Today, physicists across the world celebrated as telescopes and observatories on Earth and in space captured a "kilonova." Two neutron stars collided 130 million light years away, sending gravitational waves, x-rays, gamma-rays, radio waves, and light waves to the Earth. But these events also serve as a new kind of tool -- a tool with the potential to answer one of the most fundamental questions in our universe: How quickly is it expanding?