‘Galaxy-sized’ observatory sees potential gravitational wave clues | CU Boulder today

Banner Image: A pulsar ejecting a plume of energy. (Credit: NASA / HST / ASU / J. Hester et al.)

Scientists used a “galaxy-sized” space observatory to find possible clues to a single signal from gravitational waves, or the powerful ripples that traverse the universe and distort the fabric of space and of time itself.

The new findings, which recently appeared in Letters from the astrophysical journal, come from a US and Canadian project called North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

For more than 13 years, NANOGrav researchers have looked at the light scattered by dozens of pulsars distributed in the Milky Way to try to detect a “gravitational wave background”. This is what scientists call the constant flow of gravitational radiation which, according to theory, spreads across the Earth steadily. The team has yet to identify that target, but it is getting closer than ever, said Joseph Simon, an astrophysicist at the University of Colorado at Boulder and lead author of the new paper.

“We found a strong signal in our data set,” said Simon, postdoctoral researcher in the Department of Astrophysical and Planetary Sciences. “But we can’t yet say that’s the bottom of gravitational waves.”

In 2017, scientists in an experiment called Laser Interferometer Gravitational Wave Observatory (LIGO) won the Nobel Prize in Physics for the very 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 violent, short-lived explosion. The gravitational waves that Simon and his colleagues seek, on the other hand, are more like the constant buzz of conversation over a crowded cocktail party.

Detecting this background noise would be a major scientific achievement, opening a new window into how the universe works, he added. These waves, for example, could give scientists new tools to study how the supermassive black holes at the center of many galaxies merge over time.

“These tantalizing early clues from a gravitational wave background suggest that supermassive black holes are likely merging, and we are floating in a sea of ​​gravitational waves rippling from mergers of supermassive black holes in galaxies across the universe.” said Julie Comerford, associate professor of astrophysical and planetary science at CU Boulder and a member of the NANOGrav team.

Simon will present his team’s results at a virtual press conference Monday at the 237th meeting of the American Astronomical Society.

Galactic lighthouses

Through their work on NANOGrav, Simon and Comerford are part of a high-stakes, albeit collaborative, international race to find the bottom of gravitational waves. Their project joins two others in Europe and Australia to form a network called the International Pulsar Timing Array.

Simon said that, at least according to the theory, the fusion of galaxies and other cosmological events produces a regular churn of gravitational waves. They are gigantic – a single wave, Simon said, can take years or more to pass 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 are looking for waves of the order of several years or decades.”

He and his colleagues had to get creative. The NANOGrav team uses ground-based telescopes not to search for gravitational waves but to observe pulsars. These collapsed stars are the beacons of the galaxy. They rotate at incredibly fast speeds, sending fluxes of radiation back to Earth in a blinking pattern that remains virtually unchanged over eons.

Simon explained that gravitational waves alter the constant pattern of light emanating from pulsars, pulling or compressing the relative distances these rays travel in space. Scientists, in other words, might be able to spot the bottom of the gravitational wave simply by monitoring the pulsars for correlated changes in when they arrive on Earth.

“These pulsars spin about as fast as your kitchen blender,” he says. “And we’re looking at deviations in their timing of a few hundred nanoseconds.”

Something there

To find this subtle signal, the NANOGrav team strives to observe 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, well over a decade.

The hard work seems to pay off. In their latest study, Simon and his colleagues report that they detected a distinct signal in their data: A common process appears to affect the light from many pulsars.

“We went through each of the pulsars one by one. I think we all expected to find a few that were the trickiest that were throwing our data away, ”Simon said. “But then we went through them all, and we said, ‘Oh my God, there’s actually something here.

Researchers still cannot say for sure what is causing this signal. They will need to add more pulsars to their data set and observe them for longer periods of time to determine if this is really the gravitational wave background at work.

“Being able to detect the bottom of the gravitational wave will be a huge step, but it’s really just a step,” he said. “The second step is to identify the causes of these waves and find out what they can tell us about the universe.”

NANOGrav is a frontier physics center of the National Science Foundation of the United States. It is co-directed by Maura McLaughlin of West Virginia University and Xavier Siemens of Oregon State University.