Everything in the universe is constantly being stretched and squeezed by gravitational waves, ripples in space-time caused by the movements of massive objects. Now, astronomers may have caught the first hints of the sea of gravitational waves that permeates everything, called the gravitational wave background.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) consortium used a so-called pulsar timing array to attempt to build a sort of map of gravitational waves. Pulsarsare neutron stars that rotate extremely rapidly and regularly, sending out beams of light that act as “ticks” in extraordinarily precise cosmic clocks.
When a gravitational wave passes through the same region of space-time that those beams of light are travelling through, it makes the light appear to take slightly more or less time to reach us, making the “ticks” from a pulsar seem irregular. Pulsar timing arrays require radio telescopes to observe the signals from many pulsars across the sky simultaneously.
“These pulsars are spinning with millisecond periods and we are able to detect changes in the time of arrival [of signals]… at the hundreds of nanosecond level,” said Joe Simon at the University of Colorado Boulder. He presented this work at a virtual meeting of the American Astronomical Society on 11 January.
The NANOGrav researchers analysed data gathered on 45 pulsars over the course of 13 years and found a gravitational wave signal that was identical across multiple pulsars. This strange, low-frequency hum could be the first evidence of the gravitational wave background.
“We are seeing incredibly significant evidence for this signal,” said Simon. “Unfortunately, we do not have definitive evidence of the cross-correlated pattern that is indicative of the gravitational wave background and only the gravitational wave background.” To prove that this signal is from the gravitational wave background, we would need to see a distinctive pattern in the gravitational waves affecting each pulsar.
Gathering the additional data necessary to find that pattern should only take about a year, Simon said, although analysing it may take longer. If the signal is in fact the gravitational wave background, it will be a useful tool for understanding the most massive objects in the universe.
Black holes merging
“This will tell us more about black holes in the universe, and especially the supermassive black holes in galactic centres,” says Nelson Christensen at the Observatory of Nice in France. “This NANOGrav signal is likely from [black hole] binaries with billions of solar masses.” As these enormous pairs of black holes merge, they emit thrums of gravitational waves powerful enough to persist throughout space-time.
This effort will build a bridge between the gravitational waves we’ve already spotted coming from smaller black holes with the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors, and those from supermassive black holes, Christensen says. Such a bridge will help us understand how different types of black holes form, how galaxies evolve with the black holes within them, and maybe even the larger mysterious forces at work in our universe like dark matter and dark energy.