Ultima — Stephen Baxter

On the planet of Per Ardua, alien artifacts were discovered -- hatches that allowed humans to step across light-years of space as if they were stepping into another room. But this newfound freedom has consequences....

As humanity discovers the real nature of the universe, a terrifying truth comes to light. We all have countless pasts converging in this present -- and our future is terrifyingly finite. There are minds in the universe that are billions of years old and now we are vulnerable to their plans for us....

It's time to fight back and take control.