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Suekdccia
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- TL;DR Summary
- Density fluctuations compatible with a homogeneous and isotropic linearly expanding universe?
I would like to ask a question about an interesting paper [1] back from the late 90's
There, the authors propose how the universe may evolve from the near future to extremely far time scales
Near the end of it (Section VI, D.), they discuss entropy and heat death: They indicate that contrary to the classical view of heat death, in Big Bang cosmology the issue is more subtle as the temperature of the universe is continually changing, so a continually expanding universe would not really arrive to thermodynamic equilibrium and thus heat death as a whole (note that this paper was written a year before we discovered the accelerated expansion of the universe, so they mostly consider scenarios with a cosmological linear expansion)
However, if we consider local pockets or regions, the expansion can turn a comoving volume into an adiabatic one, so at that local level entropy would reach a maximum value. So according to this paper, while global heat death would not be attained, local or "cosmological" heat death could occur
Then they consider the case for the different main possible geometries of the universe:
If it's closed it would probably end up in a big crunch, so heat death wouldn't happen
If it's flat density perturbations of larger and larger scales could enter the horizon allowing the production of entropy so heat death would be avoided even at that local level
The last case is an open universe: Here heat death could happen as density fluctuations become "frozen" at a finite length scale (although they give some caveats i.e. that the Bekenstein bound does not directly constrain entropy production in this case, so actually is an open question without definitive conclusions).
Once summarized, I have question about this paper which is the following one:
I think this argument, if you read V.B., is based on suggesting the local/observable density parameter being different from the density parameter on a larger scale. I would say this is not the standard assumption when thinking about questions like this, rather we assume global homogeneity. So how can these authors have overlooked that? How can this argument that the universe wouldn't reach heat death globally (or even locally if it was flat) be compatible with the assumption that the universe is globally homogeneous and isotropic?
[1]: https://arxiv.org/abs/astro-ph/9701131
There, the authors propose how the universe may evolve from the near future to extremely far time scales
Near the end of it (Section VI, D.), they discuss entropy and heat death: They indicate that contrary to the classical view of heat death, in Big Bang cosmology the issue is more subtle as the temperature of the universe is continually changing, so a continually expanding universe would not really arrive to thermodynamic equilibrium and thus heat death as a whole (note that this paper was written a year before we discovered the accelerated expansion of the universe, so they mostly consider scenarios with a cosmological linear expansion)
However, if we consider local pockets or regions, the expansion can turn a comoving volume into an adiabatic one, so at that local level entropy would reach a maximum value. So according to this paper, while global heat death would not be attained, local or "cosmological" heat death could occur
Then they consider the case for the different main possible geometries of the universe:
If it's closed it would probably end up in a big crunch, so heat death wouldn't happen
If it's flat density perturbations of larger and larger scales could enter the horizon allowing the production of entropy so heat death would be avoided even at that local level
The last case is an open universe: Here heat death could happen as density fluctuations become "frozen" at a finite length scale (although they give some caveats i.e. that the Bekenstein bound does not directly constrain entropy production in this case, so actually is an open question without definitive conclusions).
Once summarized, I have question about this paper which is the following one:
I think this argument, if you read V.B., is based on suggesting the local/observable density parameter being different from the density parameter on a larger scale. I would say this is not the standard assumption when thinking about questions like this, rather we assume global homogeneity. So how can these authors have overlooked that? How can this argument that the universe wouldn't reach heat death globally (or even locally if it was flat) be compatible with the assumption that the universe is globally homogeneous and isotropic?
[1]: https://arxiv.org/abs/astro-ph/9701131