If the existence of cosmic strings is confirmed, it would be huge. They could help uncover the Holy Grail of physics: a theory of everything. And in the far future, they might even enable us to time travel.

The theories that predict their existence suggest that cosmic strings are thinner than an atomic nucleus. Prof Ken Olum, of Tufts University, describes them as astronomically long tubes. “They would most likely exist in the form of either loops or long strings that go on and on forever,” he says.

But while the strings are super thin, they can pack in a lot of mass. “A typical loop, which may be about 10–20 light-years long, can contain around the equivalent of the mass of tens of thousands of stars,” says Olum. But it’s thought they’d shrink over time as the tubes wiggle, radiating gravitational waves.

Confusingly, it’s also thought that there are two different types of cosmic strings, although they’d behave in very similar ways. One type, called cosmic superstrings, is predicted by string theory, an unproven attempt at a theory of everything, suggesting the Universe is ultimately made up of tiny, vibrating strings.

Cosmic superstrings would essentially be the fundamental strings that produce all the matter in the Universe, stretched out.

The other type is envisioned as a scar from the early Universe. At its earliest stage, the cosmos was in a completely different state from the one it’s in now. It was extremely hot and dense.

Physicists believe that the four forces of nature – electromagnetism, gravity, and the strong and weak forces of the atomic nucleus – were merged into one single force at this point. But we don’t know much about the strange physics going on at the time.

After a period of cooling and extremely rapid expansion, known as inflation, the Universe changed ‘phase’ to become a very different place. This involved particles forming and the forces breaking off from each other.

Such phase transitions happen in water, as it freezes or evaporates. J Richard Gott, professor emeritus of astrophysics at Princeton University, says you could think of the phase transition as a 'melting' of high-density energy in space. “When snow melts, you can be left with a few snowmen still standing,” he explains.

And that’s, essentially, what cosmic strings are: areas of very high-energy density left behind during a cosmic transition from higher to lower energy.

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Freezing is also a good analogy. If you fill an ice cube tray with water, you can see imperfections form while it freezes, including cracks and air bubbles. Cosmic strings would emerge in a similar way.

Physicists suspect cosmic strings could still be around today. However if we actually encountered one, which is unlikely, it might be unpleasant. A cosmic string travelling close to the speed of light and hitting Earth head-on would cut the planet in half, just like a wire cutting through a ball of clay. But one half would then immediately slam back into the other, according to David Chernoff, a professor of astronomy at Cornell University.

In the best-case scenario, we could experience the equivalent of a magnitude 7 earthquake, says Chernoff. But it could produce a seismic event of up to around 12.6 magnitude – considerably bigger than the largest earthquake in history.

Searching the sky

The hunt is on to find cosmic strings. One approach is to look for spooky copies of the same galaxy or other bright objects. Because of their high density, the strings would cause gravitational lensing. This happens when heavy objects warp the space-time around them and alter the path of light as it passes by.

A cosmic string would bend space-time into a cone-like structure. So if we were looking at a galaxy from Earth and a cosmic string appeared vertically in front it, the light from the galaxy would be split – travelling on each side of the string. Meaning we should see two identical images of the same galaxy, a small distance apart.

There have already been reported sightings of this kind. In 2003, a team of astronomers reported the potential discovery of a cosmic string, but it was later proven that the two galaxies they had seen were not identical. Another such claim was made last year. But Olum warns that recent research suggests this method of spotting cosmic strings is far from ideal.

Cosmic strings were originally thought to be much denser than scientists believe today, having what physicists call high tension, or mass, per unit length. In fact, they were once thought to be responsible for forming structures such as galaxy clusters by gravitational attraction.

However, observations by Olum and others suggest the strings should be fairly light. That’s because very dense strings should cause certain anomalies in the cosmic microwave background, the light left over from the Big Bang.

But those haven’t been spotted, suggesting the strings, if they exist, must be about 10,000 times lighter than scientists initially thought. And they couldn’t be the main cause of matter clumping together in the Universe. But it also means they’d warp space-time so subtly, that you wouldn’t actually be able to see two distinct images of galaxies, due to the gravitational lensing.

Chernoff has come up with a more promising method of spotting them called gravitational microlensing. Instead of looking at galaxies, which appear as fuzzy objects, he wants to spot lensing by observing individual stars.

