The search for cosmic strings

Our universe exploded into being, expanded at a fantastic speed and cooled. Perhaps too quickly. Some physicists believe the rapid cooling might have cracked the fabric of the universe.

These hairline fractures may still be threaded through space-time. Dubbed cosmic strings, mathematical models see them as invisible threads of pure energy, thinner than an atom but light-years long. The huge amount of energy they contain also makes them extremely heavy; a few centimetres of cosmic string might weigh as much as Mount Everest.

Proponents of cosmic strings, like Thibault Damour, a theoretical physicist at the Institute of Advanced Scientific Studies near Paris, are persuaded by the maths that keeps predicting their existence. “The fact strings come up all the time makes me confident that they exist,” he says.

However, as time capsules of the early universe, cosmic strings should retain fantastic energies – more than a billion times greater than those released by smashing particles at the Large Hadron Collider, says Ken Olum, a theoretical physicist at Tufts University in Boston, who has contemplated cosmic strings for 20 years. “You can’t build an accelerator to test physics at that scale.”

Neither can any of our astronomical instruments detect these vanishingly thin, intergalactic filaments. For some physicists, a theory that can’t be tested is not worth pursuing. It places cosmic strings in the same category as “string theory”, their controversial namesake at the other extreme of the size scale. String theory invokes vibrating strings tinier than any subatomic particle as the building blocks of the universe. For Matthew Bailes, an astrophysicist at Swinburne University of Technology in Melbourne, cosmic strings are a  “mathematical curiosity” or worse, “an exotic fantasy”.

All that may be about to change. The nascent era of gravitational wave astronomy – just two years old – may finally deliver a tool to test the existence of cosmic strings. We can’t see them but gravitational wave detectors might be able to hear the thrums and snaps created as they whip through space.

You might wonder how the emptiness of space could be cracked. It helps to picture the universe through the eyes of a quantum field theorist. Neo in The Matrix was close. He saw his world as a diaphanous fabric of greenish ones and zeroes. Quantum field theorists see the universe as a fabric of all-pervading fields.

Fields fill space like a fluid, and what we call ‘particles’ are ripples within the fluid. A photon is a ripple in the electromagnetic field (which we experience as light), an electron a ripple in the ‘electron field’, a Higgs boson a ripple in the Higgs field, and so on. “There is nothing else except fields,” is the way retired Princeton physicist Freeman Dyson once put it.

British field theorist Tom Kibble, who died in June 2016, came up with the idea of cosmic strings in 1976. He was musing about the first split second after the Big Bang when the universe underwent a rapid expansion, then cooled rapidly. This, he suggested, caused a phase change in the quantum fields, like water freezing to ice.

In a block of ice, some regions can freeze with their crystals in different orientations, rather like tiles being laid simultaneously at different ends of a room. Where they meet, they don’t fit together smoothly, resulting in a crack. Likewise Kibble surmised that the quantum phase changes in the early universe would have caused the fields to align in different orientations, again causing cracks – cosmic strings.

Some of Kibble’s past predictions have paid off. He independently predicted the existence of a fundamental particle that imparts mass to all others, now known as the Higgs boson. The discovery of that particle in 2012 won the Nobel prize.

Cosmic strings, however, were particularly problematic to put to the test. They would only appear at the edges of vast regions about as big as the observable universe. That is why, in Kibble’s original 1976 scheme, he wrote that “looking for cosmic strings directly would be pointless”.

There the story of cosmic strings might have ended, but for a remarkable calculation by the Ukrainian physicist Alexander Vilenkin about five years later.

By the early 1980s most cosmologists accepted the Big Bang theory – the idea the universe had evolved from the expansion of a uniformly hot, dense state. But the idea had one big problem: the lumpy distribution of galaxies. The simple theory of galaxy formation holds that they formed from clouds of hydrogen that condensed under the pull of gravity. That, however, should yield evenly spaced galaxies. Furthermore, the earliest galaxies formed too quickly to be explained by this process. So how did we get a lumpy universe?

Vilenkin was thinking about this problem when he picked up on an aside in Kibble’s 1976 paper: when a cosmic string wriggling in the void crossed itself, it would chop off a self-contained ‘loop’. These loops would be light-year-sized hula-hoops in space – and enormously heavy. Vilenkin ran the numbers, and realised the number of cosmic loops that would have existed in the early universe was curiously close to the number of galaxies. Perhaps, he reasoned, a cosmic loop could seed a young galaxy, much like a grain of sand seeds a pearl.

The idea caused great excitement among physicists. Stephen Hawking wrote papers on how the loops might collapse to form black holes. Others got interested in how they bend and twist in space. Some even worked out how cosmic strings might be detected: if the loops were abundant in the early universe, they would have left a pattern on the radiation left over from the Big Bang – the so-called cosmic microwave background.

