You may think you are sitting still, peacefully reading this column. In fact you are awash in waves of spacetime that periodically stretch you and squeeze you this way and that. Born in distant cosmic cataclysms, these waves travel through the Universe like ripples on a pond.
On September 14 last year we caught our first glimpse of one: a ripple emitted more than a billion years ago by the merging of two massive black holes. Historic though this was, that signal, recorded in the impossibly tiny motions of mirrors in the Laser Interferometer Gravitational-wave Observatory (LIGO) experiment, was merely the beginning.
Just as light’s electromagnetic spectrum stretches from high-energy gamma rays to languid radio signals, gravitational waves also span the range from high to low frequencies. And just as we have telescopes tuned to pick up the entire electromagnetic spectrum, so too tools are being developed to span the gravitational wave spectrum and usher in the era of gravitational wave astronomy.
LIGO, with its mirrors placed four kilometres apart, can pick up the high-frequency end – it’s optimised for waves whose crests pass by at 10 to a 1,000 times per second, that is 10 to 1,000 Hz. Waves in this frequency can be generated by a pair of fast-circling neutron stars or a relatively small pair of black holes on the brink of merging. Humans can hear soundwaves in this frequency range, which is why the quick-rising waveform that LIGO detected is referred to as a “chirp”. An even higher-frequency gravitational wave would be generated by a supernova (an exploding star) and would also be detectable at the very upper range of LIGO’s sensitivity.
But the ripples that make up the constant low hum of spacetime occur at lower frequencies. Some are generated by non-merging binary systems, many of which reside in our own galactic backyard. Pairs of neutron stars and white dwarfs circling at a leisurely pace produce gravitational waves from once a second to once in several hours. These frequencies are too low, and thus their wavelengths too long, to be detected with LIGO. To pick up longer wavelengths, longer arms are needed: eLISA, with mirrors a million kilometres apart, will be up to the task. This space-based version of LIGO, with a set of mirrors orbiting the Sun in a triangular configuration, is set to be launched in 2034.
Such a space-based instrument could also detect mergers between supermassive black holes, which produce lower frequencies than the merely massive black holes recently detected by LIGO. Since every large galaxy (including our own) seems to host a supermassive black hole in its centre, picking up this kind of signal will allow us to watch galaxies grow by consuming one another in the distant Universe.
At even lower frequencies of one crest per 10s of years, we may be able to detect the most massive of supermassive black holes as they orbit each other before merging, giving us even more insight into the growth of galaxies. To do this, we let the Universe build our instrument for us: a pulsar timing array. A pulsar is a collapsed, fast-spinning star that, like a cosmic lighthouse, emits regular pulses with each rotation. Their rotation is so regular that their pulses are the most accurate clocks in the Universe. By stretching and shrinking the space between pulsars and Earth, low frequency gravitational waves can change pulse arrival times. So in theory when these clocks lose their timing, that’s the hallmark of a gravitational wave.
These pulsar timing arrays should also allow us to pick up primordial gravitational waves left over from the Big Bang. They are still permeating the cosmos as ultra-long, ultra-low-frequency waves in spacetime, stretched out by the expansion of the Universe. In 2014, a microwave telescope called BICEP2 seemed to find swirly traces of these waves imprinted in the left-over radiation from the Big Bang. These swirls turned out to be dominated by Milky Way dust. But the waves might still be hidden there, waiting for our experiments to do a better job of teasing them out.
Our exploration of the invisible Universe with gravitational wave astronomy is only beginning, but its future looks bright.
Katie Mack is an astrophysicist at North Carolina State University.
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