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.
More than 40 trillion gallons of rain drenched the Southeast United States in the last week from Hurricane Helene and a run-of-the-mill rainstorm that sloshed in ahead of it — an unheard of amount of water that has stunned experts.
That’s enough to fill the Dallas Cowboys’ stadium 51,000 times, or Lake Tahoe just once. If it was concentrated just on the state of North Carolina that much water would be 3.5 feet deep (more than 1 meter). It’s enough to fill more than 60 million Olympic-size swimming pools.
“That’s an astronomical amount of precipitation,” said Ed Clark, head of the National Oceanic and Atmospheric Administration’s National Water Center in Tuscaloosa, Alabama. “I have not seen something in my 25 years of working at the weather service that is this geographically large of an extent and the sheer volume of water that fell from the sky.”
The flood damage from the rain is apocalyptic, meteorologists said. More than 100 people are dead, according to officials.
Private meteorologist Ryan Maue, a former NOAA chief scientist, calculated the amount of rain, using precipitation measurements made in 2.5-mile-by-2.5 mile grids as measured by satellites and ground observations. He came up with 40 trillion gallons through Sunday for the eastern United States, with 20 trillion gallons of that hitting just Georgia, Tennessee, the Carolinas and Florida from Hurricane Helene.
Clark did the calculations independently and said the 40 trillion gallon figure (151 trillion liters) is about right and, if anything, conservative. Maue said maybe 1 to 2 trillion more gallons of rain had fallen, much if it in Virginia, since his calculations.
Clark, who spends much of his work on issues of shrinking western water supplies, said to put the amount of rain in perspective, it’s more than twice the combined amount of water stored by two key Colorado River basin reservoirs: Lake Powell and Lake Mead.
Several meteorologists said this was a combination of two, maybe three storm systems. Before Helene struck, rain had fallen heavily for days because a low pressure system had “cut off” from the jet stream — which moves weather systems along west to east — and stalled over the Southeast. That funneled plenty of warm water from the Gulf of Mexico. And a storm that fell just short of named status parked along North Carolina’s Atlantic coast, dumping as much as 20 inches of rain, said North Carolina state climatologist Kathie Dello.
Then add Helene, one of the largest storms in the last couple decades and one that held plenty of rain because it was young and moved fast before it hit the Appalachians, said University of Albany hurricane expert Kristen Corbosiero.
“It was not just a perfect storm, but it was a combination of multiple storms that that led to the enormous amount of rain,” Maue said. “That collected at high elevation, we’re talking 3,000 to 6000 feet. And when you drop trillions of gallons on a mountain, that has to go down.”
The fact that these storms hit the mountains made everything worse, and not just because of runoff. The interaction between the mountains and the storm systems wrings more moisture out of the air, Clark, Maue and Corbosiero said.
North Carolina weather officials said their top measurement total was 31.33 inches in the tiny town of Busick. Mount Mitchell also got more than 2 feet of rainfall.
Before 2017’s Hurricane Harvey, “I said to our colleagues, you know, I never thought in my career that we would measure rainfall in feet,” Clark said. “And after Harvey, Florence, the more isolated events in eastern Kentucky, portions of South Dakota. We’re seeing events year in and year out where we are measuring rainfall in feet.”
Storms are getting wetter as the climate change s, said Corbosiero and Dello. A basic law of physics says the air holds nearly 4% more moisture for every degree Fahrenheit warmer (7% for every degree Celsius) and the world has warmed more than 2 degrees (1.2 degrees Celsius) since pre-industrial times.
Corbosiero said meteorologists are vigorously debating how much of Helene is due to worsening climate change and how much is random.
For Dello, the “fingerprints of climate change” were clear.
“We’ve seen tropical storm impacts in western North Carolina. But these storms are wetter and these storms are warmer. And there would have been a time when a tropical storm would have been heading toward North Carolina and would have caused some rain and some damage, but not apocalyptic destruction. ”
Associated Press climate and environmental coverage receives support from several private foundations. See more about AP’s climate initiative here. The AP is solely responsible for all content.
It’s a dinosaur that roamed Alberta’s badlands more than 70 million years ago, sporting a big, bumpy, bony head the size of a baby elephant.
On Wednesday, paleontologists near Grande Prairie pulled its 272-kilogram skull from the ground.
They call it “Big Sam.”
The adult Pachyrhinosaurus is the second plant-eating dinosaur to be unearthed from a dense bonebed belonging to a herd that died together on the edge of a valley that now sits 450 kilometres northwest of Edmonton.
It didn’t die alone.
“We have hundreds of juvenile bones in the bonebed, so we know that there are many babies and some adults among all of the big adults,” Emily Bamforth, a paleontologist with the nearby Philip J. Currie Dinosaur Museum, said in an interview on the way to the dig site.
She described the horned Pachyrhinosaurus as “the smaller, older cousin of the triceratops.”
“This species of dinosaur is endemic to the Grand Prairie area, so it’s found here and nowhere else in the world. They are … kind of about the size of an Indian elephant and a rhino,” she added.
The head alone, she said, is about the size of a baby elephant.
The discovery was a long time coming.
The bonebed was first discovered by a high school teacher out for a walk about 50 years ago. It took the teacher a decade to get anyone from southern Alberta to come to take a look.
“At the time, sort of in the ’70s and ’80s, paleontology in northern Alberta was virtually unknown,” said Bamforth.
When paleontogists eventually got to the site, Bamforth said, they learned “it’s actually one of the densest dinosaur bonebeds in North America.”
“It contains about 100 to 300 bones per square metre,” she said.
Paleontologists have been at the site sporadically ever since, combing through bones belonging to turtles, dinosaurs and lizards. Sixteen years ago, they discovered a large skull of an approximately 30-year-old Pachyrhinosaurus, which is now at the museum.
About a year ago, they found the second adult: Big Sam.
Bamforth said both dinosaurs are believed to have been the elders in the herd.
“Their distinguishing feature is that, instead of having a horn on their nose like a triceratops, they had this big, bony bump called a boss. And they have big, bony bumps over their eyes as well,” she said.
“It makes them look a little strange. It’s the one dinosaur that if you find it, it’s the only possible thing it can be.”
The genders of the two adults are unknown.
Bamforth said the extraction was difficult because Big Sam was intertwined in a cluster of about 300 other bones.
The skull was found upside down, “as if the animal was lying on its back,” but was well preserved, she said.
She said the excavation process involved putting plaster on the skull and wooden planks around if for stability. From there, it was lifted out — very carefully — with a crane, and was to be shipped on a trolley to the museum for study.
“I have extracted skulls in the past. This is probably the biggest one I’ve ever done though,” said Bamforth.
“It’s pretty exciting.”
This report by The Canadian Press was first published Sept. 25, 2024.
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.”
