HOW long is a piece of string? In this case, as long as the entire universe is wide. But then cosmic strings aren’t your average twine. Thinner than a proton and stretching for billions of light years, they would propagate through space at nigh-on the speed of light, fuelled by – and containing – the barely imaginable energies that existed at the big bang, where they originated.
Or at least that’s the idea. Cosmic strings are among the most extreme phenomena theorists have ever dreamed up, and yet there are sound reasons to consider them. They are “one of the most plausible, testable predictions of unified theories”, says , a physicist at the University of Edinburgh, UK – referring to theories that seek to describe how the fundamental forces of nature were merged, and then diverged, in the very early universe.
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We are yet to find any clear evidence for the existence of cosmic strings. But in the past year, astronomers have spotted a slew of anomalies – from inexplicably large galaxies to strangely shaped clusters – that suggest we may finally be within touching distance of these enigmatic threads. What’s more, if one of these hints does hold up, we now have the tools to confirm the existence of cosmic strings once and for all, and even to begin distinguishing between their different forms.
That would count as an “enormous” breakthrough, says at the University of Oxford, as it could help theorists decide between the various unified theories on the table. “It would be a fundamentally new constituent of the universe,” he says.
Our best description of the known particles and forces in nature, the standard model of particle physics, doesn’t lead to cosmic strings. However, many enduring puzzles, such as the nature of dark matter – which seems to glue galaxies together – or the unusual shape-shifting behaviour of elusive particles called neutrinos, has led theorists to look for extensions to the standard model. And cosmic strings do naturally pop out of many of these extended versions, broadly classed quantum field theories – which depict particles as excitations in a background field rather than as the discrete entities we are used to. In that sense, cosmic strings, though at first glance exotic, are quite banal to those seeking a more complete mathematical description of nature.
Generally speaking, these alternative theories of particle physics imagine that at the beginning of the universe, three of the forces of nature – electromagnetism and the strong and weak nuclear forces – manifested as a single force before separating. As the cosmos cooled, this unified quantum field underwent a sudden “phase transition”, a bit like the one that happens when water freezes into ice. Like the natural cracks that appear in ice cubes, similar defects, in the form of one-dimensional strings, could have occurred as the unified field gave way to its constituent parts.
Confusingly, there is another category of cosmic strings that might exist. These “cosmic superstrings” are related to superstring theory, which hopes to unite the two disparate pillars of modern physics – quantum mechanics and general relativity – into a theory of quantum gravity. In superstring theory, everything in the universe is made of minuscule one-dimensional vibrating strings. These aren’t in themselves cosmic strings. However, if some of these strings were blown up to cosmological scales by inflation, a period of sudden expansion early in the universe’s history, they would have become cosmic superstrings.
Galactic oddities
With so many different theories manifesting cosmic strings, what they would look like today is very much up for grabs. Some would stretch across the observable universe, with ripples of energy travelling along them at close to the speed of light so that they kink and fold into a tangled web. When the strings collide with each other or folded back on themselves, smaller string loops would break off. These loops could also intersect with themselves, breaking off even smaller loops that would add to the jumble.
If this network does exist, however, it should leave its mark on the cosmos by tugging on anything else that has mass. For example, when cosmic strings were as an extension to the standard model, by Tom Kibble at Imperial College London, they seemed to offer an explanation of how galaxies and clusters were seeded in the early universe. The idea was that matter would naturally have been drawn to the intense gravity of cosmic strings, setting off a cascade of structure formation in the early universe, as matter merged and clumped together.
However, in the early 1990s, cosmic strings fell out of favour among theorists. When physicists measured the ancient radiation left over from the early universe, known as the cosmic microwave background (CMB), they found that it was very smooth – varying only very slightly across the entire sky. This didn’t fit with the idea that cosmic strings account for a large part of the universe’s energy. To take a rough analogy, if the surface of a swimming pool is still, you can be pretty sure there is no orca or manatee throwing its weight around down below.
The CMB’s smoothness means that, at most, a tenth of the universe’s energy is wrapped up, so to speak, in these cosmic string structures. “They can’t have been the dominant disturbance that caused the formation of galaxies,” says , a cosmologist at Princeton University. “But they could still be there, and may be detectable, if you look closely enough.”
Peebles is encouraged by some unusually shaped galaxy clusters he identified in a . Even if cosmic strings had little impact on the overall distribution of galaxies today, individual strings could still have deformed individual clusters of galaxies. Imagine a cosmic string cutting through a spherical galaxy cluster like a piece of cheese wire that moves at almost the speed of light. The string’s gravity would distort the cluster, leaving a somewhat-skewed hamburger shape in its wake. “This slight flattening would be common to a large region of space, so you’d look at a lot of galaxies and see if there was any tendency to form one of these sheets,” he says.
Hints of a flat pattern among galaxies near our own had been noted since the 1950s, but recent astronomy surveys have made the case stronger. In his paper, Peebles writes that an analysis of two distinct groupings of galaxies appears to show the requisite squishing. Each cluster, one of which contains our own Milky Way, is about 30 times as long as it is wide.
This flattening, Peebles argues, is hard to reconcile with the standard model of cosmology used to explain the structure of the universe, known as the lambda-CDM model – the Greek letter lambda denoting the hypothetical force that is thought to be accelerating the expansion of the universe, and CDM being cold dark matter. If the finding holds up, Peebles argues, “thoughts will turn to the passages of long, nearly straight cosmic strings”.
New observations of ancient galaxies offer further hints of cosmic strings, as their power to pull matter together could explain how such galaxies grew so quickly. In February, the James Webb Space Telescope (JWST) spotted a handful of surprisingly massive galaxies that formed only 500 million to 800 million years after the big bang, an observation that also challenges the lambda-CDM model. The smoothness of the CMB means that cosmic strings can’t be the main driver of galaxy formation, but they can still have some clout, says at McGill University in Montreal, Canada. “If you have an additional source of structure formation, which cosmic strings can provide, then you can help the situation,” he says.
