A ‘low pitch hum’ of gravitational waves flowing through the milky way was discovered for the first time.
North American Nanohertz Observatory for Gravitational Waves (NANOGrav) detected the gravitational background (GWB) using observational data from an array of pulsars. Â Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo collaborations had detected gravitational waves before.
Albert Einstein predicted the gravitational waves for the first time in 1916.
Though the cause is not known yet, according to Stephen Taylor, a gravitational wave astrophysicist at Vanderbilt University, Tennessee, who co-led the research, the detected signal is consistent with the theoretical expectations of gravitational waves emerging from the most massive black holes in the universe weighing as much as billions of suns.
The Discovery of GWB
Astronomers studied pulsars- rotating neutron stars that emit pulses of radiation- to discover the gravitational wave background.
Factors such as dust and gas in the interstellar medium, etc. effects timing of pulses. Gravitational waves stretch and compress the space-time fabric between earth and pulsars, leading to arrival of light flashes earlier or later than normal.
Scientists collected data related to timing of pulses for 15 years. This was done using radio telescopes like Arecibo Observatory (collapsed in 2020) in Puerto Rico, Green Bank Observatory in West Virginia, Karl G. Jansky Very Large Array in New Mexico, and the Canadian Hydrogen Intensity Mapping Experiment (CHIME) in Canada. The difference between pulses’ actual arrival time and predicted arrival time was calculated.
Likeliest Source of GWB
The likeliest source for this gravitational wave background is believed to be pairs of submissive blackholes caught in a death spiral.
During the merging of two galaxies, supermassive blackholes can meet up and orbit one another and later their orbits can get tightened as gas and stars pass between the black holes and steal energy. This stops when blackholes get too close.
In the light of final parsec problem, only rare groups of three or more supermassive blackholes result in mergers.
As they orbit one another, supermassive blackholes can as powerful as gravitational waves until they collide eventually.
Luke Kelley of the University of California, Berkeley, chair of NANOGrav’s astrophysics group, said that nothing can stop two blackholes from merging once they get close enough to be seen by pulsar timing arrays. The existence of gravitational wave background backs up this prediction and potentially puts the final parsec problem to rest.
The team hopes to explore all potential sources of the gravitational wave background.
