I reckon the sun running out of fuel is pretty obvious, but the end or not end of the universe is mostly speculation, it relies mostly on what dark energy is, and how it behaves in the future, which they don't know anything about. They can't see it, they only assume it must be there because the universe appears to be expanding faster, based on the red shift from supernovas, but that's a very far line to draw from that, to there's this percentage of dark energy in the universe, and it has this amount of push opposing gravity. I've seen them try to do math with it, it's stupid, they don't know what it is. They don't know whether it will be sucked into black holes, and whether it has it's own gravity, if it's subatomic particles, or all these crazy properties they pretend it might has, all based on pretty much nothing at all. I reckon it is like all other energy, and does have gravity, and will be sucked into black holes, adding to their gravity, while removing all propulsion, which will happen in particular when all the stars burn out, and the large ones turn into black holes.
Again, this is my own speculation, but I reckon an endless chain of causality is required for the big bang to have happened, and that wasn't the beginning, as there could be no beginning, and no end. Einstein was silenced on his cycling universe paper, which wasn't published in English until five years ago. I've been saying this stuff for ten years, it's what I first started writing about when I first got online. I'm not a physicist, my view is physicists are mostly talking nonsense, and the whole thing has been corrupted and manipulated by religion. They sent death threats to Einstein, they changed his last words to God doesn't play dice, and the universe must have had a beginning, even though he wrote long books saying the opposite. He was kind of Buddhist, which I am too, because it makes the most sense, compared to nonsensical, unscientific garbage, which the majority of scientists of his time believed in, fundamentally.