After five detections over the last few years, gravitational waves are in danger of becoming boring news. But it's worth keeping in mind just how groundbreaking a discovery it is. After all, these are distortions in the very fabric of spacetime. The precise instruments at the LIGO and Virgo facilities were responsible for these past detections, but now astronomers plan to look for ripples from supermassive black hole collisions using natural detectors in space: pulsars, the "cosmic lighthouses" of the sky.

The first four gravitational wave detections were given off from the mergers of two black holes, each a few dozen times more massive than our Sun. The fifth and most recent event was created by a collision between two neutron stars, meaning the waves were accompanied by light signatures for the first time.

All of these events were picked up by ground-based facilities like the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) in the US, or the Virgo facility in Italy. These observatories make their detections by beaming lasers down long tunnels and precisely measuring tiny distortions in the beam. Extreme care is taken to remove all possible interference, so that the only way the laser can be affected is when gravitational waves wash over it and physically warp the local fabric of spacetime.

These detectors have managed to pick up ripples from black holes and neutron stars with masses several dozen times larger than the Sun, but waves from much bigger cosmic cataclysms, like collisions between two galaxies, have so far eluded detection. That's because they ripple out at a much lower frequency, and a new study has outlined where and how to start looking.

"Observing low-frequency gravitational waves would be akin to being able to hear bass singers, not just sopranos," says Joseph Lazio, co-author of the study.

The researchers say pulsars are key to tapping into the songs of these intergalactic Barry Whites. Pulsars are dense neutron stars that rapidly rotate, emitting electromagnetic signals like clockwork. The regularity and strength of those signals has earned them the nickname "cosmic lighthouses," and programs are currently using huge arrays of known pulsars to search for gravitational waves.

These pulsar timing arrays are based on the same principle as LIGO and Virgo. Essentially, if enough is known about each pulsar, then their signals can be predicted incredibly precisely, so if those signals are delayed by even the tiniest fraction of time, then that could indicate that a gravitational wave has rolled through. Importantly, this method could be more tuned towards lower frequency waves.

"A difference between when the pulsar signals should arrive, and when they do arrive, can signal a gravitational wave," says Chiara Mingarelli, lead author of the new study. "And since the pulsars we study are about 3,000 light-years away, they act as a galactic-scale gravitational-wave detector."

Lower frequency gravitational waves would be given off by collisions between supermassive black holes – objects that are up to billions of times more massive than those behind previous detections. These monsters lurk at the center of many galaxies, and so their collisions might mark the grand finale of two galaxies merging into one.

In order to predict when and where these mergers will occur, the new study scoured the sky for the most likely candidates. These would include galaxies that are most likely to host two supermassive black holes, and then which of those are most likely to merge. To figure all this out, the team used data from the 2 Micron All-Sky Survey (2MASS), and combined it with galaxy merger rates pulled from the Illustris simulation project.

Of the 5,000 galaxies they studied, the astronomers narrowed in on about 90 that probably have pairs of supermassive black holes in the process of merging. The team also worked out the windows of time for how long we had to measure them, and found that it depends on the size of the objects. The bigger black holes produce stronger gravitational waves but happen faster, meaning we "only" have about 4 million years to spot them. Smaller mergers meanwhile can happen over more than a hundred million years.

"By expanding our pulsar timing array over the next 10 years or so, there is a high likelihood of detecting gravitational waves from at least one supermassive black hole binary," says Mingarelli.

The researchers hope that the array can also teach us about how galaxies are formed and what happens when they merge – which could be useful information, considering we're currently on-course for a collision with our neighboring Andromeda galaxy.

The research was published in the journal Nature Astronomy.

Source: NASA JPL

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