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The biggest. The farthest. The most energetic. A new detection of gravitational waves from two colliding black holes has racked up multiple superlatives.
What’s more, it also marks the first definitive sighting of an intermediate mass black hole, one with a mass between 100 and 100,000 times the sun’s mass. That midsize black hole was forged when the two progenitor black holes coalesced to form a larger one with about 142 solar masses. It significantly outweighs all black holes previously detected via gravitational waves, ripples that wrinkle spacetime in the aftermath of extreme events.
“This is the big guy we’ve been waiting for, for the longest time,” says Emanuele Berti, a physicist at Johns Hopkins University who was not involved with the research. One of the behemoth’s two progenitors was itself so massive that scientists are pondering how to explain its existence.
Detected on May 21, 2019, the gravitational waves originated from a source about 17 billion light-years from Earth, making this the most distant detection confirmed so far. It’s also the most energetic event yet seen, radiating about eight times the equivalent of the sun’s mass in energy, says astrophysicist Karan Jani of Vanderbilt University in Nashville, a member of the LIGO Scientific Collaboration. “I hope it deserves its own entry in the record book.”Sign Up For the Latest from Science News
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The new event dethrones the previous record-holder, a collision that occurred about 9 billion light-years away that radiated about five solar masses worth of energy, and created a black hole of 80 solar masses (SN: 12/4/18).
Researchers with LIGO, or the Advanced Laser Interferometer Gravitational-Wave Observatory, in the United States and Advanced Virgo in Italy reported the new detection September 2 in two papers in Physical Review Letters and the Astrophysical Journal Letters.
While scientists know of black holes with tens of solar masses and others with millions or billions of solar masses, the intermediate echelon has remained elusive. Previous purported sightings of intermediate mass black holes have been questioned (SN:1/22/16).
But, for the new event, “there’s no doubt,” says astrophysicist Cole Miller of University of Maryland at College Park, who was not involved with the study. “This demonstrates that there is now at least one intermediate mass black hole in the universe.”
The black hole’s two progenitors were themselves heftier than any seen colliding before — at about 85 and 66 times the mass of the sun. That has scientists puzzling over how this smashup came to be.Normally, physicists expect that the black holes involved in these mergers would each have formed in the collapse of a dying star. But in the new event, the larger of the pair is so big that it couldn’t have formed that way. The known processes that go on within a star’s core mean that stars that are the right mass to form such a big black hole would blow themselves apart completely, rather than leaving behind a corpse.
Instead, it might be that one or both of the colliding black holes formed from an earlier round of black hole mergers, within a crowded cluster of stars and black holes (SN: 1/30/17). That would make for a family tree that began with black holes lightweight enough to form from collapsing stars.
But there’s a problem with the multiple-merger explanation. Each time black holes merge, that coalescence provides a kick to their velocity, which would normally launch the resulting black hole out of the cluster, preventing further mergers.
However, mergers as massive as the new event seem to be very rare, given that LIGO and Virgo have detected only one. That means, Miller says, “my gosh, you’re allowed to invoke a tooth fairy,” a relatively unlikely process. Perhaps, he says, the kick might sometimes be small enough that the black holes could stay within their cluster and merge again.
The May 21 gravitational wave event had previously been publicly reported as an unconfirmed candidate, to allow astronomers to look for flashes of light in the sky that might have resulted from the collision. Some researchers had suggested that the waves might have been associated with a flare of light from the center of a distant galaxy (SN: 6/25/20). But that galaxy is significantly closer than the distance now pinpointed in the new papers, at about 8 billion light-years from Earth rather than 17 billion, making the explanation less plausible.
The longer LIGO and Virgo observe the heavens, the more the bounty of unusual events can be expected to grow, Miller says. “We are going to have a set of ‘gosh, didn’t expect that’ type of events, which are thrilling to think about and extremely informative about the universe.” MoreThe secret to directly detecting dark matter might be blowin’ in the wind.
The mysterious substance continues to elude scientists even though it outweighs visible matter in the universe by about 8 to 1. All laboratory attempts to directly detect dark matter — seen only indirectly by the effect its gravity has on the motions of stars and galaxies — have gone unfulfilled.
