In physical cosmology, the content of the Universe is modeled by the stress-energy-momentum tensor of perfect fluid, with energy density $\rho(t)$ and pressure $P(t)$. I'm wondering, why not use ideal gas instead？
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1$\begingroup$ I suppose the difference would be that an ideal gas exhibits heat conduction and viscosity, while a perfect fluid does not. Most cosmological models (or at least the simpler ones that were developed first historically) are isotropic and homogeneous, which means that even if your matter was capable of heat conduction and viscosity, they wouldn't occur. E.g., no temperature gradients, so no heat conduction. $\endgroup$– user4552Commented Sep 24, 2017 at 3:08
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2 Answers
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I believe that the spin of particles, which Poplawski associates with torsion (in such papers as 2010's "Cosmology with torsion", 2011's "Big bounce from spin and torsion", and--in collaboration with Desai--2015's "Non-parametric reconstruction of an inflaton potential"), might be more plausibly incorporated into cosmology with a fluid model. His cosmology, based on the formation of local universes (one of which would include our observable region) through one or more bounces of mass after the gravitational collapse of any rotating star into a black hole, has the interesting effect of approximately balancing expansion against contraction, which could exempt it from the Borde-Guth-Vilenkin Theorem that would otherwise limit the eternality of the resulting multiverse to the future, rather than permitting a multiverse eternal both to the future and to the past. Although dependent on the Einstein-Cartan version of General Relativity and not requiring an inflaton field, it is considered, in a currently-unchallenged section of the Wikipedia article "Inflation (cosmology)", to be a model of inflationary cosmology.
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I imagine that for cold dark matter and the currently rather cold matter and negligible radiation content of the universe then P=0 would be appropriate - the "dust" model. Probably this wouldn't work for earlier phases in history, when temperature and pressure must have been very high, both for matter, radiation and dark matter. Having said that I can't quantify this - maybe P=0 is good right back to end of inflation.
I assume of course that you are using a cosmological constant to represent "dark energy" - if not, you will need to add stress-energy whose equation of state is unknown. (That for the inflationary period is of course also unknown.)
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$\begingroup$ @akrasia, can you explain your answer a bit more? Why wouldn't this work for "earlier phases in history"? $\endgroup$ Commented Aug 11, 2014 at 18:21
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$\begingroup$ This seems wrong to me. P=0 doesn't mean we have something other than an ideal gas. It simply means that the motion of the particles is nonrelativistic, so that in relativistic units, with c=1, the pressure is negligible compared to the density. $\endgroup$– user4552Commented Sep 24, 2017 at 3:04