Banner image: A pulsar that pushes an energy out. (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 the powerful ripples that run through the universe and twist the space and time of matter.
The new findings that appeared recently in The Astrophysical Journal Letters, comes from an American and Canadian project called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).
For over 13 years, NANOGrav researchers have pored over the luminous flux of dozens of pulsars scattered across the Milky Way galaxy to try to detect a “gravitational wave background”
“We have found a strong signal in our data set,” said Simon, a postdoctoral researcher at the Department of Astrophysics and Planetary Sciences. “But we can not yet say that this is the gravitational wave background.”
In 2017, researchers at an experiment called the Laser Interferometer Gravitational-Wave Observatory (LIGO) won the Nobel Prize in Physics for the first ever direct detection of gravitational waves. These waves were created when two black holes threw each other about 130 million light-years from Earth, generating a cosmic shock that spread to our own solar system.
This event corresponded to a pelvic accident – a violent and short-lived explosion. The gravitational waves that Simon and his colleagues are looking for, on the other hand, are more like the constant hum of conversation at a crowded cocktail party.
Discovering that background noise would be an important scientific achievement, opening a new window for the functioning of the universe, he added. These waves could, for example, give scientists new tools to study how the supermassive black holes at the center of many galaxies merge over time.
“These tempting first hints of a gravitational wave background suggest that supermassive black holes are likely to melt and that we are bubbling in a sea of gravitational waves curling from supermassive black hole fusions in galaxies across the universe,” said Julie Comerford, associate professor of astrophysics and planetary. science at CU Boulder and NANOGrav team member.
Simon will present his team’s results at a virtual press conference on Monday at the 237th meeting of the American Astronomical Society.
Through their work with NANOGrav, Simon and Comerford are part of a high-profile, albeit collaborative, international race to find the background to the gravity wave. Their project joins two others out of Europe and Australia to form a network called the International Pulsar Timing Array.
Simon said that at least according to the theory, fusing galaxies and other cosmological events produce an even sword of gravity waves. They are humane geese – a single wave, Simon said, can take years or even longer to pass the earth by. For this reason, no other existing experiments can detect them directly.
“Other observatories are looking for gravitational waves that are in 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 get creative. The NANOGrav team uses telescopes on the ground not to look for gravitational waves, but to observe pulsars. These collapsed stars are the lighthouses of the galaxy. They spin at incredibly fast speeds, sending rays of radiation that wind towards the Earth in a flashing pattern that for the most part remains unchanged over eons.
Simon explained that gravitational waves change the constant pattern of light coming from pulsars, pulling or squeezing the relative distances that these rays move through space. In other words, scientists may be able to spot the background of the gravitational wave by simply monitoring pulsars for correlated changes in the time when they arrive at Earth.
“These pulsars rotate just as fast as your kitchen blender,” he said. “And we’re looking at deviations in their timing in just a few hundred nanoseconds.”
To find the subtle signal, the NANOGrav team strives to observe as many pulses as possible for as long as possible. So far, the group has observed 45 pulses for at least three years and in some cases for well over ten years.
The hard work seems to pay off. In their latest study, Simon and his colleagues report that they have detected a clear signal in their data: Some common process seems to be affecting the light coming from many of the pulses.
“We went through each of the pulses one by one. I think we all expected to find a couple who were the fragile ones who threw away our data, ”Simon said. “But then we came through them all and we said, ‘Oh my God, there’s actually something here.'”
Researchers still cannot say with certainty what is causing the signal. They will need to add more pulsars to their datasets and observe them for extended periods of time to determine if it is actually the gravitational wave background in the workplace.
“Being able to detect the gravitational wave background will be a big step, but it’s really only step one,” he said. “Step two is to find out what is causing these waves, and find out what they can tell us about the universe.”
NANOGrav is an American National Science Foundation Physics Frontiers Center. It is co-directed by Maura McLaughlin of West Virginia University and Xavier Siemens of Oregon State University.