system
Given the following post, respond as if you're commenting on Reddit:
human
What are our best theories about what was before the Big Bang, and what will come after the Heat Death of the Universe?
gpt
There is absolutely no evidence, and there theoretically cannot be, of anything at all from before the big bang.

Likewise, presuming that the universe doesn't start contracting (which evidence so far suggests it won't), then the answer to the "After heat death" part of your question is - nothing.  

TL;DR - we know absolutely nothing about pre-big-bang.  there will be nothing happening after the heat death of the universe.