Two Supermassive Black Holes Will Shake Space & Time When They Merge. New Study Explains Why

Locked in a cosmic dance, two supermassive black holes, nine billion light years away from Earth, appear to be orbiting each other every two years.

Merging Black Holes (Photo: Getty Images)

New Delhi: Locked in a cosmic dance, two supermassive black holes, nine billion light years away from Earth, appear to be orbiting each other every two years. The two giant cosmic objects have masses that are hundreds of millions of times larger than that of our sun, and are separated by a distance roughly 50 times that which separates the Sun and Pluto.

When the black holes merge in roughly 10,000 years, the titanic collision is expected to shake space and time itself, according to a new study. When they do so, they will send gravitational waves across the universe.

The study, led by astronomers at California Institute of Technology, United States, was recently published in The Astrophysical Journal Letters.

Black Holes Observed In A Blazar

The researchers have discovered evidence for this scenario taking place within a fiercely energetic object known as a quasar, the study said. Quasars are active cores of galaxies in which a supermassive black hole is siphoning material from a disc encircling it, and in some quasars, the supermassive black hole creates a jet which shoots out near the speed of light.

In the new study, the astronomers observed a quasar designated as PKS 2131-021. This quasar belongs to a subclass of quasars called blazars, in which the jet is pointing toward the Earth. 

Though it was already known that quasars could possess two orbiting supermassive black holes, finding direct evidence for this has proved difficult. 

Why Is The New Pair Of Black Holes Special?

The authors argued in the study that  PKS 2131-021 is now the second known candidate for a pair of supermassive black holes caught in the act of merging. While it takes the PKS 2131-021 pair two years to complete an orbit, the first candidate pair of supermassive black holes caught in the act of merging orbit each other at greater distances, the study said. The first candidate pair is part of a quasar called OJ 287, and the two black holes circle each other every nine years.

Researchers have performed radio observations of PKS 2131-021 for more than 45 years. The astronomers said in the study that a powerful jet emanating from one of the two black holes within PKS 2131-021 is shifting back and forth due to the pair's orbital motion. As a result, periodic changes are caused in the quasar's radio-light brightness. 

As many as five different observatories registered these oscillations. By combining the radio data, the astronomers obtained a newly perfect sinusoidal light curve. This was unlike anything observed from quasars before, the study said.

Sandra O'Neill, lead author of the study, said that when the astronomers realised that the peaks and troughs of the light curve detected from the recent times matched the peaks and troughs observed between 1975 and 1983, they know something "very special" was going on, according to a statement issued by California Institute of Technology.

What Happens When Monstrous Black Holes Merge?

Most galaxies, including our Milky Way galaxy, possess monstrous black holes at their cores. The study said that when galaxies merge, their black holes "sink" to the middle of the newly formed galaxy. The black holes eventually join together to form an even more massive black hole.

Black holes increasingly disturb the fabric of space and time as they spiral towards each other, the study said. This sends out gravitational waves, which Albert Einstein had first predicted more than 100 years ago.

The National Science Foundation's LIGO (Laser Interferometer Gravitational-Wave Observatory), managed jointly by California Institute of Technology and Massachusetts Institute of Technology (MIT), detects gravitational waves from pairs of black holes up to dozens of times the mass of our Sun. 

The study said that the supermassive black holes at the centres of galaxies are millions to billions of times the mass of the Sun, and give off lower frequencies of gravitational waves than those detected by LIGO.

Detecting Merging Black Holes With Masses Millions Of Times That Of The Sun

The authors noted in the study that pulsar timing arrays, which consist of an array of pulsing dead stars precisely monitored by radio telescopes, should be able to detect the gravitational waves from supermassive black holes of this heft in the future. 

Merging black holes whose masses are 1,000 to 10 million times greater than the mass of our Sun would be detected by the upcoming Laser Interferometer Space Antenna (LISA) mission. 

The study said that no gravitational waves have been registered from any of these heavier sources so far. PKS 2131-021 provides the most promising target yet, the study said.

The best option to detect coalescing supermassive black holes are light waves.

OJ 287, the quasar harbouring the first candidate pair of supermassive black holes caught in the act of merging, also exhibits periodic radio-light variations, the study said.

The black holes within PKS 2131-021 orbit each other every two years and are 2,000 astronomical units apart. This is about 50 times the distance between the Sun and Pluto. One astronomical unit is the distance between Earth and the Sun.

The supermassive black holes in PKS 2131-021 are 10 to 100 times closer than the pair in OJ 287.

Sebastian Kiehlmann, the study's second author, said that their "study provides a blueprint for how to search for such blazar binaries in the future".