Timelike Infinity — Stephen Baxter

WAR BETWEEN THE AGES
Once mankind reached glorious heights, even immortality. Now it is oppressed by the Qax--until the loose end of a time tunnel, long since set adrift, whips back through the solar system.

In one desperate leap, a band of rebels escapes into the past in a makeshift craft. Enraged, the Qax and their great, sentient ships, the Spline, set off in pursuit.

While the rebels find allies living fifteen hundred years in the past and the Qax plot the destruction of the fugitives, a diplomat from Earth must make a fatal choice.... In the light of Jupiter, in a vast spasm of violence, the battle for Earth's future has begun. And to the victor goes Time itself....