Ring
From back cover Harper paperback June 1996:

Michael Poole's *wormholes* constructed in the orbit of Jupiter had opened the galaxy to humankind.  Then Poole tried looping a wormhole back on itself, tying a knot in space and ripping a hole in time.

It worked.  Too well.

Poole was never seen again.  Then from far in the future, from a time so distant that the stars themselves were dying embers, came an urgent SOS -- and a promise.  The universe was doomed, but humankind was not.  Poole had stumbled upon an immense artifact, light-years across, fabricated from the very *string* of the cosmos.

The Universe had a door.  And it was open....
Time travel, Fiction, Fiction, science fiction, general
Stephen Baxter