Greg Egan The Clockwork Rocket

In Yalda's universe, light has mass, no universal speed, and its creation generates energy; on Yalda's world, plants make food by emitting light into the dark night sky. And time is different: an astronaut might measure decades passing while visiting another star, only to return and find that just weeks have elapsed for her friends. On the farm where she lives, Yalda sees strange meteors that are entering the planetary system at an immense, unprecedented speed - and it soon becomes apparent that more of this ultra-fast material is appearing all the time, putting her world in terrible danger. An entire galaxy is about to collide with their own. There is one hope: a fleet sent straight towards the approaching galaxy, as fast as possible. Though it will feel like weeks back home, on board, millennia will pass before the collision, time enough to raise new generations, and time enough to find a way to stop the ultra-fast material. Either way, they have a chance to save everyone back on the home world.