Binary black holes, gravitational waves and numerical relativity

We had an excellent turn-out for yesterday’s superb Institute of Physics seminar even though we are in the last hectic week of the teaching semester (thanks to the organisational skills of the WIT maths/physics seminar group). The talk ‘Binary black holes, gravitational waves and numerical relativity’ was given by Dr Joan Centrella, head of the Gravitational Astrophysics Laboratory at NASA’s Goddard Space Flight Centre. Dr Centrella is a distinguished relativist, well known for her work in the simulation of black hole mergers and she certainly didn’t disappoint.

The lecture started with an overview of massive black holes, intermediate black holes and gravitational waves. Just as general relativity predicts that a large mass will curve spacetime, it predicts that moving mass will cause ripples in the curvature of spacetime – known as gravitational waves. Of course, such disturbances will be extremely difficult to detect due to the weakness of the gravitational interaction. Indeed, while many of the spectacular predictions of general relativity have been verified (the bending of light in a gravitational field, time dilation in a gravitational field, black holes and even the expanding universe) the direct detection of gravitational waves is possibly the last great test of relativity. The speaker explained that the best chance of seeing the phenomenon directly is by studying the most explosive events known: black hole mergers.

There was a brief description of the indirect observation of gravitational waves, in particular the Hulse-Taylor pulsar. This is a binary pulsar found in 1974, whose orbit has been observed to be gradually shrinking due to the radiation of energy by gravitational waves: the two stars will merge in about 300 million years. Interesting that Hulse got the Nobel for work done while still a postgraduate, while Jocelyn Bell was overlooked for her discovery of pulsars – see post on IoP meeting below.

Centrella then gave an overview of direct searches for gravitational waves, both earth-bound (LIGO) and space-based (LISA). LIGO, the Large Interferometer Gravitational Wave Observatory, is basically a huge Michelson interferometer, complete with laser source, beam splitter and mirrors – the arms of the interferometer are several kilometers in length! LISA, the Laser Interferometer Space Antenna, is an astounding project: a joint NASA/ESA mission, it will consist of three separate mini-spacecraft, each with its own laser source, maintained in an equilateral triangle that will form a giant Michelson interferometer in space. Minute disturbances in spacetime by a passing gravitational wave will be measured as tiny changes in relative arm length (having taken all other factors into account). A crucial difference between the two systems is the target: while LIGO searches for intermediate black hole events, LISA will search for massive BH events (a much stronger source in a different region of the spectrum).

LIGO (California)

LISA (artist’s impression)

Dr Centrella then described her own field: the use of numerical methods and algorithims to solve the equations of general relativity for the particular case of relativistic binary systems and their associated gravitational waves. She gave a great overview of historic problems in the area and recent breakthroughs in the field, from the puncture method to the Lazarus approach. I won’t attempt to summarize this part of the talk, but there is a nice overview of the field here and I should have a link to the slides from the talk in a day or two.

Dr Centrella with a scale model of one of the LISA spacecraft

All in all, this was a superb lecture, courtesy of the Institute of Physics. It was clear the audience enjoyed the lecture thoroughly and there were plenty of queries at question time – indeed the lecture would have continued for another hour had we not whisked the speaker off for dinner. In answer to my own question on the detection of gravitational waves from the Big Bang itself, Dr Centrella pointed out that one would certainly to see expect a signal from cosmic inflation – however these waves would be in a very different region of the spectrum from that studied by either LIGO or LISA. ..

Update: Joan has been in contact to say you can get a review article she wrote on the subject for the Scidac Review here; she has also done a podcast for Sky and Telescope with movies of the simulations here. She also has two comments and corrections to the text above; rather than paraphrase them I have put them verbatim in the comments section!

Update II: there is a wonderful article on gravitational waves and the early universe by Craig Hogan in the June 2007 edition of Physics World, which you can access here if you’re a member