Scientists have discovered gravitational waves arising from a merger of black holes, suggesting that the resulting black hole has settled into a stable, spherical shape. These waves also show that the combo black hole could be much larger than previously thought.
When the gravitational wave event known as GW190521 was first discovered on May 21, 2019, it was believed to be the result of a merger between two black holes, one with a mass equal to just over 85 suns and the other with a mass equal to about 66 suns. Scientists believed that the merger therefore created approximately 142 solar mass daughter black hole.
Yet newly studied spacetime vibrations of the black hole created by the merger, which ripples outward as the void dissolves into a proper spherical shape, seem to indicate that it is more massive than initially predicted. Instead of possessing a mass of 142 solar masses, calculations say it should have a mass equal to about 250 times that of the sun.
These results could ultimately help scientists test better general relativity, Albert Einstein‘s theory from 1915 gravity, who first introduced the concept of gravitational waves and black holes. “We’re really exploring a new frontier here,” says Steven Giddings, a theoretical physicist at the University of California. said in a statement.
Related: How dancing black holes get close enough to merge
Gravitational waves and general relativity
General relativity predicts that objects with mass distort their structure room and time – unified as a single, four-dimensional entity called “spacetime” – and that “gravity” as we perceive it comes from the curvature itself.
Just as a bowling ball placed on a stretched rubber sheet causes a more extreme “dent” than a tennis ball, a black hole causes more curvature in spacetime than a star, and a star causes more curvature than a planet. In fact, in general relativity, a black hole is a point of matter so dense that it causes a curvature of spacetime so extreme that, at a boundary event horizoneven light is not fast enough to escape the dent within.
However, this is not the only revolutionary prediction of general relativity. Einstein also predicted that when objects accelerate, they should ring the fabric of spacetime with so-called ripples gravitational waves. And again, the more massive the objects involved, the more extreme the phenomenon. This means that when dense bodies such as black holes orbit each other and continually accelerate due to their circular motion, the spacetime around them sounds like an excited bell, buzzing with gravitational waves.
These ripples in spacetime carry the angular momentum away from the spiral black holes, which in turn causes the black holes’ relative orbits to narrow, pulling them together and increasing the frequency of the emitted gravitational waves. The black holes move closer and eventually merge, creating a daughter black hole and sending out a high-frequency “chirp” of gravitational waves that reverberate through the cosmos.
But there was one thing Einstein got wrong about gravitational waves. The great physicist believed that these ripples in spacetime would be so faint that they would never be detected here. Soil after a crossing the universe for millions or even billions of light years.
Nevertheless, in September 2015 the twin detectors of the Laser interferometer gravitational wave observatory (LIGO), based in Washington and Louisiana, proved Einstein wrong. They discovered GW150914, gravitational waves associated with merging black holes at about 1.3 billion spots. light years away. The gravitational wave signal was detected as a change in the length of one of LIGO’s 4-kilometer-long laser arms, equivalent to one-thousandth of the width of a proton.
Remarkably, LIGO and its fellow gravitational wave detectors, Virgo in Italy and KAGRA in Japan, have since detected many more such events, reaching the point where they detect one gravitational wave event every week. Although, even among this plethora of gravitational wave detections, GW190521 stands out.
A special gravitational wave event
The merger frequency of the two black holes behind the GW190521 signal, located at a distance of as much as 8.8 billion light-years from Earth, was so low that the frequency only became high enough during the last two orbits of the black holes to reach the sensitivity limits of LIGO and Virgo.
The team behind this new research – which is not part of the LIGO/Virgo Collaboration – wanted to know what information about the violent collision and merger of these black holes might be hidden in this signal.
They found that when the black holes collided, the resulting black hole was created with a crooked shape. Black holes are only stable if they have a spherical shape, meaning that within milliseconds of the merger, the daughter black hole should take the shape of a sphere.
Just as the shape of a bell determines the frequency at which it sounds, the team said that as this new black hole changed shape and stabilized, the frequencies of the gravitational waves it emitted shifted. These so-called ‘ring down’ gravitational waves contain information about the mass of the daughter black hole as well as the speed at which it is spinning.
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This means that gravitational waves from such a merger provide scientists with an alternative way to measure the properties of merging black holes, as opposed to the traditional method of using the gravitational waves created during the spiraling process.
The team found two separate ring-down frequencies in the gravitational wave signal GW190521, which, when considered together, give the created black hole a mass of 250 solar masses. That means it is significantly more massive than estimated from the spiral gravitational waves. The detection of these gravitational waves was shocking even to the team behind these findings.
“I never thought I would ever experience such a measurement in my life,” says Badri Krishnan, co-author of the study and a physicist at Radboud University.
The team’s research is detailed in an article published Nov. 28 in the journal The physical assessment letters.