On 29 June, four separate groups of scientists made the announcement1–4 that promises to shake up astrophysics: they found strong hints of long gravitational waves biasing the Galaxy.
Gravitational waves are ripples in the fabric of space-time that are created when large masses accelerate. They were first detected in 2015, but the latest evidence indicates a ‘monster’ ripple with a wavelength of 0.3 parsec (1 light year) or more; the waves detected so far have wavelengths of tens to hundreds of kilometers.
Here Nature reports what this monster gravitational wave might mean for our understanding of the cosmos, and how the field might evolve.
How do the newly revealed gravitational waves differ from those already discovered by astronomers?
Gravitational waves were first detected by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington State. They felt the ripples created by the two black holes spinning around each other and merging. LIGO and its European counterpart Virgo have reported dozens of similar events.
For the latest results, the authors relied on special beacon stars called millisecond pulsars. The teams tracked changes over more than a decade in the distances between Earth and millisecond pulsars in the Milky Way, comparing signals from an array of dozens of beacon stars. These pulsar timing arrays (PTAs) are sensitive to waves 0.3 parsec long or longer.
And while LIGO and Virgo see evidence of the final stages of individual merger events — regularly spaced waves coming from a specific direction across the sky — the four PTA collaborations have so far seen only a ‘stochastic background’, a constant scramble in random directions. It can be compared to the random falling of water on the surface of a lake caused by rain.
What is the source of waves?
The most likely explanation for the stochastic background seen by PTAs is that it is produced by multiple pairs of supermassive black holes orbiting each other in the hearts of distant galaxies, says Sarah Burke- Spolaor, an astrophysicist at West Virginia University in Morgantown.
Most galaxies are thought to contain a monstrous black hole, with a mass millions or billions of times that of the Sun. And astronomers know that throughout the history of the Universe, many galaxies have merged. Thus, some galaxies must have ended up with two massive black holes, known as a black-hole binary.
Monster gravitational waves have been detected for the first time
The researchers also calculated that in the tight center of such a galactic merger, each black hole would transfer some of its momentum to the surrounding stars, slinging them at high speeds or simply dragging them along. . As a result, the two black holes eventually slow down and end up orbiting each other at distances of about 1 parsec, explains Chiara Minngarelli, a gravitational-wave astrophysicist at Yale University in New Haven, Connecticut. .
However, paired black holes closer to each other than 1 parsec can contribute to the PTA signal. “They have to be separated by a milliparsec to emit visible gravitational waves,” Mingarelli said. Theories explaining how this happens are speculative, however, and whether binaries can do this is an open question, known as the final-parsec problem. “If you don’t get past the final-parsec problem, you won’t receive any gravitational waves,” Mingarelli said.
Scientists will now seek to verify that the PTA signal is indeed coming from a binary supermassive black hole. If that is confirmed, it would be evidence that supermassive black holes can be close to each other in nature.
That result will be of fundamental importance, says Monica Colpi, an astrophysicist at the University of Milan-Bicocca in Italy – showing that thousands of black-hole binaries across the Universe have somehow ‘solved’ the ultimate parsec problem. “This is the discovery that such a population exists.”
What does a binary black hole mean for LISA, Europe’s planned space-based detector?
Supermassive-black-hole pairs that are about to emit gravitational waves will collide and merge. That’s because gravitational waves themselves will carry energy and momentum away from black holes, turning their orbits into spirals. In hundreds to tens of thousands of years, each of the pairs will end up colliding.
How gravitational waves can solve some of the Universe’s deepest mysteries
Colpi said this could be good news for the Laser Interferometer Space Antenna (LISA), a trio of probes the European Space Agency plans to launch in the mid-2030s.
As black holes rotate inward, the frequencies of their gravitational waves will increase and, in some cases, enter LISA’s sensitivity spectrum. LISA will be sensitive to wavelengths between 3 million km and 3 billion km — shorter than the wavelengths that PTAs can detect, though still longer than what ground-based detectors can detect. So LISA will see some of these mergers during its mission.
Black-hole mergers also help explain how some black holes have grown so large: they are themselves the result of earlier mergers.
Is there anything else in binary black holes that produces a stochastic background?
There are a plethora of strange physics theories that predict a similar omnidirectional background of waves coming from all directions in space. These sources may make up part or even the majority of the signal. Possibilities include some form of dark matter and even cosmic strings, hypothetical infinitesimally thin defects in the curvature of space-time. Cosmic strings can develop kinks, which can eventually burst, producing gravitational waves.
One of the most exciting alternative explanations is a cosmic gravitational-wave background that originated in the early Universe, Burke-Spolaor said. Telescopes that see across the electromagnetic spectrum — from radio waves to γ-rays — are limited in how far they can peer, and thus how far into the past they can see. This is because, before galaxies and stars existed, the cosmos was filled with opaque ionized gas. This blocks astronomers’ view of what happened in the Universe during its first 400,000 years or so.
But gravitational waves can travel in any medium. As a result, any such waves created since the first moment after the Big Bang can still be present and detected as part of a stochastic background, providing a window into the extreme physics of the Big Bang. “That’s amazing to me,” Burke-Spolaor said. “Who knows what’s back there.”
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