The idle time on hundreds of thousands of computers is being harnessed to look for evidence of pulsars closely orbiting other stars or black holes — and you can join too. The search, part of a distributed-computing program called Einstein@Home, could turn up extreme pairs of astrophysical objects that would put general relativity to the most stringent tests yet.
Modelled after SETI@Home, which searches through radio noise for signals from intelligent extraterrestrial civilizations, Einstein@Home uses the idle time on the computers of some 200,000 volunteers to hunt for evidence of gravitational waves. These ripples in the fabric of space-time should be emitted by dense, fast-orbiting objects such as pairs of neutron stars or colliding black holes, according to Einstein's general theory of relativity. So far no gravitational waves have ever been observed, though there's excellent indirect evidence for them. Einstein@Home has been searching for their signals in data from the Laser Interferometer Gravitational wave Observatory (LIGO) and the British-German GEO-600 gravitational wave observatory.
Now, the software has been expanded to comb radio noise from the Arecibo Observatory in Puerto Rico to look for binary star systems with at least one pulsar – a spinning neutron star that sends strong radio beams from its magnetic poles.
Pulsars sometimes orbit other stars. When they orbit very closely, they become ideal laboratories to test a number of predictions of general relativity and potentially reveal any deviations from this long-held theory of the nature of gravity.
So far, just nine pulsars have been found orbiting other neutron stars. The closest such pair, in which the objects circle each other every 2.4 hours, was used in 2008 to measure how much a star's spin axis wobbles as a result of the gravitational twisting of space. The observation, made over a period of four years, agreed with the prediction of general relativity to within 13%.
Finding neutron stars in even tighter embraces would allow astrophysicists to test the theory with greater precision. "The more compact the binary the better," says Jim Cordes of Cornell University in Ithaca, New York, a member of a collaboration that has been searching Arecibo radio data for such pairs.
To search for the extreme binaries, Einstein@Home will use a program to anaylze the lighthouse-like radio blips of pulsars, whose beams periodically sweep across the Earth, for periodic deviations. If the pulsar is orbiting another star, the blips' timing and wavelengths will be subtly compressed and stretched as the pulsar approaches and recedes from us. By comparing Arecibo's data with a host of simulated neutron-star scenarios, computers can find the best fit and determine whether the pulsar has a companion.
So far, Cordes and his colleagues have had no luck finding such pairs with orbital periods shorter than 2.4 hours. But the team's computing power has been limited to discovering pulsars that take 50 minutes or more to orbit a companion.
Increasing the computing time with Einstein@Home will allow more comparisons to be made between the data and the simulations to find the best fit. This could reveal pulsars that dance around their partners with periods of 11 minutes or perhaps less.
The search could also reveal even more exotic pairings, like a pulsar in orbit around a black hole. "That would be a tremendous discovery because we don't know of any of those yet," says Cordes. "Then we can use the pulsar to study space-time around the black hole, and if the geometry is right, we might even be able to say something about the event horizon" (the black hole's "surface" below which no matter or light can escape).
Although Einstein@Home was set up to find evidence of gravitational waves, the pulsars it finds will not be detectable by existing gravitational-wave experiments, says project leader Bruce Allen of the Albert Einstein Institute in Hannover, Germany. Today's detectors are designed to pick up faster and more dramatic ripples in space-time, such as those released when two neutron stars merge. But Allen says the neutron star binaries found by Einstein@Home could be detectable by the proposed European-US space mission LISA, which is expected detect the effects of tens of thousands of close binary stars of many kinds. "Any systems we find in this way would probably become a calibration source for LISA," Allen says.
In the meantime, Cordes hopes bringing volunteers into the radio search will raise visibility for Arecibo, which faces an uncertain future. The US National Science Foundation (NSF) is set to open a competition to find an organization to manage Arecibo for another five years, beginning in 2010.
But it is unclear how much funding NSF itself will put up to run the facility. "I think the wild card in all this is how much money NSF is going to offer," Cordes says. "The fear is that they wouldn't put in all that's needed, so that whoever does manage it would have to come up with [the rest]," a prospect that could hamstring the world's largest radio telescope in uncertain economic times.