The evolution of a network of strings produced at a grand-unification phase transition in an expanding universe is discussed, with particular reference to the processes of energy exchange between the strings and the rest of the universe. This is supported by numerical calculations simulating the behavior of strings in an expanding universe. It is found that in order that the energy density of the strings does not come to dominate the total energy density there must be an efficient mechanism for energy loss—the only plausible one being the production of closed loops and their subsequent decay via gravitational radiation.

• Received 14 October 1983

DOI:https://doi.org/10.1103/PhysRevD.29.1557