Star light, star bright

Chernoff’s proposal involves looking at stars in the Milky Way and nearby galaxies and monitoring their brightness. The theory is, if a string passes between us and a star, the light from the star would be gravitationally lensed by the string.

For a short period of time, two unresolved images of the star would appear in the sky – though they wouldn’t be separately discernible. But there would be a sudden and detectable doubling of the star’s brightness. In fact, a string passing in front of a star should increase its brightness by a factor of two for a few days, before it goes back to normal, explains Chernoff.

While he's started investigating some data, he’s yet to find a cosmic string. The approach is considered speculative. How likely we are to find strings this way will depend on how fast they’re moving and whether they’re clustered in galaxies – things we don’t know for sure. But there’s optimism among physicists. “This is an interesting idea,” says Olum, a view supported by Gott.

Chernoff himself is cautiously optimistic. “We’re actually in a golden age for this experiment,” he says. “The National Science Foundation, NASA and ESA are building these experiments to do exactly what we need, which is to repeatedly measure the light from large numbers of stars.”

He highlights the Vera C Rubin Observatory, currently under construction in Chile, as promising. It will allow us to make a time-lapse movie of the sky, looking at 17 billion never-before-seen stars.

The Vera C Rubin Observatory could be a vital tool in the search for cosmic strings. Photo credit: aurore simonnet/nanograv, vera rubin observatory

If we do discover a string, the approach would be perfect for studying it. “Once you've detected a microlensing event, there’s a whole bunch of other things that you can immediately do,” says Chernoff, adding that the crucial advantage is that we’ll know its exact location – pinpointed by an individual star. “So all the gravitational wave detectors could then be focused on a point in the sky.”

The most promising data so far, however, comes from gravitational waves. Last year, the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, a collaboration between teams based in the US, Europe, Australia and China, reported evidence for a background of gravitational waves with long wavelengths.

These were spotted by observing pulsars, rotating objects that emit pulses of radiation at regular intervals. But gravitational waves with long wavelengths, such as those thought to be produced by cosmic strings, can warp space-time in a way that makes the radiation from the pulsars irregular, allowing scientists to detect them.

The most likely explanation for the result is pairs of supermassive black holes orbiting each other as galaxies merge. That’s because we know that black holes actually exist, unlike cosmic strings.

But is it the whole story? Long-wavelength gravitational waves kicking around in the galaxy may also come from other sources, including cosmic strings. The signal seen by NANOGrav may in fact be a cocktail of different types of gravitational waves.

When the team analysed the data, they were rather surprised. “The model of black holes doesn’t fit well,” says Olum who is part of the collaboration. The team went on to try and match the signal to various astrophysical sources and discovered that cosmic superstrings, the kinds predicted by string theory, fit the data well – better than black holes or the alternative type of cosmic strings.

The finding has led to a lot of excitement about the possibility that NANOGrav has discovered the first evidence for string theory. “We actually got a detection,” says Gott. “That means we’ve seen something.”

Olum remains cautious, however. “It could very well just be black hole binaries,” he argues. “The next data set may show something different.” Indeed, more data is likely to make the signal stronger and more reliable. But for that, we have to wait a few years.

Further ahead, the planned Laser Interferometer Space Antenna, LISA, will launch into space during the 2030s and continue the search for gravitational waves. “LISA is better matched to detect gravitational waves from cosmic strings than either NANOGrav or LIGO,” says Chernoff.

Illustration of the LISA Pathfinder space probe — The LISA probe could provide more telling evidence for the existence ofcosmic strings. Photo credit: Science Photo Library

Finding cosmic strings would be revolutionary – giving us important hints about what a deeper theory of physics would look like.

Not only may their detection give observational support for string theory, they might also provide a means to test so-called 'grand unified theories', which aim to describe the very young Universe, in which three of four fundamental forces (apart from gravity, you need a theory of everything for that) are united.

“There’s a huge number of grand unified models,” says Olum. “So even if we can just rule out some of them, that would help people know where to focus their attention.”

