Closed strings as imaginary D-branes

Sen has recently drawn attention to an exact time-dependent boundary conformal field theory with the space–time interpretation of brane creation and annihilation. An interesting limit of this BCFT is formally equivalent to an array of D-branes located in imaginary time. This raises the question: what is the meaning of D-branes in imaginary time? The answer we propose is that D-branes in imaginary time define purely closed string backgrounds. In …