#43 Ringworld by Larry Niven

I thought I had read this many years ago but turns out I was wrong. Two humans and two aliens go to the core of the universe to investigate a bright spot at the request of one of the aliens governments. The pay being an advanced space drive that will allows humans and the other alien race to be able to escape the explosion at the center of the universe.  It turns out that that the bright spot is a large ring around a sun that is inhabited by humans that have lost their tech.  There are several more books set in this universe and I am pretty sure I read one of them a long time ago so I think at some point I will be reading the rest of them.