Scientists have discovered gravitational waves stemming from a black hole merger event that suggest the resultant black hole settled right into a stable, spherical shape. These waves also reveal the combo black hole could also be much larger than previously thought.
When initially detected on May 21, 2019, the gravitational wave event often called GW190521 was believed to have come from a merger between two black holes, one with a mass corresponding to just over 85 suns and the opposite with a mass corresponding to about 66 suns. Scientists believed the merger subsequently created an roughly 142 solar mass daughter black hole.
Yet, newly studied spacetime vibrations from the merger-created black hole, rippling outward because the void resolved right into a proper spherical shape, appear to suggest it’s more massive than initially predicted. Slightly than possess 142 solar masses, calculations say it must have a mass equal to around 250 times that of the sun.
These results could ultimately help scientists higher test general relativity, Albert Einstein‘s 1915 theory of gravity, which first introduced the concept of gravitational waves and black holes. “We’re really exploring a brand new frontier here,” Steven Giddings, a theoretical physicist on the University of California, said in an announcement.
Related: How dancing black holes get close enough to merge
Gravitational waves and general relativity
General relativity predicts that objects with mass warp the very fabric of space and time — united as a single, four-dimensional entity called “spacetime”” — and that “gravity” as we perceive it arises from the curvature itself.
Just as a bowling ball placed on a stretched rubber sheet causes a more extreme “dent” than a tennis ball would, a black hole causes more curvature in spacetime than a star does, and a star causes more curvature than a planet does. In reality, a black hole, usually relativity, is a degree of matter so dense it causes curvature of spacetime so extreme that, at a boundary called the event horizon, not even light is fast enough to flee the inward dent.
This is not the one revolutionary prediction of general relativity, nonetheless. Einstein also predicted that when objects speed up, they need to set the very fabric of spacetime ringing with ripples called gravitational waves. And again, the more massive the objects involved, the more extreme the phenomenon is. This implies when dense bodies like black holes spiral around each other, always accelerating because of their circular motion, spacetime rings around them like a struck bell, humming with gravitational waves.
These ripples in spacetime carry away angular momentum from the spiraling black holes, and that, in turn, causes the black holes’ mutual orbits to tighten, drawing them together and increasing the frequency of the gravitational waves emitted. Spiraling closer and closer, the black holes finally merge, making a daughter black hole and sending a high-frequency “chirp” of gravitational waves echoing out through the cosmos.
But there was one thing Einstein got unsuitable about gravitational waves. The nice physicist believed that these ripples in spacetime can be so faint that they might never be detected here on Earth after traveling across the universe for tens of millions, and even billions, of sunshine years.
Yet, in Sept. 2015, the dual detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) based in Washington and Louisiana showed Einstein was incorrect. They detected GW150914, gravitational waves related to merging black holes positioned around 1.3 billion light-years away. The gravitational wave signal was detected as a change within the length of one among LIGO’s 2.5 miles (4 kilometers) long laser arms, corresponding to a thousandth the width of a proton.
Remarkably, since then, LIGO and its fellow gravitational wave detectors, Virgo in Italy and KAGRA in Japan, have detected many more such events, reaching the purpose at which they’re detecting one gravitational wave event each week. Although, even amongst this cornucopia of gravitational wave detections, GW190521 stands out.
A special gravitational wave event
The merging frequency of the 2 black holes behind the GW190521 signal, that are positioned as distant as 8.8 billion light-years from Earth, was so low it was only throughout the final two orbits of the black holes that the frequency became high enough to achieve the sensitivity limits of LIGO and Virgo.
The team behind this latest investigation — which shouldn’t be a part of the LIGO/Virgo Collaboration — desired to know what information concerning the violent collision and merger of those black holes could also be locked away on this signal.
They found that the quick the black holes collided, the resultant black hole was created with a lopsided shape. Black holes are only stable once they have a spherical shape, meaning that inside milliseconds of the merger, the daughter black hole would need to assume the form of a sphere.
Just as the form of a bell determines the frequency at which it rings, the team said that as this latest black hole modified shape and stabilized, the frequencies of the gravitational waves it rang out were shifted. These so-called “ring down” gravitational waves contained information concerning the mass of the daughter black hole and in addition the speed at which it’s spinning.
Which means ring-down gravitational waves from such a merger offer scientists an alternate technique to measure the properties of merging black holes, in contrast to the normal approach to using the gravitational waves created throughout the spiraling process.
The team found two separate ring-down frequencies within the gravitational wave signal GW190521, which, when considered together, give the created black hole a mass of 250 solar masses. Meaning it’s considerably more massive than estimated by utilizing the spiraling gravitational waves. The detection of those ringdown gravitational waves was shocking even to the team behind these findings.
“I never thought I’d ever see such a measurement in my lifetime,” Badri Krishnan, co-author of the research and a physicist at Radboud University, said.
The team’s research is detailed in a paper published on Nov. 28 within the journal The Physical Review Letters.