After five detections over the last few years, gravitational waves are in danger of becoming boring news. However, none have been observed yet coming from supermassive black holes. This is because while bigger galaxies have bigger black holes, the mergers also occur fairly quickly.
Collision of two blackholes.
All of these events were picked up by ground-based facilities like the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) in the United States, or the Virgo facility in Italy. These observatories assemble their perception by disseminating lasers down long tunnels and exactly calculating minute deformities in the beam. Extreme care is taken to remove all possible interference, so that the only way the laser can be affected is when gravitational waves wash over it and physically warp the local fabric of spacetime.
The astronomers strategize to aim for ripples from supermassive black hole clashes using natural locater in space: pulsars, the "cosmic lighthouses" of the sky.
The researchers say pulsars are key to tapping into the songs of these intergalactic Barry Whites.
A pulsar is a rapidly rotating neutron star which emits electromagnetic signals. The regularity and strength of those signals has earned them the nickname "cosmic lighthouses", and programs are now using huge arrays of known pulsars to search for gravitational waves.
These pulsar timing arrays are based on the same principle as LIGO and Virgo. This in turn shortens the time required to observe gravitational waves. Importantly, this method could be more tuned towards lower frequency waves.
"They absolutely dwarf the black hole mergers detected by LIGO", said lead author Chiara Mingarelli, who is also a research fellow at Flatiron Institute's Center for Computational Astrophysics. "And since the pulsars we study are about 3,000 light-years away, they act as a galactic-scale gravitational-wave detector", she added. Using an array of well-known pulsars can help scientists detect even miniscule abnormalities, like gravitational waves from merging supermassive black holes.
Whenever two galaxies collide their dark supermassive hole spirals to form to merge with each other. To figure all this out, the team used data from the 2 Micron All-Sky Survey (2MASS), and combined it with galaxy merger rates pulled from the Illustris simulation project.
Out of the 5,000 galaxies that they studied, the scientists narrowed the list down to 90 pairs of supermassive black holes, the most massive of which are expected to merge within the next four million years. The most recent observation of a gravitational wave was generated by the merger of two neutron stars, which are the collapsed cores of large stars - they're the smallest and, at the same time, densest stars we know of.
In a research done by the NASA, it has been predicted that in the next ten years, scientists might be able to detect gravitational waves from at least one black hole binary.