COSMIC STRINGS

Cosmic strings are hypothetical ‘scars’ running across the cosmos. Nobody yet knows if they exist, but if they do, they could tie everything we know about the Universe together in one unified theory. The thing is, astronomers may have just uncovered the first evidence of their existence

by MIRIAM FRANKEL

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.

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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.