String-dominated universe

A system of cosmic strings may evolve either to a scaling configuration, in which the persistence length scales with the horizon distance, or to a string-dominated universe. The former alternative provides the basis of an attractive theory of galaxy formation. This paper is concerned mainly with the latter alternative. It is shown that, provided the strings are formed later, and hence are lighter, than in a conventional grand unified theory, it is possible that our Universe became string dominated rather recently, at about ${10}^4$ yr. Such a universe would behave very like a matter-dominated one. Strings would constitute the bulk of the dark matter required to make Ω=1, but would be very hard to detect.