Black holes by the dozens challenge theorists

doi:10.1126/science.370.6517.648

GSTDTAP > 气候变化

DOI	10.1126/science.370.6517.648
	Black holes by the dozens challenge theorists
	Adrian Cho
	2020-11-06
发表期刊	Science
出版年	2020
英文摘要	As children once did before Christmas, astrophysicists have been eagerly awaiting the arrival of a catalog. This one lists not toys, but cataclysmic collisions of objects such as black holes—the ghostly ultraintense gravitational fields left behind when massive stars collapse. Last week, it arrived, courtesy of the world's network of gravitational wave detectors. “It's very exciting!” says Selma de Mink, an astrophysicist at Harvard University. “There will definitely be a flurry of papers rushing to take the first stabs at the data.” The observations come from three huge L-shaped optical instruments called interferometers that sense gravitational waves, the infinitesimal stretching of space triggered by the collisions. Two belong to the Laser Interferometer Gravitational-Wave Observatory (LIGO), instruments with arms 4 kilometers long in Louisiana and Washington that in 2015 first sensed the chirp of gravitational waves from two black holes spiraling into each other. Virgo, an interferometer near Pisa, Italy, with 3-kilometer-long arms, joined the hunt in 2017. LIGO and Virgo had already spotted 11 events including a merger of two neutron stars, which shed light on how the universe forges heavy elements. Now, the team has cataloged 37 additional black hole mergers, another neutron star merger, and one possible merger of a black hole and neutron star. The catalog and statistical analyses, posted 28 October on the physics preprint server arXiv, “are superimportant,” says Carl Rodriguez, an astrophysicist at Carnegie Mellon University who was not involved in the work. “With an individual event, there's only so much you can do in comparing to astrophysical models. But with a catalog you can not only begin to constrain the theory, you can start to understand the landscape.” For example, by analyzing the 47 black hole mergers together researchers can probe how the black holes spin—a potential clue to how the pairs came together. If both black holes spin in the same sense that they orbit each other, they might have formed from a pair of stars born together in the same spinning cloud of gas. If one or both black holes spins in a different direction—and especially if they spin in the opposite sense of the orbit—the black holes probably formed separately and paired up later. It's hard to tell how the spins of a single pair of black holes align from the burst of gravitational waves emitted during the merger, says Maya Fishbach, an astrophysicist and LIGO member from Northwestern University. But by analyzing the events en masse, scientists have teased out evidence that at least some of the mergers involve reversed spins, suggesting black hole pairs form in more than one way. “It seems like there might be multiple things going on,” Fishbach says. Rodriguez has argued that the black holes could form separately in knots of old stars called globular clusters and then pair up. He notes that the overall rate of black hole mergers that LIGO and Virgo see seems to roughly match the rate his model predicts. “I shouldn't toot my own horn—but I totally am going to,” he says. But he notes that because of the uncertainties, the data are also consistent with such a mechanism producing only one-quarter of the mergers. Analyses of all the events also show that when it comes to black holes, “the diversity is surprisingly large,” says Frank Ohme, a gravitational wave astronomer at the Max Planck Institute for Gravitational Physics. From details of the mergers' chirplike signals, scientists can calculate the masses of the colliding black holes. They expected to find a “mass gap” between about 45 and 135 solar masses—the result of particle physics processes that should blow apart stars within a certain mass range before they can collapse into black holes. However, LIGO and Virgo have spotted mergers involving black holes within the gap, including one weighing 85 solar masses. De Mink, who models the evolution of black hole pairs from binary star systems, says the mass gap is “such a clear prediction from the models that it's hard to believe” it's not there. Similarly, scientists expected another forbidden range below five solar masses, based not on theoretical modeling, but on observations of individual black holes peacefully orbiting normal stars. But one black hole falls below that limit. Researchers can even probe how the number of black hole mergers may have changed over cosmic time, Fishbach says. The new data show the rate 8 billion years ago was no more than 10 times what it is now, she says, tightening the limit by a factor of 10,000. Such data will challenge theorists as never before, says Ilya Mandel, a theoretical astrophysicist at Monash University. They can no longer simply whip up models to account for oddball events, but have to explain the entire set of observations. “When you have a whole population things become a lot harder,” Mandel says. “Your model not only has to reproduce the events that you do see, but also not predict events that you don't see.” The LIGO and Virgo teams owe their bounty to their detectors' ability to spot ever fainter, more distant events. And they want to keep adding to it. “We've answered a lot of questions we didn't even know we had,” Fishbach says, “but we raised even more. This is just the beginning of the science.”
领域	气候变化 ; 资源环境
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条目标识符	http://119.78.100.173/C666/handle/2XK7JSWQ/301975
专题	气候变化资源环境科学
推荐引用方式 GB/T 7714	Adrian Cho. Black holes by the dozens challenge theorists[J]. Science,2020.
APA	Adrian Cho.(2020).Black holes by the dozens challenge theorists.Science.
MLA	Adrian Cho."Black holes by the dozens challenge theorists".Science (2020).