In this artist’s interpretation, a pair of supermassive black holes (top left) emits gravitational waves that ripple through the fabric of space-time. Those gravitational waves compress and stretch the paths of radio waves emitted by pulsars (white). By carefully measuring the radio waves, a team of scientists recently made the first detection of the universe’s gravitational wave background. Credit: Aurore Simonnet for the NANOGrav Collaboration
Following 15 years of data collection in a galaxy-sized experiment, scientists have “heard” the perpetual chorus of gravitational waves rippling through our universe for the first time—and it’s louder than expected.
The groundbreaking discovery was made by scientists with the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) who closely observed stars called pulsars that act as celestial metronomes. The newly detected gravitational waves—ripples in the fabric of space-time—are by far the most powerful ever measured: They carry roughly a million times as much energy as the one-off bursts of gravitational waves from black hole and neutron star mergers detected by experiments such as LIGO and Virgo.
Most of the gigantean gravitational waves are probably produced by pairs of supermassive black holes spiraling toward cataclysmic collisions throughout the cosmos, the NANOGrav scientists report in a series of new papers appearing today in The Astrophysical Journal Letters.
“It’s like a choir, with all these supermassive black hole pairs chiming in at different frequencies,” says NANOGrav scientist Chiara Mingarelli, who worked on the new findings while an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics (CCA) in New York City. “This is the first-ever evidence for the gravitational wave background. We’ve opened a new window of observation on the universe.”
[embedded content]Credit: National Science Foundation
The existence and composition of the gravitational wave background—long theorized but never before heard—presents a treasure trove of new insights into long-standing questions, from the fate of supermassive black hole pairs to the frequency of galaxy mergers.
For now, NANOGrav can only measure the overall gravitational wave background rather than radiation from the individual “singers.” But even that brought surprises.
“The gravitational wave background is about twice as loud as what I expected,” says Mingarelli, now an assistant professor at Yale University. “It’s really at the upper end of what our models can create from just supermassive black holes.”
The deafening volume may result from experimental limitations or heavier and more abundant supermassive black holes. But there’s also the possibility that something else is generating powerful gravitational waves, Mingarelli says, such as mechanisms predicted by string theory or alternative explanations of the universe’s birth. “What’s next is everything,” she says. “This is just the beginning.”
An artist’s rendering of gravitational waves from a pair of close-orbiting black holes (visible on the left in the distance). The waves are passing by several pulsars and the Earth (on the right). Credit: Keyi “Onyx” Li/U.S. National Science Foundation
A galaxy-wide experiment
Getting to this point was a years-long challenge for the NANOGrav team. The gravitational waves they hunted are different from anything previously measured. Unlike the high-frequency waves detected by earthbound instruments such as LIGO and Virgo, the gravitational wave background is made up of ultra-low-frequency waves. A single rise and fall of one of the waves could take years or even decades to pass by. Since gravitational waves travel at the speed of light, a single wavelength could be tens of light-years long.
No experiment on Earth could ever detect such colossal waves, so the NANOGrav team instead looked to the stars. They closely observed pulsars, the ultra-dense remnants of massive stars that went supernova. Pulsars act like stellar lighthouses, shooting beams of radio waves from their magnetic poles. As the pulsars rapidly spin (sometimes hundreds of times a second), those beams sweep across the sky, appearing from our vantage point on Earth as rhythmic pulses of radio waves.
The Very Large Array in New Mexico gathered data that contributed to the detection of the universe’s gravitational wave background. Credit: NRAO/AUI/NSF
The pulses arrive on Earth like a perfectly timed metronome. The timing is so precise that when Jocelyn Bell measured the first pulsar radio waves in 1967, astronomers thought they might be signals from an alien civilization.
As a gravitational wave passes between us and a pulsar, it throws off the radio wave timing. That’s because, as Albert Einstein predicted, gravitational waves stretch and compress space as they ripple through the cosmos, changing how far the radio waves have to travel.
For 15 years, NANOGrav scientists from the United States and Canada closely timed the radio wave pulses from dozens of millisecond pulsars in our galaxy using the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico. The new findings are the result of a detailed analysis of an array of 67 pulsars.
“Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” says Maura McLaughlin of West Virginia University, co-director of the NANOGrav Physics Frontiers Center. “These results are made possible through the National Science Foundation’s (NSF’s) continued commitment to these exceptionally sensitive radio observatories.”
