Should the universe be modeled by perfect fluid or ideal gas？ 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？
 A: 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.  
A: 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.)
