Unlike black holes, merging neutron stars produce both gravitational and electromagnetic waves, such as X-rays, radio waves and visible light. While gravitational waves can measure the distance between the neutron-star merger and Earth, electromagnetic waves can measure how fast its whole galaxy is moving away from Earth. This creates a new way to measure the Hubble constant. However, even with the help of gravitational waves, it's still tricky to measure the distance to neutron-star mergers?"that's, in part, why current gravitational-wave based measurements of the Hubble constant have an uncertainty of ~16%, much larger than existing measurements using other traditional techniques.