. There is a non zero probability you could quantum tunnel right through the walls of a bank vault, but, you might plausibly attribute the crime to a Boltzmann brain. While all things possible can happen, not all things that can happen do happen - nor are they compelled to do so.

Well...there´s Murphy´s law:smile:
 
  • #10
Chronos said:
Division by zero [or multiplication by infinity] turns probabilities into gibberish. We routinely renormalize predictive models to fit observed outcomes. There is a non zero probability you could quantum tunnel right through the walls of a bank vault, but, you might plausibly attribute the crime to a Boltzmann brain. While all things possible can happen, not all things that can happen do happen - nor are they compelled to do so.

So it is plausible to assume that a Boltzmann brain will indeed never happen nowhere and we may dismiss its probability?
 
  • #11
durant35 said:
So it is plausible to assume that a Boltzmann brain will indeed never happen nowhere and we may dismiss its probability?
If the universe is infinite [and the parts are suitably independent] and is roughly the same everywhere then it is almost certain that a Boltzmann brain will happen somewhere. The "almost certain" part is because the fact that a thing can happen does not make it a logical certainty that it will happen. It only makes it a statistical certainty.
 
  • #12
jbriggs444 said:
If the universe is infinite [and the parts are suitably independent] and is roughly the same everywhere then it is almost certain that a Boltzmann brain will happen somewhere. The "almost certain" part is because the fact that a thing can happen does not make it a logical certainty that it will happen. It only makes it a statistical certainty.

Isn't it the case, as Chronos mentioned, that using probability in combination with infinity makes no sense? So it makes no sense to say 'anything that can happen eventually will happen'?
 
  • #13
durant35 said:
Isn't it the case, as Chronos mentioned, that using probability in combination with infinity makes no sense? So it makes no sense to say 'anything that can happen eventually will happen'?
Probability makes sense even with infinite sets. There is a formal way of assigning a probability measure to subsets of infinite sets. For instance, https://en.wikipedia.org/wiki/Measure_(mathematics)
 
  • #14
jbriggs444 said:
Probability makes sense even with infinite sets. There is a formal way of assigning a probability measure to subsets of infinite sets. For instance, https://en.wikipedia.org/wiki/Measure_(mathematics)

So in that case, you are implying that there are infinite Boltzmann brains in the far future (since the universe is infinite in time). What would you suggest to get rid of the paradox/absurdity?
 
  • #15
durant35 said:
So in that case, you are implying that there are infinite Boltzmann brains in the far future (since the universe is infinite in time). What would you suggest to get rid of the paradox/absurdity?
While the universe may be infinite in time, it is not uniform in time. There is no implication that if you wait long enough that a Boltzmann brain will form.

By contrast, the universe does seem to be uniform in space.

The intended meaning was that if you consider a universe with infinite spatial extent [with suitable caveats about uniformity and independence], a Boltzmann brain is nearly certain to already exist somewhere. Of course, that somewhere is hideously unlikely to be within our observable universe, since the observable universe is decidedly finite in spatial extent.
 
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  • #16
jbriggs444 said:
While the universe may be infinite in time, it is not uniform in time. There is no implication that if you want long enough that a Boltzmann brain will form.

By contrast, the universe does seem to be uniform in space.

The intended meaning was that if you consider a universe with infinite spatial extent [with suitable caveats about uniformity and independence], a Boltzmann brain is nearly certain to already exist somewhere. Of course, that somewhere is hideously unlikely to be within our observable universe, since the observable universe is decidedly finite in spatial extent.

Ah, my bad, now I understand.

Thank you. :biggrin:
 
  • #17
jbriggs444 said:
While the universe may be infinite in time, it is not uniform in time. There is no implication that if you wait long enough that a Boltzmann brain will form.

By contrast, the universe does seem to be uniform in space.

The intended meaning was that if you consider a universe with infinite spatial extent [with suitable caveats about uniformity and independence], a Boltzmann brain is nearly certain to already exist somewhere. Of course, that somewhere is hideously unlikely to be within our observable universe, since the observable universe is decidedly finite in spatial extent.

But when you say already, what do you mean?
Each region of space may wait forever and still not yield a BB or another unlikely event if the universe is uniform in time. How is not being uniform in time compatible with being uniform in space in this context?
 
  • #18
durant35 said:
But when you say already, what do you mean?
By "already" I mean "at a time coordinate earlier than now". To make that meaningful, I am assuming co-moving coordinates.

How is not being uniform in time compatible with being uniform in space in this context?
We actually observe a universe which is not uniform in time but which is uniform in space. What's the problem?
 
  • #19
jbriggs444 said:
We actually observe a universe which is not uniform in time but which is uniform in space. What's the problem?

But wouldn't that mean that at every moment there are BB-s that are brought into existence simply because at every moment the space is infinite. So going forward in time would mean that actually the number of BBs increases?
 
  • #20
durant35 said:
But wouldn't that mean that at every moment there are BB-s that are brought into existence simply because at every moment the space is infinite. So going forward in time would mean that actually the number of BBs increases?
No.

