On 29 June, four separate teams of scientists made an announcement1–4 that promises to shake up astrophysics: they had seen strong hints of very long gravitational waves warping the Galaxy.
Gravitational waves are ripples in the fabric of space-time that are generated when large masses accelerate. They were first detected in 2015, but the latest evidence hints at ‘monster’ ripples with wavelengths of 0.3 parsecs (1 light year) or more; the waves detected until now have wavelengths of tens to hundreds of kilometres.
Here Nature reports what these monster gravitational waves could mean for our understanding of the cosmos, and how the field could evolve.
How do the newly announced gravitational waves differ from those astronomers had already found?
Gravitational waves were first spotted by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington State. They sensed the ripples produced by two black holes spiralling into each other and merging. LIGO and its counterpart Virgo in Europe have since 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 the signals from arrays of dozens of the beacon stars. These pulsar timing arrays (PTAs) are sensitive to waves that are 0.3 parsecs long or more.
And whereas LIGO and Virgo spot evidence of the last stages of individual merger events — regularly spaced waves coming from one definite direction in the sky — the four PTA collaborations have so far found only a ‘stochastic background’, a constant jostling in random directions. This is comparable to the random sloshing of water on the surface of a pond caused by the rain.
What is the origin of the waves?
The most likely explanation for the stochastic background seen by PTAs is that it is produced by many 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 harbour one such monster black hole, with a mass millions or billions of times that of the Sun. And astronomers know that throughout the Universe’s history, many galaxies have merged. So, some galaxies must have ended up with two supermassive black holes, known as a black-hole binary.
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Researchers also have calculated that in the crowded centre of such a galactic merger, each black hole would transfer some of its momentum to surrounding stars, slinging them out at high speed or simply dragging them around. As a result, the two black holes would eventually slow down and end up orbiting each other at distances of around 1 parsec, explains Chiara Mingarelli, a gravitational-wave astrophysicist at Yale University in New Haven, Connecticut.
Only paired black holes that got much closer to each other than 1 parsec would contribute to the PTA signal, however. “They need to be separated by a milliparsec to emit detectable gravitational waves,” says Mingarelli. Theories that explain how this would happen are speculative, however, and whether the binaries can do this has been an open question, known as the final-parsec problem. “If you don’t overcome the final-parsec problem, then you don’t get any gravitational waves,” says Mingarelli.
Scientists will now seek to verify that the PTA signal does indeed come from binary supermassive black holes. If that could be confirmed, it would be evidence that supermassive black holes can come very close to each other in nature.
That result would 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 final-parsec problem. “It would be the discovery that such a population exists.”
What would such binary black holes mean for LISA, Europe’s planned space-based detector?
Supermassive-black-hole pairs that got close enough to emit gravitational waves would eventually collide and merge. That’s because the gravitational waves themselves would carry energy and momentum away from the black holes, turning their orbits into spirals. In hundreds to tens of thousands of years, each of the pairs would end up colliding.
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Colpi says 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 the black holes spiral inwards, the frequencies of their gravitational waves will increase and, in some cases, enter LISA’s spectrum of sensitivity. LISA will be sensitive to wavelengths of between 3 million km and 3 billion km — shorter than the wavelengths that can be detected by the PTAs, although still much longer than those seen by ground-based detectors. So LISA could see several of these mergers during its mission.
Black-hole mergers could also help to explain how some of the black holes have grown so large: they are themselves the result of earlier mergers.
Could something other than binary black holes be producing the stochastic background?
There is a plethora of exotic-physics theories that predict a similar omnidirectional background of waves coming from all directions in space. These sources could constitute part or even most of the signal. The possibilities include certain types of dark matter and even cosmic strings, hypothetical infinitesimally thin defects in the curvature of space-time. Cosmic strings could develop kinks, which could eventually snap, producing gravitational waves.
One of the most exciting alternative explanations is a cosmic gravitational-wave background originating from the early Universe, says Burke-Spolaor. Telescopes that see across the electromagnetic spectrum — from radio waves to γ-rays — are limited in how far away they can peek, and thus in how far into the past they can see. This is because, long before galaxies and stars existed, an opaque ionized gas filled the cosmos. This blocks astronomers’ view of what happened in the Universe during its first 400,000 years or so.
But gravitational waves can travel across any medium. As a result, any such waves created since the first instant after the Big Bang could still be around and be detectable as part of a stochastic background, providing a window into the extreme physics of the Big Bang. “That is just amazing to me,” says Burke-Spolaor. “Who knows what’s back there.”