What are the alternative theories for the end of the expanding universe?

[durant35]

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?

 

[phinds]

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.

 

[Chronos]

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.

 

[durant35]

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)?

 

[phinds]

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.

 

[Chronos]

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.

 

[durant35]

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?

 

[Chronos]

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.

 

[Chiclayo guy]

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

 

[durant35]

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?

 

[jbriggs444]

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.

 

[durant35]

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'?

 

[jbriggs444]

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)

 

[durant35]

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?

 

[jbriggs444]

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.

 

[durant35]

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.

 

[durant35]

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?

 

[jbriggs444]

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?

 

[durant35]

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?

 

[jbriggs444]

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".

 

[durant35]

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?

 

[jbriggs444]

So what's your opinion/argument on us being BBs?

Not worth worrying about.

 

[durant35]

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?

 

[jbriggs444]

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.

 

[durant35]

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?

 

[jbriggs444]

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.

 

[durant35]

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?

 

[Gerhard Mueller]

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.

 

[Buzz Bloom]

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

 

[Chronos]

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.