Cosmic Strings

The appeal of string theory is in the simplicity of the idea. The devil, as usual, is in the details that follow. But one-dimensional objects are common in physical systems, and sometimes have little to do with string theory as the candidate theory of everything. The Lund string-model for example is an effective description for the fragmentation of color flux-tubes resulting in hadronization. And then there's cosmic strings.

Cosmic strings are stable, macroscopic, one-dimensional objects of high energy density that might be created in the early universe. It was originally suggested by Kibble in 1976 that such objects could form from symmetry-breaking phase transitions in quantum field theory that would take place when the universe was young and hot. These strings then form a network of (infinitely) long strings and loops that evolves with the expansion of the universe. It was thought for a while that strings might seed the density perturbations leading to the large-scale structures we see today, but this turned out not be consistent with the increasingly better data. While we know now that cosmic strings cannot have dominated in the early universe, some of them might still have been present, and still be present today.

The topic raised to new attention when it was found that cosmic strings might alternatively also be created in a string theory scenario in the early universe and then grow to macroscopic sizes. That is interesting because cosmic strings have a bunch of possibly observable consequences. For the purposes of testing string theory, the question is of course if one could distinguish a cosmic string created by ordinary quantum field theory from a cosmic super-string-theory-string.

Two of the most outstanding observables are that cosmic strings create peculiar gravitational lensing effects and can, while they move around, create cusps that release bursts of gravitational waves. There are other, more subtle, signatures, such as the creation of small non-Gaussianities in the cosmic microwave background (CMB) and some influence on the CMB tensor-modes, but the gravitational lensing and gravitational wave bursts have so far gotten the most attention due to the already good experimental prospects of detecting them.

For what the lensing is concerned, every now and then a candidate is found where the lens might have been a cosmic string, though none of them has survived scrutiny. Like CSL-1, that later turned out to be merely two similar galaxies in close vicinity. In any case, the gravitational lensing wouldn't allow us to tell whether we're looking at a super-string or not.

There are however differences between fundamental and non-fundamental cosmic strings that have been pointed out during the last years. These stem from the presence of additional spatial dimensions in super-string theory. These have the consequence of altering the evolution of the string network, resulting in a denser network today, that might give one the hope that bursts of gravitational radiation are more likely to occur. Recently though, a more detailed study has been done, examining the motion of the string and the gravitational radiation emitted by taking into account the additional dimensions:

Effect of extra dimensions on gravitational waves from cosmic strings
By Eimear O'Callaghan, Sarah Chadburn, Ghazal Geshnizjani, Ruth Gregory, Ivonne Zavala
Phys. Rev. Lett. 105:081602 (2010).
arXiv:1003.4395 [hep-th]

In their analysis, the researchers found that the presence of compactified extra dimensions larger than the width of the string dampens the gravitational wave emission. The effect depends on the the number of extra dimensions, and the damping can be several orders of magnitude. While this is interesting in the sense that the signal carries information about the sort of string one is dealing with, it means unfortunately that the signal is also far less likely to be detected at all. The strength of the damping depends also on the ratio of the width of the string and the size of the extra-dimensions, though this dependence is hidden within the model and not obvious from the results. I wrote to one of the authors of the above paper, Ruth Gregory, and was explained that simulating the dynamics of a thick string was quite a challenge which is why they had to resort to an empirical model.

A signal of cosmic strings would be tremendously exciting either way. But so far the prospects of being able to unambiguously assign such a signal to string theory seem slim.