Banner image: Pulsar emitting a stream of energy. (Credit: NASA / HST / ASU / J. Hester et al.)
Scientists have used a galaxy-sized space observatory to find possible hints of a unique signal from gravitational waves or powerful waves that move in the universe and deform the very fabric of space and time.
The new discoveries that appeared recently in Astrophysical Journal Letters, a welcome from an American and Canadian project called the North American Gravitational Wave Observatory Nanohertz (NANOGrav).
For more than 13 years, NANOGrav researchers have studied the flow of light from dozens of pulsars distributed in the Milky Way galaxy to try to find the “gravitational background of the wave.”
“We found a strong signal in our data set,” said Simon, a doctoral researcher in the Department of Astrophysics and Planetary Sciences. “But we still can’t say that’s the background of the gravitational wave.”
In 2017, scientists in an experiment called the Observatory for Gravitational Wave Laser Interferometers (LIGO) won the Nobel Prize in Physics for the first direct detection of gravitational waves. These waves were created when two black holes collided about 130 million light-years from Earth, generating a cosmic shock that spread to our own solar system.
This event was the equivalent of a cymbal crash – a loud and short-lived explosion. The gravitational waves that Simon and his colleagues are looking for, in contrast, are more like the constant hum of a crowded cocktail.
Discovering this background noise would be a great scientific achievement and would open a new window on the workings of the universe, he added. These waves, for example, could give scientists new tools to study how supermassive black holes in the centers of many galaxies merge over time.
“These tempting first hints about the gravitational background of the wave suggest that supermassive black holes are likely to merge and that we are struggling in a sea of gravitational waves shaking by supermassive mergers of black holes in galaxies around the universe,” said Julie Commerford, an associate professor. astrophysical and planetary science at CU Boulder and a member of the NANOGrav team.
Simon will present the results of his team at a virtual press conference on Monday at the 237th meeting of the American Astronomical Society.
Through their work on NANOGrav, Simon and Comerford are part of a high stakes, albeit a joint international race to find the background of the gravitational wave. Their project joins two others from Europe and Australia to build a network called the International Pulsar Timing Array.
Simon said that, at least in theory, merging galaxies and other cosmological events lead to a constant flow of gravitational waves. They are humus – a wave, Simon said, could take years or more to cross Earth. For this reason, no other existing experiment can detect them directly.
“Other observatories are looking for gravitational waves of the order of seconds,” Simon said. “We’re looking for waves that are on the order of years or decades.”
He and his colleagues had to be creative. The NANOGrav team uses telescopes on the ground not to look for gravitational waves, but to observe pulsars. These collapsed stars are the headlights of the galaxy. They rotate at incredibly fast speeds, sending streams of radiation that are thrown to Earth in a flashing pattern that remains largely unchanged for eons.
Simon explained that gravitational waves change the stable pattern of light coming from pulsars by pulling or squeezing the relative distance that these rays travel through space. In other words, scientists can detect the background of a gravitational wave simply by observing pulsars for correlated changes in the time they arrive on Earth.
“These pulsars spin as fast as your kitchen mixer,” he said. “And we’re looking at time deviations of only a few hundred nanoseconds.”
To find this subtle signal, the NANOGrav team strives to monitor as many pulsars as possible for as long as possible. To date, the group has observed 45 pulsars for at least three years, and in some cases over a decade.
Hard work seems to pay off. In their latest study, Simon and colleagues reported finding a different signal in their data: Some common processes appear to affect the light coming from many pulsars.
“We walked one by one through each of the pulsars. “I think we were all expecting to find a few who were throwing away our data,” Simon said. “But then we went through all of them and said, ‘Oh my God, there’s actually something here. “
Researchers are still unable to say for sure what causes this signal. They will need to add more pulsars to their data set and monitor them for longer periods to determine if this is actually the background of the gravitational wave of work.
“Being able to detect the background of the gravitational wave will be a huge step, but in fact it is only the first step,” he said. “The second step is to determine the causes of these waves and discover what they can tell us about the universe.”
NANOGrav is the Center for Physical Boundaries of the US National Science Foundation. It was directed by Maura McLaughlin of the University of West Virginia and Xavier Siemens of Oregon State University.