COSMIC string bears little relation to its humble domestic namesake. Forming lines of energy billions of light years long, it is narrower than a proton, and so dense that a piece 1 metre long weighs as much as an entire continent. Cosmic string is convulsed by ancient energies. It moves at almost the speed of light, shaking the fabric of space and time. It is not suitable for tying up small parcels.
In recent years, physicists have found that their theories are determined to spin out some kind of cosmic string. It also keeps cropping up in all sorts of experiments aimed at modelling the early universe. Although we have never seen it, many cosmologists are now convinced it is out there.
“Our best theories of the early universe seem determined to include cosmic string”
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Cosmic string would be handy stuff: stirring up magnetic fields, illuminating the early universe and firing out energetic particles (see “Useful stuff, string”). If we found a strand, it would tell us something about the fundamental nature of reality. We might discover that cosmic strings are giant cousins of the tiny “superstrings” thought to make up all matter. Or we might find we have unearthed a relic of the big bang, a place where the chaos of those earliest moments persists to this day.
It all hinges on the original theory of cosmic string being right. The idea dates back to 1976 when theoretical physicist Tom Kibble of Imperial College London suggested that string-like defects were formed in the aftermath of the big bang. As the hot young universe cooled down, it went through a number of sudden phase changes, like steam condensing to water and then freezing. The original melee of heavy particles “froze out” to become today’s universe of relative calm. Kibble realised that such rapid freezing ought to leave defects behind – when something freezes quickly it usually does so messily. Flash-frozen ice is white because it is full of defects, caused by rapidly growing crystals haphazardly pushing up against each other.
The idea appealed to cosmologists in the 1980s, as they thought these defects might have formed the seeds of galaxies and galaxy clusters. Especially promising were one-dimensional defects, or cosmic strings – their gravity seemed to be about the right strength to do the job. However, enthusiasm began to ebb in the late-1990s when astronomers started to get a good look at the cosmic microwave background. This radiation from 400,000 years after the big bang reveals the distribution of matter at that time, and various experiments carried by balloons and on the ground showed that the pattern was too complicated to have been imprinted by cosmic strings – a conclusion confirmed by NASA’s (WMAP) a few years later. Rather, the pattern fitted the predictions of inflation, the idea that the early universe underwent a violent expansion that stretched microscopic quantum fluctuations into what eventually became the galaxy clusters and superclusters of today. Suddenly there was no pressing need for string to tie cosmological models together, and for a while it was left on the shelf.
Now cosmologists have found a reason to open the cupboard and play with the stuff again. It turns out that the microwave background does not forbid the stuff; it only puts an upper limit on how much there can be. Cosmic string could still be responsible for about 10 per cent of the universe’s structure.
More compellingly, our best theories of the early universe – attempts to explain that first fraction of a microsecond – seem determined to include some cosmic string.
When the universe was a mere 10-35 seconds old, physicists think three of today’s forces of nature were jumbled together in one giant superforce. As space cooled, the superforce split into the strong nuclear force (binding quarks together), weak nuclear force (which triggers certain kinds of radioactive decay) and electromagnetism. This happened in a series of sudden changes that are thought to have sparked and then ended inflation. That superforce included some force fields that we no longer notice. In the heat of the young universe they were free to change, but as space cooled they froze and adopted fixed values.
Ways with string
Strings form if one of those fields gyrates, like a compass needle able to point in different directions. Such a field might point north in one region of space, east in another, south-south-west in a third. Where these areas meet, the field values will try to equalise smoothly, but this is not always possible. There will always be some regions in space where the compass ends up pointing outwards in all directions (see Diagram). These regions join to form a cosmic string where the quantum field remains in a random jitter, hemmed in by a range of irreconcilable values around it. So at its heart, the string remains in the original hot, dense state of the big bang.
Did the original superforce really include gyrating fields? In 2003, Mairi Sakellariadou at King’s College London, and two colleagues looked at a wide range of possible grand unified theories – physicists’ attempts to describe the superforce. They found that 80 to 90 per cent of these theories do indeed have the right kinds of field to make strings. “It’s very typical that strings formed at end of inflation,” says Sakellariadou.
