Gravitational Waves Could Solve Hubble Crisis
Illinois-Chicago researchers propose using gravitational wave 'standard sirens' from black hole and neutron star mergers to resolve the Hubble tension (67 vs 73 km/s/Mpc disagreement). Building on GW170817 detection, LIGO-Virgo-KAGRA could provide independent distance measurements without relying on traditional cosmic ladder methods.
The current crisis in cosmology stems from a fundamental disagreement on the universe's expansion rate, known as the Hubble constant. Measurements of the early universe, based on the cosmic microwave background radiation from the Planck satellite, indicate an expansion rate of about 67 km/s/Mpc. However, measurements of the local, "late" universe using a "cosmic distance ladder" of stars and supernovae consistently yield a higher value of around 73 km/s/Mpc. This "cosmic distance ladder" relies on a series of steps to measure ever-greater distances. Astronomers use techniques like parallax for nearby stars and then calibrate "standard candles" such as Cepheid variable stars and Type Ia supernovae to extend their measurements deeper into the cosmos. Any uncertainties in the lower rungs of the ladder can propagate upwards, leading to potential systematic errors. Gravitational waves from colliding compact objects like neutron stars and black holes offer a completely independent way to measure cosmic distances, bypassing the cosmic distance ladder entirely. The waveform of the gravitational waves itself directly encodes the distance to the event, a method known as "standard sirens." The first "standard siren" with an electromagnetic counterpart, a binary neutron star merger named GW170817, was detected on August 17, 2017. By identifying the host galaxy, NGC 4993, astronomers could measure its redshift and, combined with the distance from the gravitational waves, calculate a value for the Hubble constant. The current network of detectors—LIGO in the U.S., Virgo in Italy, and KAGRA in Japan—is in its fourth observing run (O4), which began in May 2023 and has already identified hundreds of new gravitational wave candidates. These include "dark sirens," typically black hole mergers without a light counterpart, which are analyzed statistically with galaxy catalogs to constrain the Hubble constant. A new technique, the "stochastic siren" method, uses the faint background hum from a multitude of distant, unresolved black hole mergers. The strength of this background signal is related to the expansion rate, providing another novel way to probe the Hubble tension. Future ground-based observatories like the Einstein Telescope and Cosmic Explorer, with arms 10 times longer than current detectors, will be operational in the 2030s. These next-generation facilities will detect millions of gravitational wave events annually, providing enough data to potentially resolve the Hubble tension with high precision. The European Space Agency's Laser Interferometer Space Antenna (LISA) mission, scheduled for launch in the mid-2030s, will be a space-based observatory with 2.5 million-kilometer-long arms. LISA will detect low-frequency gravitational waves from sources like supermassive black hole mergers, opening an entirely new window to study the universe's expansion and composition.
