The Expansion of the Universe

If you write down the equations from relativity, then what you see is that space time is a substance and it's stretching as the universe expands. And so gravitational waves, ripples in space time that predicted by Einstein's theory are pretty hard to spot. But if we're hoping we might get a detection next year, it will probably be because two giant black holes in a galaxy far, far away collided. That was just enough to cause a ripple that we could just about detect.