Scientists monitor how ripples in spacetime affect the timing of signals from pulsars. (Credit: Aurore Simonnet for NANOGrav Collaboration)
Astrophysicists have found the best evidence yet for a low-frequency “hum” of gravitational waves rippling through the cosmos, based on 15 years’ worth of ultra-precise measurements checking the timing of radio pulses from distant stars.
The evidence, newly published in the Astrophysical Journal Letters, comes from several teams of researchers working in the U.S. and Canada as well as Europe, India, Australia and China.
The teams monitored radio emissions from a total of 115 ultra-dense, spinning stars known as pulsars. Nearly 70 of those pulsars were observed by the North American Nanohertz Observatory for Gravitational Waves, known as NANOGrav.
“This is key evidence for gravitational waves at very low frequencies,” Vanderbilt University’s Stephen Taylor, who co-led the search and is the current chair of the NANOGrav Collaboration, said today in a news release. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe.”
A specialist checks the alignment of a test beam at the Laser Interferometer Gravitational-wave Observatory. (NSF Photo)
After three years of upgrading and waiting, due in part to the coronavirus pandemic, the Laser Interferometer Gravitational-wave Observatory has officially resumed its hunt for the signatures of crashing black holes and neutron stars.
“Our LIGO teams have worked through hardship during the past two-plus years to be ready for this moment, and we are indeed ready,” Caltech physicist Albert Lazzarini, the deputy director of the LIGO Laboratory, said in a news release.
Lazzarini said the engineering tests leading up to today’s official start of Observing Run 4, or O4, have already revealed a number of candidate events that have been shared with the astronomical community.
“Most of these involve black hole binary systems, although one may include a neutron star,” he said. “The rates appear to be consistent with expectations.”
One such event, called S230518h, was detected last week. Researchers say that if they can confirm the data, the event was most likely caused by the merger of a faraway black hole and a neutron star.
The twin LIGO gravitational-wave detectors at Hanford, Wash., and Livingston, La., will be joined for O4 by the Virgo detector in Italy as well as the KAGRA observatory in Japan. Virgo is scheduled to take part in the run starting later this year. KAGRA will parallel LIGO’s observations for the next month, take a break for some upgrades, and then rejoin the run.
Three views of a black hole, from left to right: Event Horizon Telescope's original image, PRIMO reconstruction and image blurred to match EHT's resolution. (Credit: Lia Medeiros et al. / ApJL, 2023)
Astronomers have used machine learning to sharpen up the Event Horizon Telescope’s first picture of a black hole — an exercise that demonstrates the value of artificial intelligence for fine-tuning cosmic observations.
The image should guide scientists as they test their hypotheses about the behavior of black holes, and about the gravitational rules of the road under extreme conditions.
Artwork depicts a tidal disruption event. (Carl Knox / OzGrav / ARC Centre of Excellence for Grav. Wave Discovery, Swinburne Univ. of Tech.)
Nine months ago, astronomers observed a flash that they said came from a mysterious object that seemed to flare with the brilliance of a quadrillion suns, located 8.5 billion light-years from Earth.
Now they say they’ve figured out what that object was.
In a pair of studies published by Nature and Nature Astronomy, researchers report that the event was probably sparked when a supermassive black hole suddenly consumed a nearby star. The event’s violent energy was released in the form of a relativistic jet of blazing-hot material that headed in Earth’s direction.
The jet didn’t do us any damage. But its bull’s-eye directionality produced a phenomenon called “Doppler boosting,” also known as the headlight effect. That made the jet’s flash look brighter than it would have if the jet went in a different direction.
Scientists say the flash, which was designated AT2022cmc when it was detected by the Zwicky Transient Facility in February, is only the fourth known example of a Doppler-boosted tidal disruption event.
This is the first image of Sagittarius A*, the supermassive black hole at the center of our galaxy. (EHT Collaboration via NSF)
After years of observation and weeks of rumor-mill rumblings, astronomers today unveiled their first image of the supermassive black hole at the center of our own Milky Way galaxy, Sagittarius A*.
Technically, the picture from the Event Horizon Telescope project doesn’t show light from the black hole itself. After all, a black hole is a gravitational singularity so dense that nothing, not even light, can escape its grip. Rather, the picture shows the “shadow” of a black hole, surrounded by the superheated, glowing gas that surrounds it.
And technically, the picture may not match what folks might see with their own eyes up close. Rather, the readings come from eight observatories around the world that combined their observations in radio wavelengths.
Nevertheless, the new view of Sagittarius A*, or Sgr A* for short (pronounced “sadge-ay-star”), serves to confirm in graphic terms what astronomers have long suspected: that our galaxy, like many others, has a supermassive black hole at its heart.
Today’s revelations follow up on the Event Horizon Telescope’s first-ever black hole image, which was released in 2019 and showed the supermassive black hole at the center of M87, an elliptical galaxy about 55 million light-years away.
Sgr A* is much closer — a mere 27,000 light-years from Earth, in the constellation Sagittarius. But there’s nothing to fear from this black hole: It’s relatively quiescent, in contrast to the galaxy-gobbling behemoths that are standard science-fiction fare.
