Another black hole smash-up
With the third such discovery, scientists declare gravitational wave astronomy is here to stay.
Scientists on the lookout for subtle disturbances in the fabric of space-time have detected the signal from a cataclysmic collision between two black holes that lie some 3 billion lightyears away, much farther two previous discoveries.
The findings by the team working with the Laser Interferometer Gravitational-Wave Observatory, or LIGO, cement the idea that gravitational wave astronomy — a cutting-edge tool to observe some of the most powerful events in the universe — is here to stay.
“We’re really moving from novelty to new observational science — a new astronomy of gravitational waves,” said MIT senior research scientist David Shoemaker, spokesman for the LIGO team.
Astronomers typically document the universe in different wavelengths of light, from visible and infrared all the way to X-rays and gamma rays.
But black holes do not emit light as far as we know, which makes them very difficult to study. By picking up deformations in spacetime, LIGO allows scientists to “hear” these mysterious phenomena, even if they can’t see them with telescopes.
The new signal, called GW170104, was picked up in the early morning hours of Jan. 4 by the twin L-shaped detectors in Hanford, Wash., and Livingston, La. The ripple was triggered after two black holes, spinning around slowly toward each other, finally succumbed to their shared gravitational tug and merged. The powerful collision resulted in the creation of a new, single black hole — and converted mass into gravitational waves in the process.
Gravitational waves are ripples in the fabric of space-time, caused by objects accelerating or decelerating through space. Their existence was predicted more than a century ago by Albert Einstein as part of his general theory of relativity, but they were thought to be so faint as to be virtually undetectable.
LIGO changed that. Last year, the collaboration announced that its twin detectors had picked up a passing distortion in late 2015 caused by two black holes crashing into each other. The resulting gravitational waves stretched one leg of each detector and squeezed the other, briefly changing their lengths and triggering a laser signal.
A second event soon followed. With the third confirmed find, announced Thursday, scientists are finally moving LIGO’s work from the examination of singular curiosities to demographic studies of the sky’s invisible denizens. Already, this third discovery is revealing that there may be some diversity in this mysterious cosmic population.
This merger between a binary pair of black holes happened around 3 billion years ago, at a distance more than double that of the first two finds (which occurred around 1.3 billion and 1.4 billion light-years from us, respectively).
Scientists believe one of the black holes held as much mass as 31.2 suns, while the other held 19.4 solar masses. When they coalesced, the new singularity weighed in at 48.7 solar masses, and the remaining mass was transformed into gravitational waves.
This puts the merger right in the middle of the same weight class as the two previously detected mergers — a class that scientists had not originally expected to encounter.
Most black holes, they had figured, were the corpses of dead stars and significantly smaller, on the order of a few times the mass of the sun. Others, the kind that anchored the hearts of the Milky Way and other galaxies, were supermassive, holding millions or even billions of solar masses.
These intermediate black holes, however, are starting to look rather common.
“It clearly establishes a new population of black holes,” said LIGO team member Bangalore Sathyaprakash of Penn State and Cardiff University.
The new merger does have one key difference, however. In the previous two events, the paired black holes seemed to have spins that were aligned with their orbital axis. This is consistent with one theory about how they were formed, which assumes that the stars that became these black holes are born, and die, in pairs.
But in the new find, the black holes’ spins were apparently not aligned. That would favor a competing theory that says the black holes may pair up much later in their life histories.
Both theories may explain a slice of the black hole binary population, said LIGO Executive Director David Reitze of Caltech. But how big is each slice? The answer could help scientists understand the complexities of both stellar and black hole formation.
The findings, described in a paper accepted to Physical Review Letters, also allowed scientists to investigate the limits of Einstein’s theory of general relativity by looking to see whether the gravitational waves underwent dispersion — a bending of the different wavelengths of light that happens when light passes through a physical medium. This is why white light splits into a rainbow of colors when it passes through a prism.
Einstein’s theories forbid this from happening to gravitational waves, and LIGO’s measurements have yet to contradict them.
For now, the LIGO team cannot localize where these black holes merge. But as more detectors come online in Europe, Japan and India, researchers will be better able to triangulate the sources.
Once that happens, scientists will be able to train their telescopes on these targets. They might be able to catch signals they had not previously known were related to black hole activity. (Though light cannot escape from a black hole once it passes the event horizon, black holes can be detected thanks in part to the superheated matter that collects around them.)
Scientists hope to eventually see more than just black hole mergers, Reitze said. The next big class of events would be the mergers of binary neutron stars, which could definitely be seen with both LIGO and traditional telescopes.
In the meantime, LIGO is set to wrap up its current observing run in late summer, right around the time that the European Virgo detector is expected to go online.
With a little bit of overlap between the two runs — and a little bit of luck — the two detectors might be able to see the same events. If so, it would allow scientists to get even better measurements of these violent cosmic phenomena.
Eventually, scientists might expect to catch a gravitational event once or twice a week, or perhaps even on a daily basis. But for now, each one is a thrill, said Marc Kamionkowski, a theoretical physicist at Johns Hopkins University who was not involved with the LIGO work.
“Five or 10 years from now, we’re going to have another event discovered, and then I’ll be, like, ‘Oh, yeah, another gravitational wave event,’ ” Kamionkowski said.
“But I’m still amazed every time they discover every one of these things. The glow from last year is still there.”