Couldn't find the exacts but..
Ok.
Let's say that humans have survived to almost the end of the universe. They face sure destruction, so they use their time travel technology to move back to near the beginning to time. Assuming they do the same thing again, then the minute the first refugee went back, there would be an trillions and trillions of them consuming all the resources in the universe simultaneously.
Because they would do this over and over again, and the past versions of them would also be doing the same thing, so you end up with millions and millions of the same civilization all using resources and taking over the same universe.
“She'd become a governess. It was one of the few jobs a known lady could do. And she'd taken to it well. She'd sworn that if she did indeed ever find herself dancing on rooftops with chimney sweeps she'd beat herself to death with her own umbrella.”
― Hogfather