Gravitational waves detected again, at furthest distance yet

Black hole

More insight into black hole pairsSXS

"We're really moving from novelty to new observational science, a new astronomy of gravitational waves", said David Shoemaker, a senior researcher at MIT and the spokesman for the 1,000-scientist global LIGO collaboration.

The third detection has been described in a new paper accepted for publication in the journal Physical Review Letters.

As you might imagine, the crashing of black holes rank as some of the Universe's most powerful events, with the first merger seen by LIGO releasing in its final second an estimated 10 times more energy as gravitational waves than the amount of light energy pouring out of every star in the Universe. Two times the mass of Earth's sun was converted directly into energy in a fraction of a second.

The signal, from two merging black holes 3 billion light-years away, is the most distant observation yet.

If the two black holes are not spinning in the same way, that hints at the way they met.

Without the outward pressure generated by nuclear fusion to offset the inward pull of gravity, the core suddenly collapses as the star is blown apart.

"We're starting to gather real statistics on binary black hole systems", Keita Kawabe of Caltech said.

Notably, this new research also offers some insight into the directions in which black holes are spinning. One theory is that two neighboring stars each explode and produce two black holes, which then spiral inward.

In a typical pair of orbiting black holes, each object spins on its own axis, just like the planets in the Solar System. LIGO's team says the third case involved black holes that weighed 31.2 and 19.4 solar masses.

"We really need to be able to improve the sensitivity further to do that", she said. According to Einstein's theory of gravity, the general theory of relativity, massive objects bend the fabric of space and create ripples when they accelerate - for example, when two objects orbit one another. Black holes are regions so dense with matter that not even photons of light can escape their gravitational pull. The researchers looked for an effect called dispersion, which occurs when light waves in a physical medium such as glass travel at different speeds depending on their wavelength; this is how a prism creates a rainbow. As users gain more experience, they advance to more challenging levels containing glitches that sophisticated computer algorithms have deemed more hard to classify.

RIT members of the LIGO Scientific Collaboration include O'Shaughnessy, Campanelli, Lousto, Yosef Zlochower, John Whelan, Hans-Peter Bischof, James Healy, Brennan Ireland, Jacob Lange, Daniel Wysocki, Andrew Williamson, Yuanhao Zhang and Monica Rizzo. "This clearly establishes a new population of black holes not known before LIGO discovered them", says LIGO scientist Bangalore Sathyaprakash of Penn State and Cardiff University. "Normally, we don't think of the nothing of space as having any properties at all, so it's quite counter intuitive that it could expand or contract or vibrate". Landry compares it to stretching the canvas of a painting.

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"Coming from a field of looking for something rare, I've always been hesitant, with one detection only, to declare victory", Katsavounidis says.

So, what are gravitational waves?

Each LIGO observatory features a pair of 2.5-mile-long vacuum tubes arranged in an L shape in which precisely tuned laser beams flash back and forth between multiple mirrors that effectively increase the distance each beam travels to almost 1,000 miles.

For OzGrav's Deputy Director, Professor David McClelland, this latest discovery makes the impetus to continue work on upgrading the LIGO detector even more compelling.

Glitches appear to volunteers as pictures on the computer screen in a wide range of morphologies, or forms, and can mimic true gravitational-wave signals.

RIT scientists helped the collaboration measure and interpret black hole spins and their alignment. So any deviation from this constancy would therefore show up as a frequency-dependent distortion of gravitational-wave signals seen by LIGO.

And that's precisely what the LIGO researchers found in the three confirmed cases to date.

"It is remarkable that humans can put together a story, and test it, for such odd and extreme events that took place billions of years ago and billions of light-years distant from us", Shoemaker said in a statement. As they presented the latest results at a press conference, LIGO researchers said that they are working on the analysis of at least six other "candidate events", but declined to provide details. In terms of mass, that places this event in the middle of the two mergers that were identified during LIGO's previous run.

The new merger does have one key difference, however.

This 3-D projection of the Milky Way galaxy onto a transparent globe shows the probable locations of three black-hole merger events, plus a fourth possible detection at lower significance (LVT151012, green). Scientists think it's likely that one scenario is dominant in the universe and accounts for nearly all observed events, since it would be unusual for multiple scenarios to produce equal numbers of events in a fine-tuned balance.

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