Those attempts have relied on the hope that dark matter has at least some other interaction with ordinary matter in addition to gravity (SN: 10/25/16). But a proposed experiment called Windchime, though decades from being realized, will try something new: It will search for dark matter using the only force it is guaranteed to feel — gravity.
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“The core idea is extremely simple,” says theoretical physicist Daniel Carney, who described the scheme in May at a meeting of the American Physical Society’s Division of Atomic Molecular and Optical Physics in Orlando, Fla. Like a wind chime on a porch rattling in a breeze, the Windchime detector would try to sense a dark matter “wind” blowing past Earth as the solar system whips around the galaxy.
If the Milky Way is mostly a cloud of dark matter, as astronomical measurements suggest, then we should be sailing through it at about 200 kilometers per second. This creates a dark matter wind, for the same reason you feel a wind when you stick your hand out the window of a moving car.
The Windchime detector is based on the notion that a collection of pendulums will swing in a breeze. In the case of backyard wind chimes, it might be metal rods or dangling bells that jingle in moving air. For the dark matter detector, the pendulums are arrays of minute, ultrasensitive detectors that will be jostled by the gravitational forces they feel from passing bits of dark matter. Instead of air molecules bouncing off metal chimes, the gravitational attraction of the particles that make up the dark matter wind would cause distinctive ripples as it blows through a billion or so sensors in a box measuring about a meter per side.
Within the Windchime detector (illustrated as an array of small pendulums), a passing dark matter particle (red dot) would gravitationally tug on sensors (blue squares) and cause a detectable ripple, much like wind blowing through a backyard wind chime.D. Carney et al/Physical Review D 2020
While it may seem logical to search for dark matter using gravity, no one has tried it in the nearly 40 years that scientists have been pursuing dark matter in the lab. That’s because gravity is, comparatively, a very weak force and difficult to isolate in experiments.
“You’re looking for dark matter to [cause] a gravitational signal in the sensor,” says Carney, of Lawrence Berkeley National Laboratory in California. “And you just ask . . . could I possibly see this gravitational signal? When you first make the estimate, the answer is no. It’s actually going to be infeasibly difficult.”
That didn’t stop Carney and a small group of colleagues from exploring the idea anyway in 2020. “Thirty years ago, this would have been totally nuts to propose,” he says. “It’s still kind of nuts, but it’s like borderline insanity.”
The Windchime Project collaboration has since grown to include 20 physicists. They have a prototype Windchime built of commercial accelerometers and are using it to develop the software and analysis that will lead to the final version of the detector, but it’s a far cry from the ultimate design. Carney estimates that it could take another few decades to develop sensors good enough to measure gravity even from heavy dark matter.
Carney bases the timeline on the development of the Laser Interferometer Gravitational-Wave Observatory, or LIGO, which was designed to look for gravitational ripples coming from black holes colliding (SN: 2/11/16). When LIGO was first conceived, he says, it was clear that the technology would need to be improved by a hundred million times. Decades of development resulted in an observatory that views the sky in gravitational waves. With Windchime, “we’re in the exact same boat,” he says.
Even in its final form, Windchime will be sensitive only to dark matter bits that are roughly the mass of a fine speck of dust. That’s enormous on the spectrum of known particles — more than a million trillion times the mass of a proton.
“There is a variety of very interesting dark matter candidates at [that scale] that are definitely worth looking for … including primordial black holes from the early universe,” says Katherine Freese, a physicist at the University of Michigan in Ann Arbor who is not part of the Windchime collaboration. Black holes slowly evaporate, leaking mass back into space, she notes, which could leave many relics formed shortly after the Big Bang at the mass Windchime could detect.
But if it never detects anything at all, the experiment still stands out from other dark matter detection schemes, says Dan Hooper, a physicist at Fermilab in Batavia, Ill., also not affiliated with the project. That’s because it would be the first experiment that could entirely rule out some types of dark matter.
