BAM! Astroboffins now have a second way of picking up black holes' collision super kicks

Doppler detections will help test Einstein's key theory

Artist's view of a binary black hole. Pic credit: NASA, ESA and G Bacon (STScI)

Gravitational waves released from black hole “super kicks” may soon be detectable, according to new research published in Physical Review Letters.

Einstein realised that gravitational waves were a product of his theory of General Relativity, which found that spacetime behaved like a fabric. When objects with mass – such as stars, galaxies or black holes – moved through this fabric, they emit gravitational waves and send ripples through spacetime.

A century after his theory, gravitational waves were experimentally confirmed. In February, scientists working at the Laser Interferometer Gravitational-wave Observatory (LIGO) finally found the signal they were searching for.

Gravitational waves from the collision of two massive black holes – one was 35 solar masses and the other was 30 – were detected. But signals from black hole “super kicks” remain elusive.

Super kicks are violent events that happen when two rapidly spinning black holes collide with each other. These collisions are energetic events; when two black holes that are rotating slowly collide, the speed of the recoil is around 170 kilometres per second. Increasing the rate of spin greatly increases the impact of the collision and in cases where black holes that are spinning quickly collide, the recoil is so strong that remnants of the black holes are flung out of the galaxy and cast into the emptiness of space.

Physicists at the University of Cambridge have developed a new method to detect these super kicks, but it will require extremely high precision observations. LIGO is not sensitive enough and researchers will have to wait until the Evolved Laser Interferometer Space Antennae (eLISA) is launched into space in 2034.

Researchers will have to inspect the gravitational wave signal very closely for signs of Doppler shift.

The Doppler effect explains why the frequency of waves changes if the source of the waves (or the observer) is moving. When the wave source is moving towards the observer, or vice versa, the waves arrive at a relatively higher frequency as the distance between source and observer decreases. But when the source moves away, it takes longer for the waves to arrive – and so their relative frequency decreases. Think of how a police car siren sounds when it's driving towards you and how the sound changes once it's gone past.

The shift between short and long wavelengths will show if the gravitational waves from super kicks are being directed towards or away from Earth.

"If we can detect a Doppler shift in a gravitational wave from the merger of two black holes, what we're detecting is a black hole kick," said Davide Gerosa, co-author of the study and theoretical physics PhD student from the University of Cambridge. "And detecting a black hole kick would mean a direct observation that gravitational waves are carrying not just energy, but linear momentum as well."

The researchers believe that eLISA will be able to measure kicks as small as 500 kilometres per second. The measurements will unlock the secrets of the black holes involved, giving information on their spin and momentum. The results will give researchers more opportunities to study General Relativity, too. Although the century-old theory is well established, many questions remain open.

"We now have two ways of detecting black holes, instead of just one – it's amazing that just a few months ago, we couldn't say that. And with the future launch of new space-based gravitational wave detectors, we'll be able to look at gravitational waves on a galactic, rather than a stellar, scale," said Christopher Moore, co-author of the study and Cambridge PhD student who was also involved in the initial discovery of gravitational waves. ®


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