The new wave of gravitational waves
About 10 billion-trillion-trillionths of a second into the start of creation in the big bang, the universe is believed to have had a brief but absurdly fast growth spurt. This episode, called inflation, was so cataclysmic that the very fabric of space and time was set juddering with gravitational waves (GWs). By comparison, the GWs that were first detected six years ago to much fanfare were small-scale affairs caused by black holes colliding. But now scientists at the European Space Agency (Esa) are setting their sights on grander targets – and are hoping they might soon be able to detect the faint echoes of the universe’s inflationary birth throes, almost 14bn years after the event, using the largest instrument ever built. Hundreds of times bigger than the Earth, Esa’s planned gravitational wave detector will float in space and look for wobbles in spacetime caused by all manner of immense astrophysical convulsions.
The first GW was identified in 2015 by the Laser Interferometer Gravitational-Wave Observatory (Ligo), an international project whose success won the 2017 Nobel prize in physics for three of its key proponents. Ligo consists of two massive detectors in the American states of Washington and Louisiana. Each deploys two tunnels 2.5 miles (4km) long, intersecting at a right angle, along which a laser beam travels to a mirror at the far end and bounces back. The returning light waves interfere with one another where the arms cross. As a GW passes, it very slightly contracts or stretches spacetime. Because that effect will be different in each arm, it changes the synchrony of the light waves, and so alters the interference of the two beams.
Ligo isn’t alone. A second GW detected on Christmas Day 2015 was later confirmed in collaboration with the European detector Virgo, based in Italy. A detector in Japan, called Kagra, began operating early last year, and others are planned in India and China.
Most GWs seen so far are apparently caused by the collision of two black holes. These are formed from stars many times more massive than our sun that have burnt out and collapsed under their own gravity. According to Albert Einstein’s theory of general relativity, which describes gravity as the warping of spacetime caused by mass, the collapse can continue until nothing remains but an almost infinitely dense “singularity”, which produces a gravitational field so intense that not even light can escape from it.
If two black holes get snared by each other’s gravity, they might circle each other and gradually spiral inward until they coalesce. General relativity predicted more than a century ago that such events would send GWs rippling out through the cosmos, although there was no direct evidence for them until the Ligo detection. Other extreme astrophysical phenomena can cause them too, such as the merging of neutron stars: burnt-out stars less massive than black holes that have stopped their collapse at the point where they are made of matter so dense that a thimbleful would weigh as much as 50m elephants.
GWs can also be produced by far more enormous objects. At the centre of our galaxy, and of many others, sits a supermassive black hole several million times the mass of our sun, formed from collapsed stars and clouds of cosmic gas and dust. Objects spiralling into these supermassive black holes generate GWs that oscillate at lower frequencies and longer wavelengths than those of the smaller black-hole merger waves seen by Ligo and Virgo.
Ground-based detectors can’t spot these – it would be like trying to catch a whale in a lobster pot. To see them, an interferometric detector would need much longer arms. That’s tricky, as each arm must be a long, straight, empty channel free from any vibration. So researchers are planning instead to make low-frequency GWs in space. The most advanced of these plans is the device now being built for Esa: the Laser Interferometer Space Antenna (Lisa).
Lisa will send laser beams from one spacecraft to bounce off a mirror freely floating inside another craft. With three spacecraft you can make an L-shaped double-armed structure like Ligo. But the arms don’t have to be at right angles: instead, Lisa will position its three spacecraft several million miles apart at the corners of a triangle, so that each corner becomes one of three detectors. The whole array will follow the Earth’s orbit, trailing our planet by about 30m miles.
To test the feasibility of doing laser interferometry in space, in 2015 Esa launched a pilot project called Lisa Pathfinder – a spacecraft that demonstrated the technology on a tiny scale. The mission, completed in 2017, “blew us away”, says Esa’s Paul McNamara, who was the project scientist managing the mission. “It met our requirements on day one, with no tweaking, no nothing.” It showed that a mirror floating inside a spacecraft could be kept incredibly still, wobbling by no more than a thousandth of the size of a single atom. To keep it that steady, the spacecraft uses tiny thrusters to push back against the force produced by the light coming from the sun.
In other words, says McNamara, “our spacecraft was way more stable than the size of the coronavirus”. Which is just as well, because Lisa will need to detect a change in arm length, due to a GW, of about a tenth the width of an atom over a million miles.
