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Astrophysicists Predict Gravitational Wave Strength From Merging Supermassive Black Holes

An artist’s impression of two black holes about to collide and merge.

Gravitational waves are ripples in the curvature of spacetime caused by accelerated masses that propagate as waves at the speed of light outward from their source. Although you don’t need a gigantic object to create gravitation waves, our instruments are only able to detect those generated by extreme acceleration of very massive objects, such as the binary orbit of black holes.

A massive black hole sits at the center of most galaxies, such as Sagittarius A* at the center of the Milky WayThe Milky Way is the galaxy that contains the Earth, and is named for its appearance from Earth. It is a barred spiral galaxy that contains an estimated 100-400 billion stars and has a diameter between 150,000 and 200,000 light-years.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Milky Way. These black holes are very heavy – their mass can be from a million to over a billion times the mass of the Sun and, as such, are appropriately known as supermassive black holes.

As galaxies move through the Universe, they will occasionally merge. When this happens, the supermassive black holes they host tend to migrate toward each other and form a binary system. As these two black holes orbit each other, they warp the fabric of space and time around them and produce gravitational wavesGravitational waves are distortions or ripples in the fabric of space and time. They were first detected in 2015 by the Advanced LIGO detectors and are produced by catastrophic events such as colliding black holes, supernovae, or merging neutron stars.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>gravitational waves which ripple out into the Universe. These gravitational waves complete one full oscillation every year or so as they travel through space and are classified as low-frequency gravitational waves.

The Universe is full of these supermassive black holeA black hole is a place in space where the pull of gravity is so strong not even light can escape it. Astronomers classify black holes into three categories by size: miniature, stellar, and supermassive black holes. Miniature black holes could have a mass smaller than our Sun and supermassive black holes could have a mass equivalent to billions of our Sun.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>black hole binary systems, and the gravitational waves they emit fill space, combining to form something known as the stochastic gravitational wave background. Scientists are trying to find a gravitational wave signal from this background using a complex network of radio telescopes called a pulsar timing array but it could be years before there is a confirmed detection.

Scientists Predict Gravitational Waves From Merging Supermassive Black Holes

Scientists predict gravitational waves from merging supermassive black holes. Credit: Carl Knox, OzGrav-Swinburne University

For this reason, cosmological simulations are often used to predict what this gravitational wave signal could look like. This type of simulation helps scientists understand the structure and history of the Universe by tracking the flow of matter and energy from a time soon after the Big BangThe Big Bang is the leading cosmological model explaining how the universe as we know it began roughly 13.8 billion years ago.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>Big Bang, up until today.
A team of researchers led by postgraduate researcher Bailey Sykes (from Monash University), alongside several OzGrav scientists, including OzGrav Associate Investigator Dr. Hannah Middleton, have recently made a new prediction for the strength of this gravitational wave signal. The new estimate is based on data from the MassiveBlack-II simulation, which simulates a massive region of space similar to a chunk of our own Universe.

The team made two estimates: one in which the supermassive black holes merge almost instantly once their host galaxies collide, and another in which the two black holes take time to sink towards each other once they pair up in a binary system. This second estimate is important as the gravitational wave output of a binary can change during this time due to the interactions of stars and gas nearby the supermassive binary.

The simulated gravitational wave signal using MassiveBlack-II is similar to other predictions in previous studies. It’s smaller than a signal currently detectable by pulsarFirst observed at radio frequencies, a pulsar is a rotating neutron star that emits regular pulses of radiation. Astronomers developed three categories for pulsars: accretion-powered pulsars, rotation-powered pulsars, and nuclear-powered pulsars; and have since observed them at X-ray, optical, and gamma-ray energies.” data-gt-translate-attributes=”[{“attribute”:”data-cmtooltip”, “format”:”html”}]”>pulsar timing arrays; however, as the sensitivity of telescope technology increases over time, it’s possible a confirmed detection could be just around the corner.

The results from the study add valuable insights to existing signal predictions and provide an important reference point for future pulsar timing arrays. Progressively more accurate estimates of the stochastic gravitational wave background can be used to further understand other astrophysical phenomena, including the interactions of stars and gas which impact merging supermassive black holes.


“An estimate of the stochastic gravitational wave background from the MassiveBlackII simulation” by Bailey Sykes, Hannah Middleton, Andrew Melatos, Tiziana Di Matteo, Colin DeGraf and Aklant Bhowmick, 14 February 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac388

“The MassiveBlack-II simulation: the evolution of haloes and galaxies to z ∼ 0” by Nishikanta Khandai, Tiziana Di Matteo, Rupert Croft, Stephen Wilkins, Yu Feng, Evan Tucker, Colin DeGraf and Mao-Sheng Liu, 24 April 2015, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stv627

Source: SciTechDaily