When you buy through links in our articles, Future and its syndication partners may earn commission.
Binary pairings of small black holes could be used by astronomers in a cosmic game of hide-and-seek to hunt for much larger, but more elusive, supermassive black hole binaries. The technique could therefore help solve the mystery of how supermassive black holes grew so quickly in the early universe.
Detecting black holes is no easy task, despite their reputation as fearsome cosmic titans. All black holes are surrounded by a one-way light-trapping boundary called an “event horizon” that prevents them from emitting light. Even the supermassive black holes at the heart of galaxies, which have masses millions or billions of times that of the sun, are only “visible” if they are feeding on the vast amounts of matter around them or tearing apart an unfortunate star.
But light, or more correctly known as “electromagnetic radiation,” is just one type of radiation. Another is “gravitational radiation,” which comes in the form of tiny ripples called “gravitational waves” that hum through space-time, and humanity is only just beginning to detect them. That means instead of playing hide-and-seek to find pairs of supermassive black holes, astronomers can listen for them.
“Our idea basically works like listening to a radio channel. We propose to use the signal from pairs of small black holes in a similar way to how radio waves carry the signal,” team leader Jakob Stegmann, a postdoctoral researcher at the Max Planck Institute for Astrophysics, said in a statement. “Supermassive black holes are music that is encoded in the frequency modulation (FM) of the detected signal.”
Relating to: Cracking! Some binary black holes can orbit each other in egg-shaped orbits
Little black hole sings soprano
Gravitational waves are a concept first proposed by Albert Einstein in his theory of general relativity, published in 1915.
General relativity proposes that gravity arises when a massive object “bends” the fabric of space and time, which Einstein had previously combined into a single four-dimensional entity (three spatial dimensions, one time dimension) that he called “space-time”.
The greater the mass, the greater the extreme point of curvature of space created by an object. This explains why planets have a greater gravitational pull than moons, why stars have a greater pull than planets, and why black holes have the greatest pull of any object.
Einstein also predicted that when objects accelerate through spacetime, its fabric would “ring” with ripples or gravitational waves. These are completely negligible for low-mass objects, but when black holes orbit each other (remembering that circular motion is acceleration), they have enough mass to produce significant gravitational waves.
As these black holes orbit each other, they emit constant low-frequency gravitational waves. These gravitational waves carry away angular momentum (or spin) and force the black holes together, a process called “inspiralling.” This increases the frequency of the gravitational waves, causing the angular momentum to be carried away faster and faster.
Until the black holes collide and merge, emitting a higher-frequency “scream” of gravitational waves.
However, Einstein predicted that these space-time fluctuations would be so weak that they would never be detected, especially because they would lose energy as they propagate through the cosmos, and black hole mergers would occur millions or even billions of light-years away.
Fortunately, we now know that Einstein was wrong.
Numerous black hole collisions have been detected since the first gravitational wave signal was detected in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO), originating from a binary black hole merger 1.3 billion light-years away.
But these detections have one thing in common. In the case of black holes, they were always pairs in the stellar-mass black hole range, with masses between three and a few hundred times that of the Sun. Supermassive black hole mergers have been elusive to terrestrial gravitational wave detectors, such as LIGO and its counterparts VIRGO in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan.
Just as our ears evolved to hear sounds at certain frequencies and not others, these instruments can only detect gravitational waves within a certain range of frequencies. Gravitational waves emitted by pairs of spinning supermassive black holes are too low in frequency for terrestrial gravitational wave detectors to “hear.”
In other words, stellar-mass binaries with gravitational waves will sound soprano, while supermassive binaries will sound baritone.
The team proposes to detect subtle variations in gravitational waves from stellar-mass black hole binaries, which are caused by interfering gravitational waves from supermassive binary stars.
These small modulations could thus help reveal supermassive black hole mergers, which are currently only detectable as a collective “background hum” using large collections of rapidly rotating neutron stars called “pulsar timing arrays”.
“The novel aspect of this idea is to use the high frequencies, which are easy to detect, to probe the lower frequencies to which we are not yet sensitive,” Stegmann said.
RELATED STORIES:
— How do some black holes get so big? The James Webb Space Telescope may have an answer to that question
— Brightest quasar ever seen is powered by a black hole that ‘eats a sun a day’
—Record broken! The Milky Way’s scariest stellar-mass black hole is a sleeping giant lurking near Earth (Video)
The proposal could also guide the design of future gravitational wave detectors, such as NASA and the European Space Agency’s (ESA) space-based detector LISA (Laser Interferometer Space Antenna).
“With LISA’s path now set, following its acceptance by ESA last January, the community needs to evaluate the best strategy for the next generation of gravitational wave detectors,” said team member and University of Zurich black hole theorist Lucio Mayer. “In particular, which frequency range they should target – such studies provide strong motivation to prioritize a deci-Hz [low-frequency] “detector design.”
The team’s research was published in the journal Nature on Monday (August 5).