Gravitational waves are ripples in the fabric of spacetime caused by cataclysmic events such as neutron stars colliding and black holes merging.
The biggest of these events, and the easiest to see, are the collisions between supermassive black holes at the centre of galaxies. So an important question is how often these events occur.
Today, Sean McWilliams and a couple of pals at Princeton University say that astrophysicists have severely underestimated the frequency of these upheavals. Their calculations suggest that galaxy mergers are an order of magnitude more frequent than had been thought. Consequently, collisions between supermassive black holes must be more common too.
That has important implications. There is an intense multimillion-dollar race to be first to spot gravitational waves, but if the researchers are correct, the evidence may already be in the data collected by the first observatories.
The evidence that McWilliams and co rely on comes from various measurements of galaxy size and mass. This data shows that in the last six billion years, galaxies have roughly doubled in mass and quintupled in size.
Astrophysicists know that there has been very little star formation in that time, so the only way for galaxies to grow is by merging, an idea borne out by various computer simulations of the way galaxies must evolve. These simulations suggest that galaxy mergers must be far more common than astronomers had thought.
That raises an interesting prospect—that the supermassive black holes at the centre of these galaxies must be colliding more often. McWilliams and co calculate that black hole mergers must be between 10 and 30 times more common than expected and that the gravitational-wave signals from these events are between three and five times stronger.
That has important implications for astronomers’ ability to see these signals. Astrophysicists are intensely interested in these waves since they offer an entirely new way to study the cosmos.
One way to spot them is to measure the way the waves stretch and squeeze space as they pass through Earth, a process that requires precise laser measurements inside machines costing hundreds of millions of dollars.
The most sensitive of these machines is called LIGO, the Laser Interferometer Gravitational Wave Observatory in Washington state, which is currently being upgraded; it is not due to reach its design sensitivity until 2018-19.
Another method is to monitor the amazingly regular radio signals that pulsars produce and listen for the way these signals are distorted by the stretching and squeezing of space as gravitational waves pass through the solar system.
So-called pulsar timing arrays largely rely on existing kit for monitoring pulsars and so are significantly cheaper than bespoke detectors.
Of course, everyone has assumed that the more sensitive bespoke detectors such as LIGO will be the first to see gravitational waves, although not until the end of the decade.
But all that changes if gravitational waves turn out to be stronger than thought. And that’s exactly what McWilliams and co predict. In fact, they say the waves are so strong that current pulsar monitoring kit ought to be capable of spotting them. “We calculate … that the gravitational-wave signal may already be detectable with existing data from pulsar timing arrays,” say the Princeton team.
Pulsar timing arrays are also increasing in sensitivity. If McWilliams and co are correct, this makes the detection of gravitational waves a near certainty within just a few years. Their most pessimistic estimate is that pulsar timing arrays will have nailed this by 2016.
“We expect a detection by 2016 with 95% conﬁdence,” they say.
That’s an extraordinary prediction and a rather refreshing one, given the general reluctance in science to nail your colours to a particular mast.
The first direct observation of gravitational waves will be one of the most important breakthroughs ever made in astronomy; the discoverer a shoo-in for a Nobel Prize.
So the stakes could not be higher in this race, and this time there is a distinct chance of an outside bet taking the honours.
Ref: http://arxiv.org/abs/1211.4590: The Imminent Detection Of Gravitational Waves From Massive Black-Hole Binaries With Pulsar Timing Arrays