Our galaxy’s black hole is thought to hold the mass of 4 million suns within an area that’s roughly as big around as Mercury’s orbit. Checking those dimensions against the image data serves as a test of relativity theory. Spoiler alert: Albert Einstein was right … again.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity,” EHT project scientist Geoffrey Bower said in a news release. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”
The EHT’s findings about Sgr A* are the subject of a special issue of The Astrophysical Journal Letters — and to whet your appetite for all that reading material, here are three videos that summarize the past, present and future of black hole imaging:
Eastlake High School senior Christine Ye focused on gravitational waves in her research. (UW Bothell Photo)
Christine Ye, a senior at Eastlake High School in Sammamish, Wash., has won the top award in the nation’s oldest and most prestigious competitions for science students, thanks to her research into the mysteries of black holes and neutron stars.
“I’m totally in shock,” the 17-year-old told me after winning the $250,000 first-place award in the 2022 Regeneron Science Talent Search. “It feels amazing.”
Ye was among 40 finalists honored on March 15 in Washington, D.C., during a live-streamed ceremony that was emceed by “Saturday Night Live” cast member Melissa Villaseñor. More than $1.8 million in all was awarded to the finalists, who were evaluated on the basis of their projects’ scientific rigor and their potential to become scientific leaders.
Ye’s award-winning research is based on an analysis of readings from the Laser Interferometer Gravitational-wave Observatory, and addresses one of LIGO’s most puzzling observations.
The analysis conducted by Ye and her co-author, Northwestern University postdoctoral fellow Maya Fishbach, determined that rapidly spinning neutron stars could get as massive as the mystery object. Their study will be the subject of a presentation next month in New York at a meeting of the American Physical Society.
Artwork shows a black hole and a neutron star circling toward a merger. (Credit: Carl Knox / OzGrav / Swinburne U.)
Gravitational-wave astronomers are confident that they’ve filled out their repertoire of cataclysmic collisions, thanks to the detection of two cosmic crashes that each involved a black hole and a neutron star.
In contrast, astronomers leave little doubt that the gravitational waves sparked by two separate events in January 2020 were thrown off by the merger of a black hole and a neutron star. They lay out their evidence in a paper published today by The Astrophysical Journal Letters.
“With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary. We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way,” Astrid Lamberts, a member of the Virgo collaboration who works at the Observatoire de la Côte d’Azur in France, said in a news release.
There’s still some mystery surrounding the detections.
This view of the M87 supermassive black hole in polarized light highlights the signature of magnetic fields. (Credit: EHT Collaboration)
Why are black holes so alluring?
You could cite plenty of reasons: They’re matter-gobbling monsters, making them the perfect plot device for a Disney movie. They warp spacetime, demonstrating the weirdest implications of general relativity. They’re so massive that inside a boundary known as the event horizon, nothing — not even light — can escape its gravitational grip.
But perhaps the most intriguing feature of black holes is their sheer mystery. Because of the rules of relativity, no one can report what happens inside the boundaries of a black hole.
“We could experience all the crazy stuff that’s going on inside a black hole, but we’d never be able to tell anybody,” radio astronomer Heino Falcke told me. “We want to know what’s going on there, but we can’t.”
A color-coded simulation of M87's black hole charts the dynamics in its surroundings. (Lia Medeiros / IAS / BH PIRE)
The first-ever picture of a black hole is the gift that keeps on giving — in the form of new insights into the dynamics behind the mysterious phenomenon and new evidence that Albert Einstein was right.
The validity of Einstein’s theory of general relativity has been proven time and time again over the course of the past century. But physicists keep coming up with new ideas for tweaking the theory’s equations in unorthodox ways.
To figure out how much leeway there could be for variations on Einstein’s theme, researchers took a closer look at the supermassive black hole at the center of the galaxy M87.
The team behind the relativity-checking research, published this week in Physical Review Letters, measured the size of the black hole’s shadow — that is, the dark central region from which light rays can’t escape, due to the gravitational pull of a singularity that’s 6.5 billion times as massive as our sun.
The predicted size of the shadow could vary, depending on which theory of gravity you go with. But in M87’s case, the size matched up precisely with Einstein’s theory.
The latest test of general relativity in the strong gravitational field regime near black holes (Psaltis et al. 2020) constrains deviations from GR by a factor of 500 better than historical constraints! Image Credit: Dimitrios Psaltis and Raquel Fraga-Encinas pic.twitter.com/HzaWC1NKZ8
“Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein’s theory of general relativity by almost a factor of 500, compared to previous tests in the solar system,” the University of Arizona’s Feryal Özel, a senior member of the EHT collaboration, said in a news release.
“Many ways to modify general relativity fail at this new and tighter black hole shadow test,” Özel said.
“Together with gravitational-wave observations, this marks the beginning of a new era in black hole astrophysics,” said lead study author Dimitrios Psaltis, a University of Arizona astronomer who recently finished his stint as the EHT collaboration’s project scientist.
Eight observatories around the world contributed to the initial round of observations for the Event Horizon Telescope project. For the EHT’s next campaign in 2021, there’ll be three more observatories on the case, in Arizona, Greenland and France.
