by When you make a purchase through links on our article, Future and its syndicate partners may earn a commission.
Astronomers could use a binary pair of small black holes in a cosmic game of “hide-and-seek” to hunt for much larger, even 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 have a one-way light-trapping boundary called an “event field” that ensures they do not emit any light. Even supermassive black holes at the heart of galaxies that are millions or billions of times the size of the sun are only “visible” if they are feasting on large amounts of surrounding matter or tearing apart an unfortunate star. .
However, light, or “electromagnetic radiation” as it is more accurately called, is only one type of radiation. Another is “gravitational radiation”, which comes in the form of tiny ripples that set up space-time hums called “gravitational waves”, which humanity is only just beginning to detect. That means instead of looking for supermassive black hole pairs in this game of hide and seek, astronomers can listen for them instead.
“Our idea basically works like listening to a radio channel. We plan to use the signal from a pair of small black holes similar to how radio waves carry the signal,” team leader Jakob Stegmann, a postdoctoral research fellow at the Max Institute Planck for Astrophysics, said in a statement. “Supermassive black holes are the music encoded in the frequency modulation (FM) of the detected signal.”
Related: Crazy! Several binary black holes may roll around each other in egg-shaped orbits
A little black hole singing soprano
Gravitational waves are a concept first proposed by Albert Einstein in general relativity, his 1915 magnum opus theory of gravity.
General relativity suggests that gravity arises when an object with mass “compacts” the fabric of space and time, previously unified by Einstein as a single four-dimensional entity (three spatial dimensions, one temporal dimension) called ” space-time.”
The greater the mass, the greater the end of the curve of space formed by an object. This explains why planets have a greater gravitational influence than Moons, why stars have a greater influence than planets, and why black holes have the greatest influence on any single object.
Einstein also predicted that when objects accelerate through space-time, they set up a “ring” of their fabric with ripples or gravitational waves. These are completely insignificant for objects with low masses, but when black holes are orbiting each other (remembering that circular motion is acceleration), they have enough mass to generate significant gravitational waves.
As these black holes orbit each other, they emit continuous low-frequency gravitational waves. These gravitational waves carry angular momentum (or spin), pushing the black holes together, a process known as “inspiration.” This increases the frequency of gravitational waves, causing angular momentum to be carried faster and faster.
That is until the black holes finally collide and merge, an event that sends out a higher frequency “scream” of gravitational waves.
However, Einstein predicted that these space-time leaks would be too small to ever be noticed, especially since they would lose energy as they propagate through the cosmos and black hole mergers occur millions or even billions of light-years away. .
Fortunately, we now know that Einstein was wrong.
Since the first gravitational wave signal was detected by the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015, resulting from a black hole binary merger 1.3 billion light years away, many such black hole collisions have been detected.
But these feelings have one thing in common. When they produced black holes, they were always pairs in the stellar mass black hole range, with masses between three and several hundred times the mass of the Sun. A supermassive black hole merger has not been associated with ground-based gravitational detectors such as LIGO and its VIRGO colleagues in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan.
Just as our ears evolved to hear certain frequencies of sound and not others, these instruments can only detect a certain frequency range of gravitational waves. The gravitational waves emitted by whirling pairs of supermassive black holes are too low a frequency to “hear.”
That is, with their gravitational waves, a stellar-mass binary sings a soprano, and a supermassive pair sings a baritone.
This team plans to detect the subtle change in gravitational waves from stellar-mass black hole binaries caused by the interference of gravitational waves from supermassive binaries.
These small modifications, then, could help reveal a supermassive black hole merger that can currently only be detected as a collective “background hum” using massive collections of rapidly spinning neutron stars on their called “pulsar time array.”
“The new aspect of this idea is to use high frequencies that are easy to detect to investigate lower frequencies that we are not yet sensitive to,” said Stegmann.
RELATED STORIES:
— How do some black holes get so big? The James Webb Space Telescope may have an answer
— The brightest quasar ever seen is powered by a black hole that eats ‘a sun a day’
— Record breaker! The Milky Way’s most monstrous stellar mass black hole is the sleeping giant lurking close to Earth (Video)
The proposal could also help guide the design of future gravitational detectors, such as NASA and the European Space Agency’s (ESA) space-based detector LISA (Laser Interferometer Space Antenna).
“Since the path for LISA is now set, following ESA’s takeover last January, the community needs to consider the best strategy for the next generation of gravitational detectors,” said a team member and University of Zurich black hole theorist Lucio Mayer. “In particular, what frequency range should they focus on – studies like this provide a strong incentive to prioritize deci-Hz [low-frequency] sensor design.”
The team’s research was published on Monday (August 5) in the journal Nature.
