Cosmic Superstrings Revisited

It is possible that superstrings, as well as other one-dimensional branes, could have been produced in the early universe and then expanded to cosmic size today. I discuss the conditions under which this will occur, and the signatures of these strings. Such cosmic superstrings could be the brightest objects visible in gravitational wave astronomy, and might be distinguishable from gauge theory cosmic strings by their network properties. Presented at the Mitchell Conference on Superstring Cosmology, GR17, Cosmo 04, and the 2004 APS-DPF Meeting. Seeing superstrings of cosmic size would be a spectacular way to verify string theory. Witten considered this possibility in the context of perturbative string theory, and found that it was excluded for several reasons [1]. Perturbative strings have a tension close to the Planck scale, and so would produce inhomogeneities in the cosmic microwave background far larger than observed. The scale of this tension also exceeds the upper bound on the energy scale of the inflationary vacuum, and so these strings could not have been produced after inflation, and any strings produced earlier would have been diluted beyond observation. Ref. [1] also identified instabilities that would prevent long strings from surviving on cosmic time scales. In recent years we have understood that there are much more general possibilities for the geometry of the compact dimensions of string theory, including localized branes, and this allows the string tension to be much lower, anything between the Planck scale and the weak scale. Also, we have found new kinds of extended object in string theory. Thus the question of cosmic superstrings (and branes) must be revisited, and this has been done beginning in refs. [2, 3]. A necessary set of conditions is: