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

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The discussion explores alternative theories regarding the fate of the universe, particularly in light of its accelerated expansion, which suggests a heat death scenario. While the "Big Crunch" has been largely dismissed, some speculate about the eventual evaporation of all matter, leading to a state reminiscent of the pre-Big Bang universe. The concept of Boltzmann brains arises, suggesting that if the universe is infinite, such entities could theoretically exist, although their probability remains highly debated and is often considered a mathematical curiosity. Participants argue about the implications of infinity on probability, with some asserting that while Boltzmann brains could exist, their formation is not guaranteed. Ultimately, the conversation emphasizes the complexities and uncertainties surrounding cosmological models and the nature of existence.
  • #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.
 
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  • #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?
 
  • #36
I did not follow the full discussion but I would like to mention (one of my favorites) that Big Rip and Quantum fluctuations theories predict that the next big bang will be in 101056 years!
Yeah! ... we got time! ...
 
  • #37
Stavros Kiri said:
I did not follow the full discussion but I would like to mention (one of my favorites) that Big Rip and Quantum fluctuations theories predict that the next big bang will be in 101056 years!

Oh, what a tremendous waste of time. I would really appreciate a theory whith a much shorter renewal time of about a hundred billion (100_000_000_000) years.

May be we should check the possibility of a coordinated explosion of all black holes, if they are not kept blown-up by products of star activity.
 
  • #38
Gerhard Mueller said:
May be we should check the possibility of a coordinated explosion of all black holes, if they are not kept blown-up by products of star activity.

Since they all have different masses and will evaporate at different times, the probability is zero.
 

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