Tails of gravitational waves and mechanics of compact binaries

by Luc Blanchet and Alexandre Le Tiec

The first law of binary black hole mechanics can be extended to include non local tail effects.

Ever since Kepler’s discovery of the laws of planetary motion, the “two-body problem” has always played a central role in gravitational physics. In Einstein’s general theory of relativity, the simplest and most “universal,” purely gravitational, two-body problem is that of a binary system of black holes. The inspiral and merger of two compact objects (i.e. bodies whose radius is comparable to their mass, in “geometric” units where G = c = 1) produces copious amounts of gravitational radiation, as was recently discovered by LIGO’s multiple detections of gravitational waves from black hole binary systems.


Alexandre Le Tiec (left) celebrates the detection of gravitational waves and Luc Blanchet (right) thinks about gravitational waves in Quy Nhon, Vietnam

In general relativity, the inspiral and onset of the merger of two compact objects is indeed universal, as it does not depend on the nature of the bodies, be they black holes or neutron stars, or possibly more exotic objects like boson stars or even naked singularities. However, the gravitational waves generated during the post-merger phase depend on the internal structure of the compact objects, and in the case of neutron stars should reveal many details about the scenario for the formation of the final black hole after merger, and the equation of state of nuclear matter deep inside the neutron stars

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The spin limit of colliding black holes

Geoffrey Lovelace

Geoffrey Lovelace is an Assistant Professor of Physics at California State University, Fullerton. As member of Fullerton’s Gravitational-Wave Physics and Astronomy Center and the Simulating eXtreme Spacetimes collaboration, his research interests focus on using computer simulations to model colliding black holes and neutron stars and the gravitational waves they emit.

A single black hole’s size limits its spin. Do colliding black holes obey this limit?

In our recent paper, published in Classical and Quantum Gravity, we take a first look at how supercomputer simulations can help reveal the answer.

A black hole is an object whose gravity is so strong that nothing, even light, can escape from inside its horizon. An isolated, uncharged black hole can be completely described by just two numbers: its spin and its horizon surface area. All of the black hole’s properties then follow from Kerr’s solution of Einstein’s equations.

Kerr’s solution implies that a single black hole can spin no faster than its horizon area times a constant: spinning any faster would destroy the horizon. Astronomers have found evidence that some black holes spin very close to the limit (but still below it). Mathematical relativists have proven that this spin limit is obeyed not only by Continue reading