String theory in cosmological spacetimes.
Hector J. de Vega, Norma G. Sanchez · CERN Document Server (European Organization for Nuclear Research) · 1995
Progress on string theory in curved spacetimes since 1992 are reviewed. After a short introduction on strings in Minkowski and curved spacetimes, we focus on strings in cosmological spacetimes. The classical behaviour of strings in FRW and inflationary spacetimes is now understood in a large extent from various types of explicit string solutions. Three different types of behaviour appear in cosmological spacetimes: {\\bf unstable, dual} to unstable and {\\bf stable}. For the unstable strings, the energy and size grow for large scale factors R \\to \\infty proportional to R. For the dual to unstable strings, the energy and size blow up for R\\to 0 as 1/R. For stable strings, the energy and proper size are bounded. (In Minkowski spacetime, all string solutions are of the stable type).Recent progress on self-consistent solutions to the Einstein equations for string dominated universes is reviewed. The energy-momentum tensor for a gas of strings is then considered as source of the spacetime geometry and from the above string behaviours the string equation of state is determined. The self-consistent string solution exhibits the realistic matter dominated behaviour R \\sim (X^0)^{2/3}\\; for large times and the radiation dominated behaviour R \\sim (X^0)^{1/2}\\; for early times. Finally, we report on the exact integrability of the string equations plus the constraints in de Sitter spacetime that allows to systematically find {\\bf exact} string solutions by soliton methods and the multistring solutions. {\\bf Multistring solutions} are a new feature in curved spacetimes. That is, a single world-sheet simultaneously describes many different and independent strings. This phenomenon has no analogue in flat spacetime and follows to the coupling of the strings with the geometry.