Tachyon Condensation and Non-BPS D-branes in a Ramond-Ramond Plane Wave   Background
  We consider string theory in maximally supersymmetric type IIB plane wave background with constant five form Ramond-Ramond flux (RR plane wave). It is argued that there exists a universal sector of string configurations independent of the null coordinate $x^-$ such that the space-time action evaluated at such a configuration is same in the RR plane wave and flat background. By naturally assuming its validity for the open strings we further argue that the D-branes extending along $x^{\pm}$ are universal in these two backgrounds. Moreover, a universal D-brane which is BPS in flat space must be tachyon-free in RR plane wave and a non-BPS D-brane should have a real tachyon whose potential is universal. Given the above observation we then proceed to describe open string theories for the non-BPS D-branes in RR plane wave. It is suggested that the light-cone Green-Schwarz fermions on the world-sheet satisfy certain bi-local boundary condition similar to that corresponding to flat space. We attempt a canonical quantisation with this boundary condition which gives rise to an open string spectrum similar to that in flat space - containing an R and NS sectors of states. In this process we encounter certain subtleties involving the R sector zero modes. Although the derivation of the R sector zero mode spectrum using the open string theory has not been completely settled, we suggest definite answer by requiring consistency with the relevant space-time interpretation. We finally generalise the above basic features of universality to all the exact pp-wave backgrounds with the same dilaton profile in a given string theory. 
Partha Mukhopadhyay