The added capacity should result in higher-fidelity images — not only of M87’s black hole, but also of Sagittarius A*, the supermassive black hole at the center of our own Milky Way galaxy.
Up-close views of black holes could well shine a light on another prediction made by general relativity, known as the no-hair theorem. This theorem states that the characteristics of black holes are completely determined by their mass, spin and electrical charge.
If the theorem is correct, all black holes with the same values for those three attributes would be identical to each other. Any other distinguishing characteristics for black holes and their history — their “hair,” metaphorically speaking — would disappear forever inside the black hole’s event horizon.
An artist's conception shows two black holes spiraling in to create an even bigger black hole. (Image credit: Mark Myers / OzGrav)
Scientists say the merger of two black holes that occurred when the universe was half its current age has created the most massive source of gravitational waves ever observed.
“This doesn’t look much like a chirp, which is what we typically detect,” Nelson Christensen, an astrophysicist at the French National Center for Scientific Research, said in a news release. “This is more like something that goes ‘bang,’ and it’s the most massive signal LIGO and Virgo have seen.”
Christensen and his colleagues say the signal, known as GW190521, appears to have come from the violent collision of two spinning black holes that were about 85 and 66 times as massive as our sun.
The merger created an even bigger black hole that’s about 142 times as massive as the sun. It also released the equivalent of eight solar masses in the form of gravitational-wave energy, in accordance with Albert Einstein’s E=mc2 formula, the scientists said.
GW190521 not only ranks as the biggest bang recorded since LIGO made its initial, Nobel-winning gravitational-wave detection in 2015. It also counts as LIGO’s first detection of a mysterious object known as an intermediate-mass black hole.
For decades, physicists have been fleshing out their theories regarding the nature and origin of black holes — concentrations of matter so massive and compact that nothing, not even light, can escape their gravitational grip.
Some black holes are thought to be created when stars up to 130 times as massive as our sun run out of their fusion fuel and collapse inward, producing black holes as big as 65 solar masses. There are also scenarios in which stars that weigh more than 200 solar masses can collapse into black holes in the range of 120 solar masses.
Not that long ago, scientists thought the physics behind gravitational collapse ruled out the creation of black holes between 65 and 120 solar masses. Instead, the collapse of midsize stars was thought to produce instability through the creation of electron-antielectron pairs — and as a result, the stars were supposed to blow themselves completely apart.
Now, the fact that one of the black holes involved in last year’s smashup was measured at 85 solar masses is complicating claims for the existence of a pair instability mass gap.
“The fact that we’re seeing a black hole in this mass gap will make a lot of astrophysicists scratch their heads and try to figure out how these black holes are made,” said Christensen, who is the director of the Artemis Laboratory at the Nice Observatory in France.
The authors of the paper published in the Astrophysical Journal Letters have already come up with one possibility: Perhaps the midsize black hole was not directly produced by a stellar collapse, but instead by an earlier merger of smaller black holes — just the sort of merger that LIGO and Virgo have been detecting over the past five years.
“This event opens more questions than in provides answers,” said Caltech physicist Alan Weinstein, a member of the LIGO collaboration. “From the perspective of discovery and physics, it’s a very exciting thing.”
Weinstein cautioned that there’s still some uncertainty about the current explanation for GW190521.
“Since we first turned on LIGO, everything we’ve observed with confidence has been a collision of black holes or neutron stars,” he said. “This is the one event where our analysis allows the possibility that this event is not such a collision. Although this event is consistent with being from an exceptionally massive binary black hole merger, and alternative explanations are disfavored, it is pushing the boundaries of our confidence. And that potentially makes it extremely exciting.”
Further observations from LIGO and Virgo could turn up something completely new in the gravitational-wave menagerie — for example, evidence for the creation of primordial cosmic strings.
The LIGO project is funded by the National Science Foundation and operated by Caltech and MIT. It relies on two gravitational-wave detectors that have been built at Hanford, Wash., and at Livingston, La., with about 2,000 miles of separation to provide a double-check on the detectors’ results.
Each detector consists of an L-shaped network of tunnels, measuring 2.5 miles on a side, with laser beams reflected back and forth within the tunnels. Gravitational waves from far-off cataclysms disturb the fabric of spacetime ever so slightly — but the detectors are sensitive enough to pick up such disturbances to within the width of a proton.
Europe’s Virgo detector, designed according to a similar scheme, provides further verification for LIGO’s gravitational-wave data and makes it easier to triangulate on the sources of the waves.
In the case of GW190521, the waves are thought to have been thrown off by a source that’s now roughly 17 billion light-years (5 gigaparsecs) away from us — at a time when the universe was about half its current age of 13.8 billion years. That makes it one of the most distant gravitational-wave sources detected so far.
Because light travels at a finite speed, it may seem counterintuitive for a signal that was sent out 7 billion years ago to be received from an object that’s 17 billion light-years away.
But in an email exchange, Weinstein told me there’s a relatively simple explanation for the seeming mismatch. “The universe has been expanding since the gravitational waves were emitted,” he wrote. “So physical distances are tricky to interpret in an expanding universe, and must be treated with care.”