Cosmic strings could also help explain a series of recent, puzzling observations, including impossibly large black holes spotted by the James Webb Space Telescope. These beasts are so huge they must have grown faster than scientists thought possible. But it’s possible they may have been “seeded by the gravitational attraction of cosmic string loops in the early Universe,” says Olum.

The possibility of time travel

Could cosmic strings be practically useful? Scientists have long known that Albert Einstein’s theory of General Relativity allows loops in time to exist – possibly enabling time travel. This is because space-time can be curved until it closes in on itself and creates a loop.

Nobody has ever seen or built one, but such loops are a valid solution to Einstein’s equations. In 1991, Gott discovered that two moving cosmic strings could produce such a loop – and are an exact solution to the equations.

Heavy objects such as cosmic strings can bend space-time to create shortcuts in space-time. That’s ultimately what a wormhole is thought to be: a region connecting areas of space very far apart. “General Relativity says you can beat a light beam geometrically in a race if there’s a shortcut,” says Gott.

If you travelled between two planets, for example, and there was a cosmic string in the middle, this would create a shortcut compared with travelling straight between the points.

If you took this route and travelled near light-speed, you’d be able to arrive at your destination faster than a light beam travelling straight between the two planets. That means when you arrived at your destination, you’d be able to watch yourself packing your bags at your starting point.

Gott showed that two cosmic strings travelling in opposite directions, and crossing each other, would warp space-time in such a way that it would create a time loop. If you travelled around the two strings, you’d go back in time.

But this would require both the spacecraft and the strings to travel at 99.99 per cent of the speed of light, which is hard to achieve. And it’s unlikely that a pair of infinite strings, which are rarer than loops, happen to exist in such close proximity.

“You’re very unlikely to find an infinite string moving that fast in the observable Universe; they typically move at around half to 70 per cent of the speed of light,” says Gott. “But the reason we're investigating this is we want to uncover clues as to how the Universe works.”

“There’s nothing wrong with his maths,” says Olum, although he remains sceptical about the possibility of ever building a time machine. “If your Universe doesn’t have this time machine already, you can't just take some matter and use it to make the cosmic strings proposed by Gott,” he explains. “The strings would have to stretch across the whole Universe. So it’s not very useful in terms of building a time machine.”

The secrets of the Universe

Gott admits it would be hard to build a time machine in this way. But he believes there may be another way. Rather than using infinitely long strings, we could use a finite string loop in the shape of a rectangle, with two very short sides and two enormously long sides (potentially thousands of light-years long).

The next step would be to let the loop collapse. “You can manipulate this loop by moving massive spaceships around it,” he suggests, adding that “we’re talking about super civilisations here."

This movement would stretch and drag the loop around until it collapsed into a black hole, bringing the sides towards each other at near light-speed. This would be an analogous situation to the time machine he presented in 1991, except that this time the solution wouldn’t be exact,
just approximate.

“So if your civilisation agrees to collapse a time loop around you, what you have to do in your spaceship is wait until the sides of the loop get close.” And they could get very close – millimetres apart. You would then fly around the strings to visit your past.

There are downsides, though. “You’ll find yourself trapped in a black hole, so you can’t get back out to tell your friends about your adventures," he says. It would also be technically challenging. According to Gott, the time loop would need a mass corresponding to about half a galaxy.

The thought is dizzying. But to most physicists, including Gott, the alluring thing about cosmic strings isn’t their potential for time travel. It’s the fact that they may help uncover the secrets of the Universe on the most fundamental scales. Let’s just hope one doesn’t pass too close and slice Earth in half before we decode those hidden secrets.

About our experts

Ken Olum is a research professor of physics and astronomy based at Tufts University, Massachusetts. His research focuses on gravitational waves and cosmic strings and has been published in Astrophysical Journal, Journal of Cosmology and Astroparticle Physics and Monthly Notices of the Royal Astronomical Society.

J. Richard Gott is a professor of astrophysics based at Princeton University, New Jersey. He studies the General Theory of Relativity and his work has been published in Monthly Notices of the Royal Astronomical Society, Astrophysical Journal and Physical Review.

Prof David Chernoff is the chair of the Department of Astronomy at Cornell University, New York. His research focuses on cosmology and quantum physics and has been published in Physical Review, Astrophysical Journal and Monthly Notices of the Royal Astronomical.

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