Like a sweater stretched too often by wide shoulders, space-time can be permanently warped by the gravitational waves that constantly ripple through it. This distortion, called gravitational wave memory, could allow us to detect waves previously beyond our reach – even if we can’t see the event that caused them.
This offers hope that we may be able to find some of the universe’s most exotic objects.
Gravitational waves are created by massive objects moving through space-time. In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) first detected a gravitational wave caused by two black holes spiralling towards one another and merging. Since then, theorists have been hard at work figuring out what such waves from other events and objects would look like.
Some of the most exotic objects in physics, such as evaporating black holes, cosmic strings and even possible extra dimensions, would induce gravitational waves at much higher frequencies than we can currently detect.
But there is still hope that these elusive objects could leave a detectable signal, say Lucy McNeill and her colleagues Eric Thrane and Paul Lasky at Monash University in Melbourne, Australia.
If two people were floating near, say, a pair of merging black holes, the space between them would grow and shrink as space-time was stretched and distorted by gravitational waves.
Once the black holes merged and the waves ceased, this oscillation would stop – but the two people would be at a different distance apart to when they started. Memory of the gravitational waves would leave them slightly further apart or closer together.
This permanent distortion of space-time creates a signal 10 to 100 times weaker and with a frequency much lower than the original one from the oscillating gravitational wave. It could also extend in all directions, even if the gravitational waves themselves were beamed in one specific direction.
The difference in frequency means that the kinds of event for which LIGO and similar detectors can spot gravitational waves will have too low a frequency for them to also pick up the memory signal.
But if there are astrophysical events that produce gravitational waves at frequencies too high for LIGO to spot, their memory signals might fall easily into the observatory’s detection range, thus allowing us to pick them up. McNeill and her colleagues call these “orphan” signals because the parent wave is not detectable.
“We can use LIGO to probe the universe for gravitational waves that were once thought to be invisible to it,” says McNeill. “LIGO definitely won’t be able to see the oscillatory stretching and contracting, but it will be able to detect the memory signature if such objects exist.”
In fact, memory signals could supply the first definitive proof that many objects that would emit high-frequency gravitational waves do exist. Even objects at the less-esoteric end of the spectrum, such as small primordial black holes, have not yet escaped the realm of theory.
“The problem is that the astrophysical situations that predict gravitational waves at those very high frequencies are quite speculative,” says Marc Favata at Montclair State University in New Jersey. “We don’t have solid evidence that such high-frequency sources exist – but they could if certain models are correct.”
Orphan memory signals could represent a ray of light for those theories, allowing researchers to find high-frequency sources without looking for the sources themselves.
“What excites me the most is that memory gives us a tool for probing a range of the gravitational-wave spectrum that was not previously accessible,” says Thrane. “Who knows what we may find?”
Journal reference: Physical Review Letters, DOI: 10.1103/PhysRevLett.118.181103