Scientists with the Laser Interferometer Gravitational-wave Observatory, or LIGO, have detected the signal from a cataclysmic collision between two black holes that lie 3 billion light-years away – much farther than the previous two discoveries.
The findings, described in a paper accepted to Physical Review Letters, cement the idea that gravitational-wave astronomy – a whole new way 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's David Shoemaker, spokesman for the LIGO scientific collaboration.
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 as two black holes, spinning around slowly toward one another, finally succumbed to each other's gravitational tug – and merged. The collision resulted in the creation of a new, single black hole.
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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 one another. A second soon followed. With the third 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. And 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 light-years away–much farther than the first two finds (which lay around 1.3 and 1.4 billion light-years from us, respectively). The two black holes appear to have held 31.2 and 19.4 solar masses respectively, and when they coalesced the new singularity weighed in at about 49 solar masses.
This puts the merger right in the middle of the same weight class as the previous two black hole mergers – a class that scientists had not 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 were supermassive, holding millions or even billions of solar masses, and anchored the hearts of galaxies (just as one does at the center of our Milky Way). Many LIGO researchers thought they'd start to see some of those smaller singularities.
These intermediate black holes, however, are starting to look increasingly common.
"It clearly establishes a new population of black holes that were not known before LIGO discovered them," said LIGO scientific collaboration 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. This is consistent with one theory of their formation, 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 with one another – favoring another theory that says the black holes may actually 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 the question is how big each slice is. The answer could help scientists understand the complexities of both stellar and black hole formation.
The findings also allowed scientists to probe the limits of Einstein's theory of general relativity further by looking to see whether the gravitational waves underwent dispersion – a bending of the wavelengths that happens when a wave passes through a physical medium. Einstein's theories forbid this from happening to gravitational waves, and so far LIGO's measurements have yet to contradict them.
Scientists hope to eventually see more than just black hole mergers, Reitze said. The next big class of events would be binary neutron star mergers – in part because these events could 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 set to go online. With a little bit of overlap between the two runs, and a little bit of luck, the two detectors just might be able to see the same events – which would allow scientists to get even better measurements of these violent cosmic phenomena.