Gravitational waves detected again, at furthest distance yet

LIGO spots a third black hole merger, tightens mass limits on gravitons

The Laser Interferometer Gravitational-Wave Observatory, more commonly known as LIGO, announced Thursday that it recently made its third detection of gravitational waves.

"We have further confirmation of the existence of stellar-mass black holes that are larger than 20 solar masses-these are objects we didn't know existed before LIGO detected them", says MIT's David Shoemaker, the newly elected spokesperson for the LIGO Scientific Collaboration (LSC), a body of more than 1,000 worldwide scientists who perform LIGO research together with the European-based Virgo Collaboration.

There was an issue with one detector having a calibration set wrong in Hanover, he said, so no alarm went off. Lett., doi: 10.1103/PhysRevLett.118.221101). In this case, the black holes can spin in any direction relative to their orbital motion. The pipeline takes the noisy data collected by the LIGO detector and processes it to estimate the precise shape of the gravitational wave signal, independent of any preconceived ideas of what those signals should look like. With this, LIGO is starting to create a new branch of science: gravitational wave astronomy.

Scientists will need more data to sort out how the black hole duos form, says physicist Emanuele Berti of the University of MS in Oxford. "A new astronomy of gravitational waves".

The Laser Interferometer Gravitational-wave Observatory (LIGO) in the United States made the detection on January 4 this year, demonstrating that a new window in astronomy has been firmly opened.

A young German scientist - part of the 1,000 who make up the LIGO Scientific Collaboration - saw it "and got us up in the middle of the night, which we love to do".

Meanwhile, the high rate of mergers disfavored a third scenario called "dynamical formation", which has the black holes forming far apart inside a dense stellar cluster.

In all three discoveries, LIGO detected gravitational waves from the energetic mergers of black hole pairs. LIGO's second detection featured two smaller black holes, 14 and eight times the mass of the sun (SN: 7/9/16, p. 8). As pairs of black holes spiral around each other, they also spin like a pair of ice skaters spinning individually while circling around each other. That leads, she said, to a delay known as the "hangup effect" that can be seen in the gravitational waveform.

This is an important detail that can teach us more about the dynamics of our universe, and so far we can only detect the spins of black holes using the brand new technique of gravitational wave measurements.

Simultaneously, the detection does not favour the competing theory according to which binary black holes form in pairs even at the start and eventually merge.

"The new event also provides new opportunities to test Einstein's theory of general relativity". Gravitational waves stretch and then contract spacetime as they pass, just as ripples on a pond move the surface up and then down.

Further, the LIGO-Virgo team was able, through the new observation, to put tighter constraints on the mass of the graviton, the hypothesized particle that supposedly mediates the gravitational force (analogous to the way that the photon, for example, mediates electromagnetic force).

"I think most people would be surprised if general relativity was 100 percent correct, but there's no evidence that it's not yet".

Any dispersion would also imply that the graviton, a theoretical particle that mediates gravitational attraction, would have mass.

The new observation grew out of improvements to LIGO's engineering sensitivity after its last observational run, which concluded in January 2016.

LIGO's next big breakthrough is expected to come from detecting collisions of binary neutron stars - the corpses of dead stars that pack a sun's worth of mass into a city-sized sphere. The new detection, called GW170104, occurred during LIGO's current observation period that began November 30, 2016, and concluded in early May, according to a news release about the discovery issued through LSU. Since the LIGO detectors run constantly, for weeks at a time, the data feed is continually updated. "We now have a second candidate in the "heavy black hole" category", said physicist and LIGO researcher Bangalore Sathyaprakash of Penn State and Cardiff University.

The LIGO-Virgo consortium believes that the addition of the Virgo instrument, slated to come online this summer, could substantially improve the ability to localize gravitational-wave sources in space. The observations will let scientists test theories under conditions that could never be recreated in a lab.

Within tens of seconds, LIGO's search algorithms automatically analyzed the signal, comparing it to waveforms characteristic of gravitational waves.

And where there are gravitational waves, there are black holes, based on the findings at the observatory in rural Livingston Parish. That makes it tough to tie a given LIGO detection event to the pinpoint observations of traditional telescopes. The leading theory is that dark matter is made up of exotic particles created soon after the big bang. More importantly, each detection has offered much more than just a "sighting".


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