Yes, with probability one, under the assumptions of a spatially infinite and isotropic universe, the rate at which BB's come into existence is infinite. But the rate at which they go *poof* is also infinite. One might expect the number to decrease rather than increase.

But the number in existence would be infinite and simply saying "number of BB's decreases" would not be sensible. Instead, one could use a term like "asymptotic density decreases".
 
  • #21
jbriggs444 said:
No.

Yes, with probability one, under the assumptions of a spatially infinite and isotropic universe, the rate at which BB's come into existence is infinite. But the rate at which they go *poof* is also infinite. One might expect the number to decrease rather than increase.

But the number in existence would be infinite and simply saying "number of BB's decreases" would not be sensible. Instead, one could use a term like "asymptotic density decreases".

So what's your opinion/argument on us being BBs?
 
  • #22
durant35 said:
So what's your opinion/argument on us being BBs?
Not worth worrying about.
 
  • #23
jbriggs444 said:
Not worth worrying about.

I am suffering from a kind of existential anxiety because of the concept, so if it's not a problem can you tell me why it is not worth worrying about?
 
  • #24
durant35 said:
I am suffering from a kind of existential anxiety because of the concept, so if it's not a problem can you tell me why it is not worth worrying about?
Part of it is practical. If true, then memory, evidence and reason are all untrustworthy and pointless. There is no meaningful future or past. Everything is pointless, so there is no point worrying about it. If false then it's false and again, there's no point worrying about it.

Part of it is the evidence. If you were a BB, the highest likelihood would be a hodge podge of jumbled memories that make no coherent sense. You might recall being a young elven girl prancing through flowers in the forests of Lothlorien and also recall being a wolf, leader of your pack coughing out your life's blood with a hunter's bullet in your heart. But, by and large, your memories are coherent. This makes it unlikely that you are a BB.
 
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  • #25
jbriggs444 said:
Part of it is practical. If true, then memory, evidence and reason are all untrustworthy and pointless. There is no meaningful future or past. Everything is pointless, so there is no point worrying about it. If false then it's false and again, there's no point worrying about it.

Part of it is the evidence. If you were a BB, the highest likelihood would be a hodge podge of jumbled memories that make no coherent sense. You might recall being a young elven girl prancing through flowers in the forests of Lothlorien and also recall being a wolf, leader of your pack coughing out your life's blood with a hunter's bullet in your heart. But, by and large, your memories are coherent. This makes it unlikely that you are a BB.
But if the universe is not uniform in time, why do most cosmologists worry that most BBs will exist in the far future, since it's not a guarantee?
 
  • #26
durant35 said:
But if the universe is not uniform in time, why do most cosmologists worry that most BBs will exist in the far future, since it's not a guarantee?
I do not want to argue for "most cosmologists". But one usefulness of the concept is as a counter-argument against certain cosmological models.

It is ludicrous to suppose that we are BB's. Anyone who goes to www.physicsforums.com and expects to see physics postings instead of white noise already realizes this on some level. Similarly, if one opens the fridge and expects to find food, steps into a car and expects the steering wheel to work, talks to other people, brushes ones teeth in the morning, puts on clothes before stepping out the front door, etc, etc).

Accordingly, any model in which BB's outnumber real brains has a problem to explain.
 
  • #27
jbriggs444 said:
I do not want to argue for "most cosmologists". But one usefulness of the concept is as a counter-argument against certain cosmological models.

It is ludicrous to suppose that we are BB's. Anyone who goes to www.physicsforums.com and expects to see physics postings instead of white noise already realizes this on some level. Similarly, if one opens the fridge and expects to find food, steps into a car and expects the steering wheel to work, talks to other people, brushes ones teeth in the morning, puts on clothes before stepping out the front door, etc, etc).

Accordingly, any model in which BB's outnumber real brains has a problem to explain.
I agree with that.

But how do cosmologists/or anybody else estimate the number of ordinary observer, or real brains as you said?
 
  • #28
Hi,
this discussion about Boltzmann Brains, which are only a statistical concept, has nothing to do with the existence of life forms in a late universe. The only possibility allowing life after all stars have been burned out is a renewal mechanism like a big bang or something else with the effect to provide new hydrogen to the universe.
 
  • #29
Chronos said:
Based on a few unproven [and likely unprovable] assumptions - all matter in the universe may eventually evaporate [don't hold your breathe, it will take an incredibly long time]. The universe will then resemble the state that existed prior to the big bang and could fluctuate itself back into existence.
Hi @Chronos:
I am confused about several parts of the quote above.

