Scientists may have found a way of using the collisions of black holes to measure the rate at which the universe is expanding. The violent mergers of black holes launch ripples in space-time called gravitational waves, and a new technique measures changes in these signals that occur as they experience the universe’s expansion. Scientists hope to use cosmic collisions between tight binary black hole pairings as what the team term ‘spectral sirens’ to provide an alternative measurement technique for the Hubble constant. Finally settling this pressing cosmological concern could reveal how the universe has evolved and how it looked in its early years. At the heart of the spectral siren method are gravitational waves.
On Earth, incredibly sensitive laser interferometers like the Laser Interferometer Gravitational-Wave Observatory (LIGO), the Italian Virgo and Japan’s Kamioka Gravitational Wave Detector (KAGRA) can measure these faint gravitationalwave signals. Since the first detection of gravitational waves in September 2015, LIGO and its partner instruments have collected data from around 100 distant mergers. Each detection gives scientists a hint at the size of the black holes involved in the merger. For instance, that first gravitational-wave detection stemmed from the collision of two black holes, each containing roughly 30 times the mass of the Sun.