With this in mind, Brandenberger lightweight strings could have had in the early universe. These have a small mass per unit length – at least by cosmic string standards – which means they have no noticeable effect on the CMB. Yet their gravity is still potent enough to explain the smattering of very large galaxies seen by JWST.
Theorists expect cosmic strings to have their greatest influence on structure formation closer to the big bang, when the string network would have been more tightly woven. That means the best evidence will be found in the very oldest galaxies, says Brandenberger. He plans to study their structure via the , known as 21-centimetre radiation, in galaxies born a mere 50 million years or so after the big bang. This radiation should become clearly visible when the of telescopes in South Africa and Australia switches on in the late 2020s. “I’m most excited right now about 21-centimetre,” says Brandenberger.
Astronomers pointed to another potential hint of the presence of cosmic strings in September – two galaxies that appear, to some observers, to be . The idea is that a cosmic string that happens to fall in between Earth and a distant galaxy could distort or duplicate the image of a single galaxy by gravitational lensing, whereby a very massive object can bend light around it. So one galaxy could appear as two identical ones. Brandenberger, however, is sceptical. “There were already two similar claims, and in both cases the ‘double images’ turned out to be two different objects which we see in the sky as close [together],” he says. Moreover, gravitational lensing can be caused by other very massive objects, such as lumps of dark matter or black holes, so a mirror image isn’t a sure sign of a cosmic string.
Gravitational waves
The trouble with all these proposals is that there are equally plausible explanations. With so much still unknown about the complicated dynamics of galaxies, cosmic strings remain an outside possibility. But there is another way in which cosmic strings may reveal themselves, possibly offering theorists richer data with which to untangle their origins. In June, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), an international collaboration, discovered a background hum of gravitational waves, which appears to be coming equally from all directions in the sky. It uses radio waves emitted by rotating neutron stars, or pulsars, as a way of detecting the slight distortions caused by passing gravitational waves, which subtly change the pulsar signals.
The leading candidate for this background hum is widely thought to be gravitational waves resulting from countless collisions of supermassive black holes scattered about the cosmos. That explanation is favoured on the grounds that we know such black hole binaries exist as a result of galaxy mergers. “If you bet on familiar astrophysics,” says Conlon, “you will win 99 per cent of the time.”
But cosmic strings, which are constantly bending and vibrating due to the tension held in them, would also emit gravitational waves. There is an outside chance that cosmic strings are responsible for the background hum, says Turok, who was one of the first researchers to investigate the physics of cosmic strings back in the 1980s. The problem is that the NANOGrav signal itself is very weak. “The jury is still out” as to whether it is even a real signal, says Turok, who is watching with interest as NANOGrav’s data becomes clearer.
In particular, he will be keeping an eye on whether the “spectrum” of the background hum has a hallmark slope that can only be explained by cosmic strings. In this case, the strength of the gravitational waves should increase in proportion to their wavelength. “Cosmic strings would make a very specific prediction of how the signal increases as you go to longer and longer waves… that’s exciting,” says Turok. However, he expects that another decade of data collection would be needed in order to see any such correlation.
Turok also cautions that calculating the cosmic string spectrum in detail has proven “much harder than people expected”. That is partly because the exact shape of the strings is unknown. As strings collide with each other and pinch off loops, they can end up “quite jagged on small scales”, he says, which makes the computations more challenging.
Quantum gravity
Cosmic strings discovered via gravitational waves could also offer a glimpse into the nature of the strings themselves. “All cosmic strings wiggle in roughly the same way, but the rate [at which energy reduces in] the wiggles is different for different species of string,” says Turok. For example, cosmic superstrings, made when the strings from string theory inflate, would lose energy as they decayed into exotic particles. They would also be less prone to getting chopped up into loops than the cosmic strings made from phase transitions in the very early universe. This is because string theory has extra dimensions that the strings wiggle around in, making them less likely to cross. All of which would affect the strength and form of the gravitational wave signal. Again, though, the actual calculations are proving to be fiendishly difficult, says Turok.
If experimentalists can pull off such a measurement, it would offer a rare observational test of unified theories of nature – including, perhaps, clues about the long-sought quantum theory of gravity. These unified theories would only reveal themselves at enormous energies, beyond the reach even of the Large Hadron Collider. “It would provide an absolutely unique insight into extremely high-energy physics, well beyond any other observation,” says Turok.
Theorists, meanwhile, have been working to see what sorts of fundamental theories are consistent with cosmic strings. In a , for example, at the University of Southern Denmark found that a class of quantum gravity theory, called “asymptotically safe” theories, can’t accommodate cosmic strings made by some extensions to the standard model of particle physics. Theorists prefer symptotically safe theories because they connect to these extensions in a self-consistent way that doesn’t lead to infinities. This might sound like bad news for the existence of cosmic strings, but Eichhorn says observations must lead the way. “If there was a strong discovery of cosmic strings, that would spell trouble” for these extensions, she says.
This would also be taken as a hint that cosmic superstrings are more likely to exist than kinds of cosmic string emerging from extended models of particle physics. “If cosmic strings existed that were actually blown-up superstrings, then clearly this would help us understand quantum gravity,” says Conlon. “It’s a big ‘if’ though!”
For Conlon, discovering cosmic superstrings would be like winning the lottery. “All these years, we’ve been thinking how to connect string theory to any kind of experiment or observation,” he says. “[Imagine] that you then find that the strings of string theory are really there, snaking across the entire universe.”
Dan Falk is a science journalist based in Toronto, Canada. His books include The Science of Shakespeare and In Search of Time
