Thermodynamics tells us that systems will seek to equilibrate. Heat flows from high temperature to low temperature. Stars burn out. It has nothing to do with whether there was a big bang, or whether or not the universe if flying apart.
Is there a source of energy being pumped into the universe to keep it from equilibrating eventually to a very cold temperature throughout?
If the universe is going towards a very cold equilibrium, did it not then have a beginning, whether big bang or some other beginning?
That’s a good question and I do not have an answer to it offhand; I’ll try to find one.