Detecting the background
In 2020, with just over 12 years of data, NANOGrav scientists began to see hints of a signal, an extra “hum” common to the timing behavior of all pulsars in the array. Now, three years of additional observations later, they have accumulated concrete evidence for the existence of the gravitational wave background.
“Now that we have evidence for gravitational waves, the next step is to use our observations to study the sources producing this hum,” says Sarah Vigeland of the University of Wisconsin-Milwaukee, chair of the NANOGrav detection working group.
The likeliest sources of the gravitational wave background are pairs of supermassive black holes caught in a death spiral. Those black holes are truly colossal, containing billions of suns’ worth of mass. Nearly all galaxies, including our own Milky Way, have at least one of the behemoths at their core. When two galaxies merge, their supermassive black holes can meet up and begin orbiting one another. Over time, their orbits tighten as gas and stars pass between the black holes and steal energy.
Eventually, the supermassive black holes get so close that the energy theft stops. Some theoretical studies have argued for decades that the black holes then stall indefinitely when they’re around 1 parsec apart (roughly three light-years). This close-but-no-cigar theory became known as the final parsec problem. In this scenario, only rare groups of three or more supermassive black holes result in mergers.
Supermassive black hole pairs could have a trick up their sleeves, though. They could emit energy as powerful gravitational waves as they orbit one another until eventually they collide in a cataclysmic finale. “Once the two black holes get close enough to be seen by pulsar timing arrays, nothing can stop them from merging within just a few million years,” says Luke Kelley of the University of California, Berkeley, chair of NANOGrav’s astrophysics group.
Pulsars are fast-spinning neutron stars that emit narrow, sweeping beams of radio waves. Credit: NASA’s Goddard Space Flight Center
The existence of the gravitational wave background found by NANOGrav seems to back up this prediction, potentially putting the final parsec problem to rest.
Since supermassive black hole pairs form due to galaxy mergers, the abundance of their gravitational waves will help cosmologists estimate how frequently galaxies have collided throughout the universe’s history. Mingarelli, postdoctoral researcher Deborah C. Good of the CCA and the University of Connecticut, and their colleagues studied the intensity of the gravitational wave background. They estimate that hundreds of thousands or maybe even a million or more supermassive black hole binaries inhabit the universe.
Alternative sources
Not all the gravitational waves detected by NANOGrav are necessarily from supermassive black hole pairs, though. Other theoretical proposals also predict waves in the ultra-low-frequency range. String theory, for instance, predicts that one-dimensional defects called cosmic strings may have formed in the early universe. These strings could dissipate energy by emitting gravitational waves. Another proposal suggests that the universe didn’t start with the Big Bang but with a Big Bounce as a precursor universe collapsed in on itself before expanding back outward. In such an origin story, gravitational waves from the incident would still be rippling through space-time.
There’s also a chance that pulsars aren’t the perfect gravitational wave detectors scientists think they are, and that they instead might have some unknown variability that’s skewing NANOGrav’s results. “We can’t walk over to the pulsars and turn them on and off again to see if there’s a bug,” Mingarelli says.
The NANOGrav team hopes to explore all the potential contributors to the newfound gravitational wave background as they continue monitoring the pulsars. The group plans to break down the background based on the waves’ frequency and origin in the sky.
An international effort
Luckily, the NANOGrav team isn’t alone in its quest. Several papers released today by collaborations using telescopes in Europe, India, China and Australia report hints of the same gravitational wave background signal in their data. Through the International Pulsar Timing Array consortium, the individual groups are pooling their data to better characterize the signal and identify its sources.
“Our combined data will be much more powerful,” says Stephen Taylor of Vanderbilt University, who co-led the new research and currently chairs the NANOGrav collaboration. “We’re excited to discover what secrets they will reveal about our universe.”
More information:
The NANOGrav 15-year Data Set: Evidence for a Gravitational-Wave Background, The Astrophysical Journal Letters (2023). DOI: 10.3847/2041-8213/acdac6
Provided by
Simons Foundation
Citation:
Clamor of gravitational waves from universe’s merging supermassive black holes ‘heard’ for first time (2023, June 28)
retrieved 29 June 2023
from https://phys.org/news/2023-06-clamor-gravitational-universe-merging-supermassive.html
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.
NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
