Phase Space is a collection of 25 SF stories by Stephen Baxter, many thematically linked to his "Manifold" trilogy (Time, Space and Origin) and other novels of cosmic scope.
"The phase space of a system is the set of all conceivable states of that system," says the first page. As with "Manifold" these stories explore possible (and significantly linked) states of Earth and the universe, alternate timelines offering different solutions to Baxter's favourite cosmological question--the Fermi Parado...