Btw, why do so many of you appear to be so certain that the heat death of this universe would be the end of everything? Doesn't . . . make it seem quite likely that this is not the only universe that exists?

Yes, but (as we understand it now, and for all practical purposes) this particular universe contains the entirety of what I will experience, what everyone I could ever know will experience, and what I can ever have an effect on. As such, I don't much care about what happens in other universes.