![]() ![]() There could be millions of axion strings floating through the Milky Way right now, and we wouldn't see them.īut the universe is old and big, and we can use that to our advantage, especially once we recognize that the universe is also backlit. How can we search for these axion strings? Models predict that axion strings have very low mass, so light won't bump into an axion and bend, or axions likely wouldn't mingle with other particles. So, if we determine that we live in an axiverse, it would be a major boon for string theory. This hypothetical axiverse, filled with a variety of lightweight axion strings, is predicted by no other theory of physics but string theory. Instead, they looked like loops and lines - a network of lightweight, nearly invisible strings crisscrossing the cosmos. In the earliest moments of the history of our cosmos, the universe went through phase transitions, changing its entire character from exotic, high-energy states to regular low-energy states.ĭuring one of these phase transitions (which happened when the universe was less than a second old), the axions of string theory didn't appear as particles. But axions-as-dark-matter have to face some challenging observational tests, so some researchers instead focus on the lighter end of the axion families, exploring ways to find them.Īnd when those researchers start digging into the predicted behavior of these featherweight axions in the early universe, they find something truly remarkable. Great! Could axions make up dark matter, which seems to be responsible for giving galaxies most of their mass but can't be detected by ordinary telescopes? Perhaps it's an open question. So, we have lots of new kinds of particles with all sorts of masses. What's more, string theory doesn't predict just one axion but potentially hundreds of different kinds, at a variety of masses, including the axion that might appear in the theoretical predictions of the strong nuclear force. What makes this hard is that we're not exactly sure how these extra dimensions curl up on themselves, and there's somewhere around 10^200 possible ways to do it.īut what these dimensional arrangements appear to have in common is the existence of axions, which, in string theory, are particles that wind themselves around some of the curled-up dimensions and get stuck. So the extra dimensions have to be teensy-tiny and curled up on themselves at scales so small that they evade normal efforts to spot them. These spatial dimensions aren't visible to the naked eye, of course otherwise, we would've noticed that sort of thing. It's proven to be an especially thorny problem to solve, due to a variety of factors, not the least of which is that, for string theory to work (in other words, for the mathematics to even have a hope of working out), our universe must have more than the usual three dimensions of space and one of time there have to be extra spatial dimensions. Now, we need to turn to string theory, which is our attempt (and has been our main attempt for 50-odd years now) to unify all of the forces of nature, especially gravity, in a single theoretical framework. At everyday low energies, this symmetry disappears, and to account for that, and out pops a new particle - the axion. ![]() However, this new symmetry only appears at extremely high energies. One solution to this puzzle is to introduce another symmetry in the universe that "corrects" for this misbehavior. But this symmetry doesn't seem to fit naturally into the theory of the strong nuclear force. ![]() There's one kind of symmetry, called the CP symmetry, that says that matter and antimatter should behave the same when their coordinates are reversed. Physicists love symmetries - when certain patterns appear in mathematics. The axion, named by physicist (and, later, Nobel laureate) Frank Wilczek in 1978, gets its name because it's hypothesized to exist from a certain kind of symmetry-breaking. First, we need to get to know the axion a little better. ![]()
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