In the last couple of years there has been some enthusiasm both in the string theory and cosmology communities regarding the possible observation of an astronomical effect of a cosmic string. A pair of images – named CSL-1 – has been observed with the right properties such that they are candidates to arise from a single object lensed by a cosmic string such that it actually appears doubled. See for example the following paper.
The recent Hubble Space Telescope observations of CSL-1 clearly shows that it is not a cosmic string, but just a pair of interacting elliptical galaxies. If there were a string lensing a single object, you would have to see a discontinuity of the picture along the string – very different from lensing by a point mass for example. And even though the shapes are quite similar, the actual shapes would have to have been much more similar.
If CSL-1 could have been interpreted as a cosmic string, surely it would have had a very important impact on our understanding of fundamental physical laws – basically, it could either indicate a field theory phase transition at high energies or that it is possible for some superstrings from the early universe to remain macroscopic and still stable. At any rate, this particular case has not falsified the idea of cosmic strings in general. This should answer the question posed above.
The hypothesis that we may observe cosmic strings has been around for long – and long before it was shown that these cosmic strings could be identified with strings from string theory (the other possibility being that cosmic strings might arise as gauge theory solitons). The idea, that a cosmic string could be a fundamental string (F-string) is actually rather new. Before 1995 it was believed that F-strings have a tension close to the Planck scale and observational data precludes such heavy strings – cosmic strings are bound to have a tension at least two orders lower. Furthermore, Witten showed that long BPS F-strings (in the heterotic string theory) are unstable and hence would never be seen.
There are of course a number of instabilities that would prevent superstrings from growing to cosmic size (but it should be remembered that there is no fundamental principle preventing strings from growing to cosmic sizes – it’s basically just a matter of how much energy the string carries).
One is related to gauge strings. For gauge strings, the U(1) symmetry is exact with a magnetic flux running along the core of the string. However, one expects that should be electric and magnetic sources for every flux, so that the string can break by creation of a monopole/anti-monopole pair.
Another instability is related to global strings: that there are no global exact symmetries in string theory (this is because black holes can destroy global charges). This implies that a domain wall will force the string to collapse. The heterotic and Type II strings are effectively global strings, because they couple to the two-form field B. The Type I string couples to no form field and it can potentially break. The Type I and Type II cosmic superstrings have appeared in brane inflation models. However, it now seems that long superstrings can be stable – they generate networks (generally of (p,q) strings), radiate and lens distant objects (this was exactly what many hoped to see in CSL-1). More precisely, in Type II and Type I string theories it has been realized that it is possible – with the advent of D-branes – to construct long superstrings which are not BPS but are nonetheless stable and potentially observable. And in warped compactifications – as in the Randall-Sundrum model – superstring tensions can be reduced by several orders of magnitude.
I should be said, that a problem might be, however, that string theory generally produces too many and too many kinds of cosmic superstrings. For example, a cosmic superstring may arise as a Dp brane wrapped on a compact (p-1) dimensional cycle and general Calabi-Yau compactifications often have a huge number of S^2 and S^3 cycles.
After the observation of CSL-1, the existence of cosmic strings might be considered less likely for some people. But as such, it was not really a blow to string theory (as Peter Woit seems to be implying), or to the possibility of observing cosmic string, or to new physics in general. Sure, it was really bad luck – think about this: how many years did it take before black holes were “observed”? Does anybody believe that black holes do not exist? Or Dark Matter, for that matter 😉 ?