Doesn't Voldemort have an interest in not allowing the heat death of the Universe to happen?

Heat death is a long ways off, and in the long run, it's extremely unlikely that Harry is unique enough to play a crucial role in stopping it. Heck, Voldemort himself could read up on all the science Harry currently knows, a task that would take him at most eleven years, mostly like less (after all, Harry did it, and at a much younger age, too), and be at least every bit as well-suited as Harry is right now at stopping heat death, and probably much more well-suited seeing as he knows more magic. (Plus, eleven years on a cosmic scale is less than the blink of an eye.) I don't think Voldemort would consider the (very probably minimal) value Harry has to offer in preventing the heat death of the universe enough positive utility to outweigh the fact that a prophecy said he would tear apart the very stars in heaven.

That is not to say, however, that this entire line of thought doesn't work. If Harry has something to offer Voldemort that he would actually value highly enough to consider letting him live, then we're in business. Off the top of my head I'm not thinking of anything, but it's a definite possibility. (Intelligence enhancement, perhaps?) Still, I don't think the "help stop heat death" offer is going to be the winning method.