New constraints on cosmic strings from the South Pole Telescope

Cosmic strings are stable, one dimensional objects of high energy density that might populate our universe. Cosmic strings can arise in quantum field theories and would form networks that extend throughout the universe. They were discussed three decades ago as a possible origin of cosmological structures, but fell out of favor when that was not compatible with data.

Cosmic strings received renewed interest however since they might appear also in the early universe if superstring theory is taken into account. No longer thought to be necessary to explain present day observational cosmology, the question is now how tightly constrained a possible contribution of cosmic superstrings is and if they may become observable in the soon future, when looked for in the right place with the right means, thus providing a long sought for hint that string theorists are on the right track. For more details, see my earlier post.

A recent paper has now put forward new constraints on the density of such string networks

Cosmic String constraints from WMAP and SPT
By Cora Dvorkin, Mark Wyman and Wayne Hu
arXiv:1109.4947

The brief summary is that the have taken into account new data from the South Pole Telescope and not found anything.

The somewhat longer summary is that cosmic string networks leave an imprint in the anisotropy of the Cosmic Microwave Background (CMB) by actively generating perturbations, even after recombination. Most importantly, they act as lenses for the CMB light, which makes a contribution to the spectrum at large multipole moments or small angular size respectively. See here for an explanation of the CMB anisotropies. The recent measurements from the South Pole Telescope have now much improved the previously available data at large multipole moments. The new data is however perfectly consistent with a string-free universe, which allowed the authors of the above paper to derive improved and tighter constraints on models with cosmic strings.

They are careful to point out however that their constraints directly apply only to the most straigh-forward model of cosmic string networks, and that there are more complicated models (in which cosmic strings are merely meta-stable or there are different types of strings) for which the constraints would look different. In any case, this is yet another negative result for the phenomenology of string theory.