Gravitational Effects of Cosmic Strings in Friedmann Universes.

Cosmic strings have been invoked recently as a possible source of the primordial density fluctuations in matter which gave rise to large-scale structure by the process of gravitational collapse. If cosmic strings did indeed seed structure formation then they would also leave an observable imprint upon the microwave and gravitational wave backgrounds, and upon structure on the very largest scales. In this work, the energy-momentum tensor appropriate to a cosmic string configuration in the flat Friedmann universe is first obtained and then used in the linearised Einstein equations to obtain the perturbations of the background spacetime and the ambient matter. The calculation is fully self-consistent to linear order because it takes into account "compensation," or the response of the ambient matter density field to the presence of the string configuration, and is valid for an arbitrarily curved and moving configuration everywhere except very close to a string segment. The single constraint is that the dimensionless string tension Gmu/c^2 must be small compared to unity, but this condition is satisfied in any theory that leads to strings of cosmological relevance. The gravitational wave spectrum and the microwave background temperature fluctuations from a single infinite straight and static string are calculated. The statistically expected fluctuations from an ensemble of such strings with a mean density equal to that found in computer simulations of the evolution of string networks is also calculated. These fluctuations are compared with the observational data on the microwave background to constrain Gmu. Lastly, the role of infinite strings in the formation of the large -scale structure on scales of tens of Megaparsecs observed in deep redshift surveys is examined. Based on the current values for the fraction of the total energy density in long strings and in subhorizon-sized loops, it would appear that gravitational collapse of matter in the wakes behind infinite strings could be responsible for the Megaparsec scale structure.