Big Freeze dilemma Let's say that the density of the universe is equal to the critical density. In this scenario it is normally said that the universe will freeze up due to entropy. Is it possible that after after billions and billions and billions(and even more) years all black holes start exploding due to Hawking radiation, therefore re-populating the universe with energy? And if so could this re-population of energy be done in such a way that the density of the universe changes?
For this latter question I think this would be impossible simply due to conservation of energy, but I tend to be wrong. 
 A: Well, again. Yes, it'll repopulate some of the energy, but the universe will keep on accelerating due to dark energy, maybe slow down a bit the acceleration, but keep expanding. 
The radiation density due to normal particle radiation will keep on decreasing as the universe expands, as 1/$a^4$. But it's mostly irrelevant, because the dark energy dominates, and goes as the exponential of time (exp(t)) [so, mathematically you have to have to figure out how those add, but basically the exponential in time dominates], and the normal (not dark) energy density keeps on decreasing. Anyway, before it was normal energy it was the mass of the black holes, and so the total mass-energy is the same, and keeps on going to zero asymptotically due to the exponential expansion. 
The universe keeps expanding and getting emptier and emptier, with black holes disappearing after about $10^{100}$ years. Whether it's a Big Freeze or Heat Death depends on whether the entropy decreases (for the freeze) or increases. Either way there will be no usable energy left, and it's over. See wiki at  https://en.m.wikipedia.org/wiki/Heat_death_of_the_universe
If you are really interested in cosmology get the Dodelson book, it'll tell you and explain the key things, even though now a bit outdated. You might be ok with it even if you have not studied much general relativity first, but you might need some. Then there'll be lots of material online that you can learn from.
