Gravitational waves can tell how black holes are born

Gravitational waves or ripples in space-time captured by space detectors could soon be used to discover when and how some of the universe's largest black holes were born, scientists say.
Scientists led by researchers at the Durham University in the UK ran the huge cosmological simulations that can be used to predict the rate at which gravitational waves caused by collisions between the monster black holes might be detected.
The amplitude and frequency of these waves could unveil the initial mass of the seeds from which the first black holes grew since they were formed 13 billion years ago and provide further clues about what caused them and where they formed, the researchers said.

The study combined simulations from the EAGLE project - which aims to create a realistic simulation of the known universe inside a computer - with a model to calculate gravitational wave signals.
Two detections of gravitational waves caused by collisions between supermassive black holes should be possible each year using space-based instruments such as the Evolved Laser Interferometer Space Antenna (eLISA) detector that is due to launch in 2034, the researchers said.
In February the international LIGO and Virgo collaborations announced that they had detected gravitational waves for the first time using ground-based instruments and this month reported a second detection.

As eLISA will be in space - and will be at least 250,000 times larger than detectors on Earth - it should be able to detect the much lower frequency gravitational waves caused by collisions between supermassive black holes that are up to a million times the mass of our Sun.

Current theories suggest that the seeds of these black holes were the result of either the growth and collapse of the first generation of stars in the universe; collisions between stars in dense stellar clusters; or the direct collapse of extremely massive stars in the early universe.
As each of these theories predicts different initial masses for the seeds of supermassive black hole seeds, the collisions would produce different gravitational wave signals.

This means that the potential detections by eLISA could help pinpoint the mechanism that helped create supermassive black holes and when in the history of the universe they formed.
"Understanding more about gravitational waves means that we can study the universe in an entirely different way," said lead author Jaime Salcido, PhD student in Durham University's Institute for Computational Cosmology.
"By combining the detection of gravitational waves with simulations we could ultimately work out when and how the first seeds of supermassive black holes formed," he said.
Gravitational waves were first predicted 100 years ago by Albert Einstein as part of his Theory of General Relativity.

The waves are concentric ripples caused by violent events in the universe that squeeze and stretch the fabric of space time but most are so weak they cannot be detected.