Even if the experiment turns up nothing, Hooper says, “the amazing thing about [Windchime] … is that, independent of anything else you know about dark matter particles, they aren’t in this mass range.” With existing experiments, a failure to detect anything could instead be due to flawed guesses about the forces that affect dark matter (SN: 7/7/22).
Windchime will be the only experiment yet imagined where seeing nothing would definitively tell researchers what dark matter isn’t. With a little luck, though, it could uncover a wind of tiny black holes, or even more exotic dark matter bits, blowing past as we careen around the Milky Way. More
Galaxies that helped transform the early universe may have been small, round and green.
Astronomers using the James Webb Space Telescope have spotted “Green Pea” galaxies dating to 13.1 billion years ago. These viridescent runts, spotted just 700 million years after the Big Bang, might have helped trigger one of the greatest makeovers in cosmic history, astronomers said at a January 9 news conference in Seattle at the American Astronomical Society’s annual meeting.
Green Peas first showed up in 2009 in images from the Sloan Digital Sky Survey, an ambitious project to map much of the sky. Citizen science volunteers gave the objects their colorful name. Their greenish hue is because most of their light comes from glowing gas clouds, rather than directly from stars.
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These galaxies are rare in the present-day universe. Astronomers think that the ones that do exist are analogs of galaxies that were more plentiful in the early universe.
“They’re a bit like living fossils,” said astrophysicist James Rhoads of NASA’s Goddard Space Flight Center in Greenbelt, Md. “Coelacanths, if you will,” referencing a fish thought to be extinct until it showed up off the coast of South Africa in 1938 (SN: 12/2/11).
These galaxies leak much more ultraviolet light, which can rip electrons from atoms, than typical galaxies do. So Green Peas dating to the universe’s first billion years or so could be partly responsible for a dramatic and mysterious cosmic transition called reionization, when most of the hydrogen atoms in the early universe had their electrons torn away (SN: 1/7/20).
Three ancient Green Peas turned up in JWST’s first image, released in July 2022 (SN: 7/21/22). The objects look red in JWST’s infrared vision, but the wavelengths of light they emit are like those of the previously discovered Green Peas. The findings were also published in the Jan. 1 Astrophysical Journal Letters.
“This helps us explain how the universe reionized,” Rhoads said. “I think this is an important piece of the puzzle.” More
A surprisingly short gamma-ray burst has astronomers rethinking what triggers these celestial cataclysms.
The Fermi Gamma-ray Space Telescope detected a single-second-long blast of gamma rays, dubbed GRB 200826A, in August 2020. Such fleeting gamma-ray bursts, or GRBs, are usually thought to originate from neutron star smashups (SN: 10/16/17). But a closer look at the burst revealed that it came from the implosion of a massive star’s core.
In this scenario, the core of a star collapses into a compact object, such as a black hole, that powers high-speed particle jets. Those jets punch through the rest of the star and radiate powerful gamma rays before the outer layers of the star explode in a supernova (SN: 5/8/19). That process is typically thought to produce longer GRBs, lasting more than two seconds.
Discovering such a brief gamma-ray burst from a stellar explosion suggests that some bursts previously classified as stellar mergers may actually be from the deaths of massive stars, researchers report online July 26 in two studies in Nature Astronomy.
The first clues about GRB 200826A’s origin came from the burst itself. The wavelengths of light and amount of energy released in the burst were more similar to collapse-related GRBs than collision-produced bursts, Bing Zhang, an astrophysicist at the University of Nevada, Las Vegas, and colleagues report. Plus, the burst hailed from the middle of a star-forming galaxy, where astronomers expect to find collapsing massive stars, but not neutron star mergers — which are generally found on the fringes of tranquil galaxies.
Another group, led by astronomer Tomás Ahumada-Mena of the University of Maryland in College Park, searched for the supernova that’s expected to follow a GRB produced by a collapsing star. Using the Gemini North Telescope in Hawaii to observe GRB 200826A’s host galaxy, the team was able to pick out the telltale infrared light of the supernova. The burst may have been so brief because its jets had just barely punched through the surface of the star before they petered out and the star blew up, Ahumada-Mena says. More