More good news comes from superstring theory, an even more ambitious attempt to unify physics because it brings gravity into the same picture as all the other forces. Confusingly sharing the “string” tag, superstring theory seemed to have little connection with cosmic strings. Superstrings are not universe-girdling giants but minuscule vibrating filaments, which are thought to form the basis of every variety of subatomic particle. Inflation might have been able to stretch a tiny superstring to astronomical size, but early calculations showed that the resulting string would be millions of times denser than “conventional” cosmic string. Its gravity would dominate the structure of the universe and scrawl a very obvious message on the microwave sky.
That changed in 2003. Superstring theory needs at least six or seven extra spatial dimensions, which can be curled up in different ways. Shamit Kachru, a theorist based at Stanford University in California, and others worked out that the extra dimensions could be warped in a way that makes the fundamental strings much lighter. Cosmic string could indeed be an inflated superstring. “Superstring theories seem now to have a natural place for these things,” Kibble says.
And there is a third method of making string. Modern versions of string theory include objects called branes, short for membranes, which can have two or more dimensions, and drift around in the multidimensional space of superstring theory.
String theorists have suggested that inflation was caused by two cosmic-scale branes spanning the universe. The forces between them pulled them together, which generated the explosive energy that drove inflation. When the branes finally collided, the wreckage they left behind was our universe threaded with string-like scars. Similar to conventional cosmic strings, these would be one-dimensional worlds filled with a brew of trapped exotic particles.
The hubbub of speculation has given new life to cosmic strings, but it would be nice to see some actual evidence for them. It has arrived from an unexpected direction.
In a few laboratories around the world, physicists are attempting to make model universes by stirring pots of liquid helium.
The best kind of helium for simulating the universe turns out to be a relatively uncommon isotope called helium-3. At two thousandths of a degree above absolute zero, helium-3 turns into a superfluid and flows with no resistance. Like space it is smooth and featureless, and it even shares some of the subtle quantum properties of the vacuum of space. Best of all, it can be put through a phase change that mimics the cooling early universe. When helium is suddenly cooled into the superfluid state, it forms a tangle of defects called vortices. In the eye of quantum mechanics, they are almost identical to cosmic strings.
“A tangle of stringy defects forms in liquid crystals, heated fluid and superconductors too”
Even colliding branes can be made from the stuff. In a recent experiment, George Pickett and his team at the University of Lancaster in the UK chilled a test tube of helium-3 to 150 microkelvin to make sure it was almost entirely in the superfluid state. They then used a strong magnetic field to change the rotation of helium atoms in the central section of the cylinder. Sandwiched between sections of unaltered helium, the slab of helium in this altered state has two flat boundaries that are mathematically like the branes of string theory. When the researchers turned off the magnetic field, they found that the altered phase shrank and the branes collided, leaving behind a tangle of string-like defects ().
Things like cosmic strings form in several other systems too, including liquid crystals, superconductors and in heated fluid just as it begins to convect. In all these cases, a phase transition produces a tangle of stringy defects.
Whip crack away
Pickett and the other laboratory researchers are mainly interested in these systems for their own sake, but for cosmologists, the defects are a bonus. “They can see strings in the lab, and that’s amazing,” says physicist Anne Davis of the University of Cambridge. Although these experiments are reassuring to proponents of cosmic string, they are limited: the universe is not actually a blob of helium or a sliver of superconductor. “We won’t get direct information about cosmic strings from these experiments – that would be too much to expect,” Kibble says.
Seeing cosmic string in space, rather than in the lab, is the only way to prove it exists. In 2003, astronomers thought they had found one. The gravity of a cosmic string should bend light in such a way as to make a galaxy behind it appear to be in two places, on either side of the string. A team led by Mikhail Sazhin of Capodimonte Astronomical Observatory near Naples, Italy, spotted what appeared to be two identical images of a single galaxy (żěè¶ĚĘÓƵ, 18 December 2004, p 30).
It turned out to be a false hope: the Hubble space telescope showed that the two images were two different, deceptively similar galaxies. Kibble thinks we’d be lucky to spot cosmic string this way, because the deflection of light it would produce is probably very slight. “I’m not holding my breath,” he says. Cosmologist Alexander Vilenkin of Tufts University in Medford, Massachusetts, is more hopeful. “Only now are observers beginning to look for strings systematically,” he says.