In November 1989 the Cosmic Background Explorer (COBE) satellite was launched – a US$140 million experiment to map the cosmic microwave background. But when the data was unveiled in 1992, the cosmos showed no hint of cosmic strings. Instead, it favoured the idea galaxies had seeded around tiny quantum fluctuations that had been imprinted when the universe was less than the size of an atom.

“That did cause people to lose enthusiasm for cosmic strings,” admits Xavier Siemens, a theoretical physicist at the University of Milwaukee, “but they were not ruled out.”

Meanwhile, Kibble’s strings were popping up in other fields of physics. In 1996, two papers in the same issue of Nature described experiments where liquid helium – a model for the early universe – had been rapidly cooled. String-like defects appeared. Other string-ish flaws were found during phase changes in liquid crystals and superconductors, exotic materials whose properties also fit Kibble’s equations. “In fact, one might say defects and ordering processes of the type Kibble discovered have been found and studied almost everywhere except in the universe,” writes physicist Neil Turok, of Canada’s Perimeter Institute, in his 2013 book Symmetry and Fundamental Physics.

The cosmic string idea also cropped up in the physics of the very small. In 2003 one systematic review published in Physical Review D concluded that almost all theories of supersymmetry – the idea that all fundamental particles have as-yet-unseen partners – predict cosmic strings of one form or another. Meanwhile Olum and others have run computer simulations showing that, if this prediction holds true, there should be at least a billion cosmic string loops sprinkled through the observable universe.

What was missing was the real-life observation. But how do you detect something thinner than an atom, as long as a galaxy, and invisible to boot?

Enter gravitational waves. In September 2015 the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves reverberating from colliding black holes. That added a new dimension to astronomers’ ability to scan the universe. “After LIGO’s discovery,” Damour says, “I immediately thought, ‘Aha! Now it would be good if cosmic strings were detected.’”

Cosmic strings can’t be seen but they might be heard. Gravitational waves are ripples in spacetime generated by massive objects moving extremely fast – like a pair of inspiralling black holes or neutron stars. Or a writhing cosmic string.

“What happens is like a whip,” explains Damour, who worked out the idea with Vilenkin in 2000. The crack of a bullwhip is actually a sonic boom caused when part of its tail moves faster than the speed of sound. Likewise, as a cosmic string loop wiggles and bounces, some parts would be whipped up to the speed of light – and emit a burst of gravitational waves. The two physicists calculated such a burst might be detectable by LIGO.

From 2005 to 2010, LIGO listened but heard no whip crack. Since September 2015, advanced LIGO, an upgraded version which is four times more sensitive, has continued the vigil.

One difficulty in detecting the crack is that it would only be emitted in a particular direction, like the beam of a flashlight. LIGO would have to be right in the path of the beam.

That is why our best hope of detecting cosmic strings is probably not from their whipcracks but from their rotations. As a loop of cosmic string spins like a hula-hoop, it would emit gravitational waves – one wave for each turn of the hoop. Since the hoops could have a circumference of light-years, it could take decades to finish a single spin.

In other words, this cosmic hula hoop would generate gravitational waves at an extremely low frequency – way too low for LIGO to detect. You need an entirely different kind of gravitational wave detector; luckily we have one waiting in the wings.

A pulsar timing array is a gravitational wave detector the size of the galaxy. Pulsars are spinning neutron stars (collapsed cores of exploded stars) emitting intense beams of light that appear to blink on and off with a precision rivalling atomic clocks. The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) has been obsessively timing a few dozen pulsars for a decade.

Any deviation from the norm could indicate a passing gravitational wave has stretched or squeezed the spacetime between us and the pulsar – causing a slight lag, or advance, in the timing.

“We’re about to open a new window on gravitational waves at low frequencies,” says Siemens, who is also director of NANOGrav. To keep tabs on pulsars across the whole sky, NANOGrav is linked with two other pulsar timing arrays, one using radio telescopes across Europe, and the other based at the Parkes Observatory, in New South Wales.

So far the searches have drawn a blank, as Siemens and Olum announced last September.

“In physics, when you don’t find something it’s not a failure,” Olum says. “It’s a success of a different kind, because it tells us something new about the universe.” The no-show of cosmic strings at certain energies can already be used to rule out some theories of supersymmetry.

The next level up in the search for cosmic strings, and perhaps our only hope of a definitive answer, will come with the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector due to launch in 2034, which will listen to the frequency band between the high-pitched chirps caught by LIGO and the sub-bass murmurs to which pulsar timing arrays are attuned.

Even if the evidence continues to come up negative, some physicists are unlikely to let go of cosmic strings. Siemens says the strings might have been formed with too low an energy to give off any signals “detectable in the near future”. Another possibility is that ancient cosmic strings radiated away their energy and faded to nothingness too quickly after the Big Bang to have left a lasting impression.

For now, cosmic strings sit on the shelf alongside other beautiful ideas that could complete our understanding of the universe, but lack empirical support. “This is the beauty and the danger of physics,” Damour says. “Sometimes things exist that we can never see.”