Cosmic string observations via black hole energy extraction

Cosmic strings which become attached to rapidly spinning black holes can extract significant amount of rotational energy and angular momentum. We will discuss the effect on primordial black holes, which are expected to have form with one or more cosmic strings attached. Although large primordial black holes are predicted to rapidly spin up due to accretion soon after forming, we argue that cosmic strings will spin them down again. As a result, the spins of primordial black holes should consequently be observed to be near zero. We will also consider the effect on a supermassive black hole of capturing a cosmic string and the possibility of observing the subsequent spin down by its effect on a pulsar orbiting the black hole.