I am confused by the "Based . . . assumptions" part. Isn't this "heat death" result based on two concepts that are both generally accepted by most physicists as real physical phenomena rather than just theoretical conjectures.
1. Any collection of bound gravitation bodies which are far enough away from other bodies such that the gravitational influence of these other bodies is insignificant on the collection of bound bodies will ultimately all collapse into a since black hole. This is because all gravitational potential energy resulting from the separation of the bodies from each other is gradually radiated away as gravitational waves.
2. A black hole will eventually evaporate due to Hawking radiation.​

I am also confused by "The universe will then resemble the state that existed prior to the big bang" in the second sentence, although it may be due to just an ambiguity regarding "resemble".
Won't the temperature of the universe when "all matter has evaporated" be infinitesimally small, which of course is much different from the extremely hot condition at the big bang.​

I am also confused by "the state that existed prior to the big bang".
I understood that with respect to a basic concept of current standard model of cosmology there is no state prior to the big bang.​

Regards,
Buzz
 
  • #30
Assuming all matter eventually evaporates [via Hawking radiation by black holes], the universe would then be empty of matter [a Milne universe],l which would [presumably] resemble the state of the universe prior to the big bang. A universe devoid of matter would have no definable 'arrow' of time by any current definition. I freely admit that is a rather simplistic and naïve perspective.
 
  • #31
Chronos said:
Assuming all matter eventually evaporates [via Hawking radiation by black holes], the universe would then be empty of matter [a Milne universe]

This doesn't seem right. The universe after all matter has evaporated via Hawking radiation is not empty of stress-energy; the stress-energy is just in the form of radiation instead of matter. In the Milne universe, there is zero stress-energy (since the Milne universe is just Minkowski spacetime in a different coordinate chart).

Chronos said:
which would [presumably] resemble the state of the universe prior to the big bang

I assume by this you mean the state of the universe prior to inflation starting? Or are you assuming some other model? If you mean the state prior to inflation starting, that's not the same as the Milne universe either (or the same as a universe filled with Hawking radiation); at least, not as I understand the current contenders for inflationary models.
 
  • #32
Why shall the future be a universe without the chance of life, though we live in a universe, which supports the existence of life? This kind of view is really depressive.
 
  • #33
Gerhard Mueller said:
Why shall the future be a universe without the chance of life, though we live in a universe, which supports the existence of life? This kind of view is really depressive.
Science evaluates things based on what experiment reveals rather than on what one would like to be true.
 
  • #34
jbriggs444 said:
Science evaluates things based on what experiment reveals rather than on what one would like to be true.
The experiment you are talking about is "nature"! And "nature" supports life. There is no experimental hint that this will change.
 
  • #35
Gerhard Mueller said:
Why shall the future be a universe without the chance of life, though we live in a universe, which supports the existence of life? This kind of view is really depressive.
How shall the universe create and preserve life in the far future if there is nothing else than cold photon gas?
 
<h2>1. What is the expanding universe theory?</h2><p>The expanding universe theory, also known as the Big Bang theory, is a scientific explanation for the origin and evolution of the universe. It states that the universe began as a singularity and has been expanding and cooling ever since.</p><h2>2. What are some alternative theories to the expanding universe?</h2><p>Some alternative theories to the expanding universe include the Steady State theory, which proposes that the universe has always existed and is continuously creating new matter, and the Oscillating universe theory, which suggests that the universe goes through cycles of expansion and contraction.</p><h2>3. How do these alternative theories explain the expansion of the universe?</h2><p>The Steady State theory explains the expansion of the universe by proposing that new matter is continuously being created to fill in the gaps left by the expansion. The Oscillating universe theory explains the expansion as a result of a previous contraction, and predicts that the universe will eventually collapse back in on itself.</p><h2>4. What evidence supports the expanding universe theory?</h2><p>One major piece of evidence for the expanding universe theory is the observation of the cosmic microwave background radiation, which is a remnant of the intense heat from the Big Bang. Additionally, the redshift of distant galaxies and the abundance of light elements in the universe also support the theory.</p><h2>5. What are the implications of the expanding universe theory for the future of our universe?</h2><p>If the expanding universe theory is correct, it suggests that the universe will continue to expand and cool, eventually leading to a state of maximum entropy and the end of all life. However, the exact fate of the universe is still unknown and may depend on the validity of alternative theories.</p>

1. What is the expanding universe theory?

The expanding universe theory, also known as the Big Bang theory, is a scientific explanation for the origin and evolution of the universe. It states that the universe began as a singularity and has been expanding and cooling ever since.

2. What are some alternative theories to the expanding universe?

Some alternative theories to the expanding universe include the Steady State theory, which proposes that the universe has always existed and is continuously creating new matter, and the Oscillating universe theory, which suggests that the universe goes through cycles of expansion and contraction.

3. How do these alternative theories explain the expansion of the universe?

The Steady State theory explains the expansion of the universe by proposing that new matter is continuously being created to fill in the gaps left by the expansion. The Oscillating universe theory explains the expansion as a result of a previous contraction, and predicts that the universe will eventually collapse back in on itself.

4. What evidence supports the expanding universe theory?

One major piece of evidence for the expanding universe theory is the observation of the cosmic microwave background radiation, which is a remnant of the intense heat from the Big Bang. Additionally, the redshift of distant galaxies and the abundance of light elements in the universe also support the theory.

5. What are the implications of the expanding universe theory for the future of our universe?

If the expanding universe theory is correct, it suggests that the universe will continue to expand and cool, eventually leading to a state of maximum entropy and the end of all life. However, the exact fate of the universe is still unknown and may depend on the validity of alternative theories.

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