Singular And Smooth Time-Dependent Orbifolds

Abstract

We consider string theory in a smooth time-dependent orbifold of Minkowski space, which is known as the ‘null-brane’, and whose limit reproduces a spacetime with a null singularity — the parabolic orbifold. We show that adding particles of small enough energy to the geometry does not cause a gravitational collapse. The null-brane is a well-behaved background of string theory where it is possible to compute string scattering amplitudes using perturbation theory. We also mention another way of making the parabolic orbifold smooth — besides considering the null-brane.