Black Hole Microstate Counting
The D-brane configuration under consideration consists of \(Q_5\) D5-branes, \(Q_1\) D1-branes, and \(Q_k\) quanta of light-like (conventionally) left-moving momentum along the common compactified dimension (similar to Kaluza–Klein modes). This is an excited BPS state. The statistical entropy is given by the number of distinct ways in which we can distribute the total momentum on the available excited states of the system. Since we have light-like momenta, we have to look for massless excitations. We use the BPS nature of our configuration to go to another point in parameter space which facilitates the calculation of the number of excited states. We consider the picture where \(N g_s \ll 1\), while our calculation of the entropy in the previous lecture was performed at \(N g_s \gg 1\).