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A double whammy of gravitational waves sparks a burst of black hole speculation

Alan Boyle
Artwork shows the merger of two black holes, which gives off a burst of gravitational waves.  (SXS Graphic)

Two detections of gravitational waves, separated by a mere 21 minutes, set off a flurry of excitement among astronomers today.

Was it a binary black-hole merger? A double observation of a single black-hole merger, created by gravitational lensing effects? A glitch affecting the analytical systems at the world’s gravitational-wave detectors? Or merely a coincidence of cosmic proportions?

“This is a genuine ‘Uh, wait, what?’ We’ve never seen that before…….’ moment in gravitational wave astronomy,” Robert Rutledge, a physicist at McGill University, tweeted today. “If you’d like to see how double-checks and confirmations and conclusions occur – pay attention, in real time. Happening now.”

As the hours ticked by, the most intriguing likelihood – that the ripples in spacetime were warped and refocused by a powerful gravitational field in accordance with general relativity – appeared to become much less likely.

A closer look at the data for the events, known as S190828j and S190828l, suggested that they did not emanate from the same location on the sky, as was earlier thought.

“The sky localizations are similar, but distinct,” Northwestern University Christopher Berry, a member of the LIGO Scientific Collaboration, said in a follow-up tweet. He noted that “you might expect them to overlap for a gravitationally lensed signal, but that doesn’t seem to be the case.”

If the astronomers sound tentative about all this, it’s because they’re working on a daily basis to make sense of results that become public almost as soon as they’re spit out by the twin detectors of the Laser Interferometer Gravitational-wave Observatory, or LIGO, at Hanford, Wash., and Livingston, La., and by the Virgo detector in Italy.

The quick turnaround helps facilitate follow-up observations in electromagnetic wavelengths, such as the telescope sightings that led to the first-ever detection of a neutron star collision in 2017. But they also fuel real-time speculation about black-hole bizarreness.

For example, astronomers oohed and ahhed earlier this month over readings that hinted at the detection of a black hole in the process of gobbling up a neutron star. That was also an “Uh, wait, what?” moment crying out for confirmation. The findings won’t be officially published unless they survive the peer-review process.

Yet another wave of speculation has arisen over the seeming detection of a collision involving a black hole with a mass up to 100 times heavier than our sun.

Some physicists say black holes shouldn’t exist in that mass range, due to the way stars of particular sizes blow up when they die. There should be “no black holes, not even a few” of that size, Quanta Magazine’s Natalie Wolchover quotes Stan Woolsey, an astrophysicist at the University of California at Santa Cruz, as saying.

“But of course we know nature often finds a way,” Woolsey added.

Will LIGO and Virgo reveal new ways in which nature finds a way? Based on the Nobel-winning discoveries we’ve already seen on the gravitational-wave front, that’s virtually certain to be the case, whether or not those new ways happen to involve S190828j and S190828l.

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