Spacetime Ripples on the Horizon?

Scientists might be closer to detecting one prediction of Einstein's theory of gravity than they thought they were.

Binary white dwarf
In this NASA illustration, a close pair of orbiting white dwarf stars throw off spiral waves of gravitational radiation.
There’s been a bit of a flurry in physics recently over the “imminent” detection of gravitational waves. Gravitational waves are ripples in the fabric of spacetime, created by accelerating masses. In a binary system, the waves move outward from the whirling pair, carrying away energy as the objects (say, two stars or black holes) spiral in toward each other and merge.

The match between the predicted loss of energy through gravitational waves and the shrinking orbit of two pulsars led to Russell Hulse and Joseph Taylor winning the 1993 Nobel Prize in Physics. But despite a committed effort, no one’s directly detected gravitational waves yet.

Some new calculations posted to the open-access research site suggest that we might be a lot closer to detection than previously thought. The predictions appear in three papers, from two different sources. And while they do agree that the gravitational wave signal might be higher than expected, they don’t agree on how high.

The papers, two by Sean McWilliams (Princeton) and his colleagues and one by Alberto Sesana (Max Planck Institute for Gravitational Physics, Germany), both look at waves created by merging supermassive black holes and how detectable they might be to a method called pulsar timing arrays.

Pulsars are the ultimate celestial clocks, sending us super-regular “pulses” of radiation from space. But the arrival time of those beats depends on the distance between us. If a gravitational wave passes between us and a pulsar, the wave will slightly stretch the space between us and the spinning star, causing the distance to grow and shrink in an oscillating way. Those changes will make some pulses arrive a little earlier and others a little later than expected, says Sesana. The errant beats would be the fingerprint of a passing gravitational wave.

Pulsar timing is particularly sensitive to binaries with periods of months or years, such as two supermassive black holes, says Scott Hughes (MIT), who works on gravitational waves but was not involved with any of the papers. These various black hole signals should overlap to create an overall signal, kind of like the gabble of voices at a cocktail party, he says.

“You can’t quite tell what any individual is saying — unless someone happens to be particularly close to you, or is particularly loud and obnoxious,” he says. “But because you know what people sound like, you know that you are hearing human voices rather than, say, a chorus of vacuum cleaners, which might have a similar volume.”

The goal is to detect this cosmic cocktail party. To estimate how loud the party is, you need to know how often galaxies and their central black holes merge.

That’s where the new papers come in. Both assessed the merger rate for galaxies in the second half or so of the universe’s history. McWilliams’s team assumed that mergers are the primary way massive galaxies grow during this cosmic epoch. They also assumed the central black holes’ growth at this time depends more on merging with each other than on frantic gas-guzzling. Given these mergers-dominant assumptions (which McWilliams says he's confident are accurate), the rate of galaxy mergers depends solely on the number of observed galaxies with particular masses.

Using these numbers, McWilliams and his colleagues found that merger rates are 10 to 30 times higher than previously estimated. That would make the gravitational wave signal from inspiraling black holes 2 to 5 times stronger, so the signal could potentially be detectable in current data, they conclude.

Sesana’s result is more cautious. He doesn’t assume that mergers dominate, and instead of going just by the number of galaxies, he also constrains his results by the number of galaxies seen in pairs. (Presumably, these are galaxies that will or are already merging.) While this constraint does add some internal inconsistency to his model, he says, it also gives an overall result that looks consistent with observations. But it also gives a lower merger rate: his signal strength estimate is a factor of 3 to 10 below current pulsar timing limits, although it still offers the chance of detection in the next few years.

So even with the more cautious scenario, we might detect gravitational waves by the end of the decade. If so, it could open a whole new eye on the universe, the same way observing at wavelengths beyond visible light did.

Listen to a 2009 podcast about merging black holes and gravitational waves.

S.T. McWilliams et al. "The imminent detection of gravitational waves from massive black-hole binaries with pulsar timing arrays." Posted to November 19, 2012.

A. Sesana. "Systematic investigation of the expected gravitational wave signal from supermassive black hole binaries in the pulsar timing band." Posted to November 22, 2012.

S.T. McWilliams et al. "Gravitational Waves and Stalled Satellites from Massive Galaxy Mergers at z<1." Posted to November 22, 2012.

One thought on “Spacetime Ripples on the Horizon?

  1. Tony Pace

    Gravitational Wave detection, real soon now:
    Wasn’t LIGO supposed to do that already? I notice that it is down for upgrades now until 2014 or so; It would be a sad note if a pair of merging Galaxies’ central Black Holes merged the day after tomorrow (i.e., “Maya Doomsday”) and its Gravity Waves dropped below detectability by the time LIGO comes back on line.
    This could be Astronomy’s reprise of Thermonuclear Power Plants. Their proponents been around about 40 years and the promise was then, “Oh, about five years hence…”, several times since whence.
    More seriously: Do we have even a rough estimate of how many galaxies there are Out There? Or an estimate of the probability of Supermassive Black Hole collisions per Year?

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