Beating heart of galaxy mergers (Image: MPI for Gravitational Physics/W. Benger-ZIB))

TAKE the pulse of the universe, and its invisible wrinkles become visible. The first direct evidence of Einstein’s gravitational waves, may already exist in records of light pulses from rapidly spinning dead stars.

Crucially, we may uncover those waves as early as 2013. New research suggests that we’ve underestimated the rate at which black holes merge, and how that changes the light from pulsars.

Gravitational waves are produced by massive, accelerating objects, such as two black holes spinning towards each other (see diagram). The heavier the black holes are, and the faster they’re moving, the more powerful those waves. As they move, the waves stretch and squash space-time like the folds of an accordion. Measuring the waves would be a powerful test of general relativity, and would offer a new wavelength for probing the universe.


But finding traces of the waves is a big challenge, in part because we haven’t had the right tools. A proposed space-based detector was cancelled due to lack of funds, and our best Earth-based detector, LIGO, is closed until 2014 for a major upgrade. Once it is running, LIGO may need another four to five years to collect enough data.

In the meantime, wave hunters have been watching for subtle changes in the timing of pulsars. These dense cores of dead stars sweep the sky with beams of radio waves ejected from their poles. If the beams point directly at Earth, the pulsars appear to blink at exceptionally regular intervals. But when a gravitational wave goes by, it distorts space-time so that the pulsar and Earth bounce towards or away from each other, changing the distance that the pulsar’s light has to travel and making its beat irregular.

Pulsar timing is most sensitive to the larger waves coming from supermassive black holes in the centres of merging galaxies. But galaxies are merging all the time, so the universe is too noisy for a lone pulsar to give a definitive signal, says Sean McWilliams of Princeton University.

Instead, observers use an array of well-characterised pulsars to see if their beats all vary in tandem – a sure sign of a passing gravitational wave. So far, this pulsar timing technique has been coming up empty. But that may be because we haven’t been looking at the data the right way, McWilliams says.

Observers depend on theoretical predictions of how often galactic black holes merge to calculate how large and complex a signal they should expect amid the noise. Current analysis methods look for a smooth data curve, based on the known merger rate. But in 2010, observers noticed that galaxies in the centres of clusters were gaining mass much faster than expected, hinting that these galaxies are growing via more frequent mergers.

Based on this new rate, McWilliams and colleagues calculated that the gravitational wave signal should be 3 to 5 times stronger (arxiv.org/abs/1211.5377) than anticipated, and that the data curve should become more complex. If analysis methods are tuned to this new curve, the team thinks gravitational waves could even be waiting in data collected but not yet analysed.

“If the most optimistic predictions of McWilliams are correct, it could be announced next year,” says Maura McLaughlin of West Virginia University, who studies pulsar-timing.

In an independent paper, Alberto Sesana of the Albert Einstein Institute in Golm, Germany, made a similar, more conservative analysis of galaxy mergers and came to nearly the same conclusion: pulsar timing arrays should be able to catch the waves in the next 3 to 10 years (arxiv.org/abs/1211.5375).

Both models leave plenty of wiggle room, McLaughlin cautions, and the observing techniques being used now aren’t perfect. “There are lots of uncertainties,” she says. But the push to find gravitational waves is important, she adds.

“Once we have gravitational waves, we’ll have this completely different tool, and we can see things we’ve never seen before because they don’t emit light. It will be really revolutionary.”

Once we have gravitational waves, we can see things we’ve never seen before. It will be revolutionary