Scientists are getting closer in race to find gravitational wave background and dark matter

Charles Q. Choi
·6-min read
 Researchers used pulsars shown in the inset to calculate dark matter's affect on how stars move within the Milky Way.
Researchers used pulsars shown in the inset to calculate dark matter's affect on how stars move within the Milky Way.

Astronomers may be getting closer to discovering as-yet hidden cosmic secrets, such as the nature of dark matter and the presence of widespread distortions in space-time, researchers reported at the 237th American Astronomical Society meeting, held virtually this week.

The existence of dark matter, an the invisible substance thought to make up more than four-fifths of all matter in the universe, may help explain a variety of cosmic puzzles, such as how galaxies can spin as fast as they do without getting ripped apart. However, much about the nature of dark matter — and even whether it exists at all — remains unknown.

To help pinpoint dark matter's properties, researchers sought to directly measure the gravitational effects that dark matter should have on the speed at which stars are moving in the Milky Way. They focused on galactic lighthouses known as pulsars, or spinning neutron stars that emit twin beams of radio waves from their magnetic poles as they rotate. (Neutron stars are the remains of large stars that perished in catastrophic explosions known as supernovas.)

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"It is a very, very small number we are trying to measure," study lead author Sukanya Chakrabarti, an astrophysicist at the Rochester Institute of Technology in New York, said during a news conference held on Monday (Jan. 11). "In terms of change in velocity, it's just a few centimeters per second, or roughly the speed of a crawling baby, and not a very fast baby at that."

Pulsars spin at very steady rates, so they can serve as precise clocks. By monitoring tiny variations in the spin of 14 pulsars, the researchers could estimate the speeds at which these pulsars are moving and thus deduce the gravitational force that dark matter is exerting on them.

The scientists found that across the galaxy, the average amount of dark matter may be slightly lower than previous estimates. The researchers also calculated the amount of dark matter contained within the volume of the Earth is just 1.63 lbs. (740 grams), Chakrabarti said. These findings in turn can help current experiments seeking to directly detect dark matter "try and understand the nature of dark matter particles," she added. For instance, this might affect how often one might expect such particles to interact with detectors.

In addition, scientists are now analyzing an unusually high number of gamma rays from the center of the Milky Way to see if they might come from annihilating dark matter particles. Previous research suggested dark matter could be made of new kinds of particle, ones that annihilate when they come in contact with each other, generating high-energy gamma rays.

Based on 11 years of data from NASA's Fermi Gamma-Ray Space Telescope, "we can say what are good candidates for dark matter," study lead author Mattia di Mauro, an astrophysicist at the National Institute for Nuclear Physics in Torino, Italy, said during the same news conference. These include weakly interacting massive particles, or WIMPS, hypothetical elementary particles that only barely interact with ordinary matter except through their gravitational pull.

"In the future, the Large Hadron Collider or other particle physics detectors could test these specific candidates," he added.

The gravitational background 

Researchers at the premiere astronomy conference also reported finding the first possible hints of a mysterious new kind of gravitational wave, cosmic ripples that warp the fabric of space and time itself.

Scientists reported the first-ever direct detection of gravitational waves in 2016 using the Laser Interferometer Gravitational-Wave Observatory (LIGO), a discovery that earned the 2017 Nobel Prize in Physics. The space-time distortions those researchers saw were created when two black holes collided with each other about 130 million light-years from Earth. Since then, LIGO has observed dozens more such signals.

But the gravitational waves that LIGO are best at detecting are the most powerful ones, loud outbursts released when extraordinarily massive objects collide with one another. Researchers now also want to detect gravitational waves that are more like the background noise of small talk at a crowded party.

In theory, merging galaxies and other cosmic events should generate such a "gravitational wave background." Detecting this steady hum could shed light on mysteries such as how galaxies have grown over time.

However, these waves are huge, posing a major challenge for detecting this gravitational wave background. Whereas existing gravitational-wave observatories on Earth are designed to search for gravitational waves on the order of seconds long, ripples from the gravitational wave background are years or even decades long.

Now researchers say they may have detected a strong signal of the gravitational wave background using a U.S. and Canadian project called the North American Nanohertz Observatory for Gravitational Waves (NANOGrav).

"We're seeing incredibly significant evidence for this signal," study lead author Joseph Simon, an astrophysicist at the University of Colorado Boulder, said during the AAS press conference. "Unfortunately, we can't quite say what it is yet."

NANOGrav uses telescopes on the ground to monitor dozens of pulsars. Gravitational waves can alter the steady blinking pattern of light from pulsars, squeezing and expanding the distances these rays travel through space. 

"As these waves pass us, the Earth gets pushed around very slightly," Simon said. "As Earth is pushed closer to pulsars in one part of the sky, those pulsars' pulses will appear a little bit sooner than expected, and pulses from pulsars in the other part of the sky appear to come a bit later."

Analyzing this pulsar light could therefore help scientists detect signs of the gravitational wave background. 

"By monitoring signals from a large number of these pulsars, we created a galaxy-size gravitational-wave detector within our own Milky Way," Simon said.

To find these subtle hints, NANOGrav scientists have attempted to observe as many pulsars as they can for as long as possible. So far, they have observed 45 pulsars for at least three years, and in some cases, for more than a dozen years.

"These pulsars are spinning about as fast as your kitchen blender," Simon said in a statement. "And we're looking at deviations in their timing of just a few hundred nanoseconds."

Now the researchers said they have detected potential evidence of a common process distorting the light from many of the pulsars. As of yet, they cannot verify whether this signal is evidence for the gravitational wave background, "but we also don't have evidence against it," Simon said.

The scientists caution they still need to look at more pulsars and monitor them for longer time periods to confirm whether the gravitational background is the cause.

If the researchers can verify they have detected the gravitational wave background, they next want to pinpoint what causes these waves and what such signals can tell scientists about the universe.

The scientists detailed their findings Jan. 11 at an online meeting of the American Astronomical Society. Chakrabarti and her colleagues detailed their findings in a study accepted in the journal Astrophysical Journal Letters. Simon and his colleagues detailed their NANOGrav findings online Dec. 24 in the journal The Astrophysical Journal Letters.

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