Massless cosmic strings in an expanding universe

Abstract

Circular massless cosmic strings which move with the speed of light in the de Sitter universe are described. Construction of the background geometry is based on parabolic isometries of the de Sitter spacetime. Microscopic circular cosmic strings may appear at the Planck epoch and then grow up to the Hubble size. We analyze: images of the strings, influence of strings on trajectories of matter, formation of overdensities, and shifts of energies of photons. These effects allow one to discriminate massless strings from their massive cousins. The present work extends our results on straight massless cosmic strings in Minkowsky spacetime to curved backgrounds.

Optical equations for null strings

“…We use name "null strings" instead of "massless strings" to keep connection to earlier publications. As we will see, the massless strings in space-times with parabolic isometries studied in [10], [11], [14] are a subclass of the null strings.…”

“…Strings in space-times with global parabolic isometries [10], [11], [14] are of special interest since they allow explicit description of backreaction effects. They were called massless strings.…”

“…By taking into account (2.10) one concludes that strings studied in [10], [11], [14] are null strings for which…”

“…Tensionless strings have zero rest mass but a finite energy per unit length. So one can also call these two types of cosmic strings, respectively, massive and massless strings [10], [11]. These names are more appropriate in studying physical effects caused by the gravitational field of the stings on the surrounding matter.…”

“…A method how to describe physical effects around massless cosmic strings has been developed in [10], [11] for strings in flat and de Sitter space-times. Here backreaction effects can be described analytically due to maximal isometries.…”