STRINGS IN STRONG GRAVITATIONAL-FIELDS

We study string propagation in curved space-time. In such a problem, the equations of motion and the string constraints are nonlinear and difficult to solve. We propose here a systematic expansion in c, the world-sheet speed of light, to solve the string dynamics. Since c is proportional to the string tension, this amounts to a large alpha' expansion. To zeroth order each point of the string moves along a null geodesic (null string), while the first order correction incorporates the string dynamics. As an illustration we apply our formalism to the Robertson-Walker geometry. In this case, it turns out that the string expands or contracts at the same rate as the whole universe.