Massless cosmic strings in an expanding universe

Circular massless cosmic strings which move with the speed of light in the de Sitter universe are described. The 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 to the Hubble radius. We analyze the images of the strings, the influence of strings on the trajectories of matter, the formation of overdensities, and the shifts of the 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 Minkowski spacetime to curved backgrounds.