Colliding Black Holes Bring Hope for Gravitational Astronomy

An international team of astronomers have come accross solutions to decades-old equations describing what happens as two spinning black holes in a binary system orbit each other and spiral in toward a collision.

The results should have significantly impact not only on the study of black holes, but also the search for elusive gravitational waves, a type of radiation predicted by Einstein’s theory of general relativity, in the cosmos.

Unlike planets, whose average distance from the sun does not change over time, general relativity predicts that two black holes orbiting around each other will move closer together as the system emits gravitational waves.

According to lead author Dr Michael Kesden of the University of Texas at Dallas:

Merging Black Holes

The energy lost to gravitational waves causes the black holes to spiral closer and closer together until they merge, which is the most energetic event in the universe, after the big bang. That energy, rather than going out as visible light, which is easy to see, goes out as gravitational waves, which are much more difficult to detect.

While Einstein’s theories predict the existence of gravitational waves, they have not been directly detected. But the ability to ‘see’ gravitational waves would open up a new window to view and study the universe.

Optical telescopes can capture photos of visible objects, such as stars and planets, and radio and infrared telescopes can reveal additional information about invisible energetic events. Gravitational waves would provide a qualitatively new medium through which to examine astrophysical phenomena.

Says co-author and PhD student Davide Gerosa, of Cambridge’s Department of Applied Mathematics and Theoretical Physics:

First Measurements of Gravitational Waves

Later this year, upgrades to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and VIRGO in Europe will be completed, and the first direct measurements of gravitational waves may be just around the corner. Around the same time, the LISA Pathfinder mission will be launched as a test mission for establishing a gravitational wave detector of unprecedented sensitivity in space.

The equations the researchers solved deal specifically with the spin angular momentum of binary black holes and a phenomenon called precession.

Just as Kepler studied the motion of the earth around the sun and found that orbits can be ellipses, parabola or hyperbolae, the researchers found that black hole binaries can be divided into three distinct phases according to their rotation properties.

Dynamics of Black Holes

The researchers also derived equations that will allow statistical tracking of such spin phases, from black hole formation to merger, far more efficiently and quickly than was possible before.

“With these tools, new insights into the dynamics of black holes will be unveiled,” said Gerosa. “Gravitational wave signals can now be better interpreted to unveil mysteries of the massive universe.”


Effective Potentials and Morphological Transitions for Binary Black Hole Spin Precession Michael Kesden, Davide Gerosa, Richard O’Shaughnessy, Emanuele Berti, and Ulrich Sperhake Phys. Rev. Lett. 114, 081103 – Published 24 February 2015

Illustration of two rotating black holes in orbit. Both, the black hole spins (red arrows) and the orbital angular momentum (blue arrow) precess about the total angular momentum (grey arrow) in a manner that characterizes the black-hole binary system. Gravitational waves carry away energy and momentum from the system and the orbital plane (light blue) tilts and turns accordingly. Credit: Graphic by Midori Kitagawa

See also