Brace yourselves for gravitational waves

[Alex K]

This by the way is also how the cosmic expansion of the universe is described mathematically. The numbers in the metric tensor simply grow by a time dependent factor called a(t). So "space" expands and distances between otherwise unbound objects increase. In this formulation, it is also clear that space doesn't have to expand into anything. It's just that the rulers change. This is all totally standard, not something I made up.