At the center of every galaxy in the universe is a supermassive black hole, millions or billions of times larger
One way to see such mergers isemission of gravitational waves by them - ripples in the fabric of space and time. Distant merging pairs of supermassive black holes emit gravitational waves as they spiral around each other. Since black holes are large, each such wave travels past the Earth for many years. Astronomers have used a technique known as the pulsar time matrix to observe gravitational waves from supermassive binary black holes. But, alas, so far to no avail.
In parallel, astronomers were looking for a collisionsupermassive black holes using light. A number of candidates for light sources have been identified by searching for quasars. Quasars are very bright and are believed to be powered by the accumulation of gas clouds on supermassive black holes.
If there are two black holes in the center of the quasar,revolving around each other (instead of one black hole), orbital motion can change the accumulation of a gas cloud and lead to a periodic change in its brightness. Hundreds of candidates have been identified through this type of searches.
“If we can find a pair of mergingsupermassive black holes, this will not only tell us how galaxies evolved, but also show the expected signal strength of gravitational waves for observers of pulsars, ”the scientists explain.
OzGrav's research aims to resolve the controversy by determining whether any of the identified quasars could be powered by colliding black holes. Verdict? Probably no.
“We have developed a new method that allowssimultaneously search for a periodic signal and measure the noise properties of a quasar, ”explains Xinjiang Zhu from OzGrav. "Therefore, it must provide a reliable estimate of the statistical significance of the detected signal."
Applying this method to one of the mostknown candidate sources, named PG1302-102, researchers have found compelling evidence for periodic variation; however, they argued that the signal is likely to be more complex than existing models.
“The conventional model for quasar noise is wrong,” adds Zhu. "The data show additional features in the random fluctuations of gas accumulation in supermassive black holes."
"Our results show that quasars are complex"- explains co-author and principal investigator of OzGrav Eric Train. "We will need to improve our models if we are going to use them to identify supermassive binary black holes."
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