Atomic Russian Dolls (by Bruce Shirley)

The Scientist was about to release the power of the atom, but it didn't matter, because in a couple of days everybody would be dead. The galaxy was being destroyed by asteroids, debris of an unimaginably-large, galactic catastrophe that would soon split the sun.

He flicked a red switch, releasing the neutron bombardment. A couple of days, nothing to a tree, yet a lifetime for a mayfly and generations for bacteria. The first alpha track appeared indicating he had split an atom. Ten years ago he would have felt like a god. Now he just wondered if there was a god, and whether it cared. Was the human race of any significance to the might of a being that ruled the universe.

A being was watching them and had watched for thousands of years. A creature so large, size meant nothing, and movement was measured in the lifetimes of stars. Despite being on the point of splitting the atom, the creature was unhappy, because he and his galaxy were soon to be destroyed by ...