Cosmic strings

Phase transitions in the early universe can give rise to a network of vacuum strings. The dynamics of strings is studied in flat space-time and in expanding universes. The cosmological evolution of strings is discussed, and in particular the black-hole production by collapsing closed loops. Estimations are made for strings of electroweak (${10}^2$ GeV) and grand unification (${10}^{15}$ GeV) mass scale. It is shown that the effect of electroweak strings is negligible, while grand unification strings can have important cosmological consequences. In particular, massive black holes and large oscillating loops can provide seeds for galaxy formation.