A new analysis of the cosmic microwave background may have revealed a hint of strings. Neil Bevis, of Imperial College London, and his team looked at the WMAP data and found they could get a better fit for the readings if they used a model that includes strings as well as inflation. However, the statistical significance of the result is slim, and it may contradict other data, so for now nobody is claiming that cosmic string has been detected.
The gravity of cosmic strings should also make its mark on another kind of background radiation: radio waves emitted by hydrogen gas when the universe was more than 400,000 years old. Benjamin Wandelt of the University of Illinois at Urbana-Champaign has pointed out that we could detect this signature – but only with an array of radio receivers covering 1000 square kilometres, which is unlikely to be funded any time soon ().
Still, finding strings is far from a lost cause. “We are more likely to see them by detecting gravitational radiation,” says Kibble. The fields within a cosmic string are under great tension, which makes them writhe. Such a dense object waving about in space would send out strong distortions called gravitational waves. Ripples running along a string can form a sharp cusp that would move close to the speed of light, sending out a whip crack of gravitational waves that would sound like nothing else in the universe.
Could we hear it? The strength of gravitational waves depends on how heavy the string is, but the prospects aren’t too bad. The , a detector based at two sites in the US, will stand a modest chance of catching the cosmic whip-crack once it is upgraded in 2013. A planned space-based detector called LISA will be even more sensitive. “If we can detect them we’ll learn a great deal about the universe and particle theory,” Vilenkin says.
Detecting the separation between lensed images or the loudness of the gravitational waves could give us a measure of the string’s mass. In the case of conventional cosmic strings, this could tell us what the temperature was when the primeval superforce split. That would provide a crucial test for any grand unified theory of physics, which will have to predict a split at the right temperature.
What if cosmic strings turn out to be giant superstrings or the relics of colliding branes? These more exotic objects can be distinguished from the conventional variety. When a conventional cosmic string crosses over itself, the theory says it always casts off a loop, which then shrinks and disappears. So random writhings through the ages would have snipped away most of the original ball of string, leaving only a few complete strings in the universe today. But strings formed from superstrings and by colliding branes have the extra dimensions of string theory to dodge around in, so they cast off loops less frequently. If these things did form in the early universe, then more of them would have survived, perhaps as many as a thousand. It is even possible that they would knot together into a taut cat’s cradle stretching across the universe.
Discovering them would be really big news. String theory has often been criticised as a theorists’ plaything, a pretty piece of mathematics unable to make any testable predictions. That perception would change pretty fast if we were to find a host of giant superstrings crisscrossing the skies.
COSMOLOGY
Cosmology – Keep up with the latest ideas in our .
Useful stuff, string
Cosmic string may be unwieldy for small parcels, but you can still use it to tie up a few loose ends.
A single piece could seed entire galaxies. Its powerful gravity would attract surrounding gas, kick-starting the process of collapse that turns gas into stars.
Although we know now that string isn’t the main catalyst for forming galaxies and larger structures in the universe, it could play one special role. Less than a billion years after the big bang, it seems that enough stars had already formed to flood the universe with ultraviolet light, which was bright enough to blast apart the hydrogen atoms in intergalactic space. Conventional models of star formation say that this should not have happened so early in the life of the universe. Maybe string did the job.
Ken Olum and Alexander Vilenkin at Tufts University in Medford, Massachusetts, have calculated that loops of cosmic string could have seeded a first generation of galaxies, enough to cast that early, ionising light.
After that, the cosmic string could have stirred up some magnetism. The origins of the relatively smooth large-scale magnetic field that pervades today’s universe is somewhat mysterious. Most mechanisms put up to explain its origins make a more tangled field. However cosmic string would create a smooth magnetic field by sweeping through the ionised gas.
Even frayed string could be handy. The high-energy particles trapped inside a cosmic string could occasionally leak out, producing the disturbingly energetic cosmic rays that hit Earth.
The cosmos, it seems, really could do with a ball of string.
