On August 8, 2017, LIGO/Virgo detected the merging of two neutron stars 130 million light years away. Just 1.7 seconds later, the Fermi Gamma Ray Space Telescope received an optical signal—a short gamma ray burst (GRB). Thus began a new era of “multimessenger astronomy.” The GRBs are very energetic explosions observed in galaxies. The neutron star merger offers the first evidence that heavy metals such as gold, platinum, and uranium were created by the collision of neutron stars in a “kilonova.” The resulting gravitational waves offer a new way of measuring the Hubble constant, which determines the rate of expansion of the universe. An important result from the neutron star merger is an extremely accurate determination of the speed of gravitational waves; they move at the speed of light. This has significant ramifications for gravitational theory. It falsifies many proposed modified gravity models.
Probing the internal composition of neutron stars with gravitational waves