Lisa’s launch won’t happen for at least a decade, though. “We have three satellites to build, each with many parts,” says McNamara. “It just takes time – that’s one of the unfortunate truths of a very complex mission.” The next milestone is the official “mission adoption”, expected in 2024. “At that point we will know the details of what the mission is, who among Esa member states and the US is contributing what, and how much it costs,” says astrophysicist Emanuele Berti, of Johns Hopkins University in Baltimore.
Japan and China are also in the early stages of planning space-based GW detectors. McNamara sees these not as competition, but as a good thing – because with more than one detector, it will be possible to use triangulation to pinpoint where the waves are coming from.
“Lisa will change GW astronomy pretty much in the same way as going beyond visible light [to radio waves, Xrays etc] was a gamechanger for ordinary astronomy,” says Berti. “It will look at different classes of GW source.” By studying supermassive black hole mergers, he says, “we hope to understand a lot about the formation of structure in the universe, and about gravity itself”. And if Lisa does see “primordial” GWs from inflation early in the big bang, that could test theories of how everything began.
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There might be another way to see low-frequency GWs that wouldn’t require a purpose-built detector at all. A collaboration called the North American Nanohertz Observatory for Gravitational Waves (NanoGrav) is using observations made with a global network of radio telescopes to look for the effect GWs have on the timing of “cosmic clocks” called pulsars.
Pulsars are rapidly spinning neutron stars that send out intense beams of radio waves from their poles, sweeping across the sky like lighthouse beams. Pulsar signals are highly regular and predictable. But “if a GW passes between the pulsar and the Earth”, says NanoGrav team member Stephen Taylor of Vanderbilt University in Tennessee, “it deforms the intervening spacetime”, causing the pulse to arrive sooner or later than expected.
In effect, the pulsars themselves become the detectors. As NanoGrav team member Julie Comerford of the University of Colorado at Boulder says, this gives the “detector” arms as long as the distance between the Earth and the pulsars: perhaps thousands of light years. Because of that immense size, the signals detectable by NanoGrav have very long wavelengths and very low frequencies, beyond even the reach of Lisa and made by gargantuan supermassive black holes billions of times as massive as the sun, which merge as entire galaxies collide. No other detector could sense these, says Taylor. Unthinkably cataclysmic though they are, such mergers are actually quite common, and NanoGrav would see a kind of hubbub made by lots of them. “All over the universe, there are pairs of supermassive black holes orbiting around each other and producing GWs,” says Comerford. “These ripples produce a sea of GWs that we are bobbing in.”
In January, a NanoGrav team led by Comerford’s postdoctoral researcher Joseph Simon in Colorado reported a possible first detection of this GW background. Although more work is still needed to verify that the signal is truly caused by GWs, Comerford calls the result “the most exciting astrophysics result I’ve seen in the last few years”.
If NanoGrav uses, in effect, a GW detector light years in size, physicist Sougato Bose of University College London believes we could make one small enough to fit inside a cupboard. His idea relies on one of the oddest effects in quantum theory, which generally describes very tiny objects like atoms. Quantum objects can be placed in so-called superpositions, meaning that their properties aren’t uniquely defined until they’re measured: more than one outcome is possible.
Quantum scientists can routinely put atoms in a quantum superposition – but such peculiar behaviour disappears for big objects such as footballs, which are either here or there whether we look or not. As far as we know, it’s not that a superposition is impossible for something that big – it’s just impossible to sustain for long enough to detect it, because the superposition is too easily destroyed by any interaction with the object’s surroundings.
Bose and his colleagues suggest that if we can create a quantum superposition of an object intermediate in size between an atom and a football – a tiny crystal about a hundred nanometres across, around the size of a large virus particle – the superposition will be so precarious that it would be sensitive to a passing GW. In effect the two possible states of a quantum superposition can be made to interfere like two light waves – and distortions of spacetime caused by a GW would show up as a change in that interference.
Bose thinks that diamond nanocrystals held within a vacuum much more empty than outer space and cooled to within a whisker of absolute zero could be kept in superpositions for long enough to do the trick. It wouldn’t be easy, but he says that all the technical challenges have already been demonstrated individually – it’s a matter of putting them all together. “I don’t see any barrier in doing it over the next 10 years or so, if enough funding was there,” he says.
If these and other developments create a boom in GW astronomy, what will we see? “When you open a new window to the universe, you usually see things you don’t expect,” McNamara says. As well as seeing more of the kinds of events that we already know to cause GWs, we might well get signals we can’t easily explain. “That,” says McNamara, “is when the fun starts.”
This will change GW astronomy in the same way as going beyond visible light was a gamechanger for ordinary astronomy
Emanuele Berti