On the interactions of black holes and cosmic strings

In the early universe, cosmic defects, such as cosmic strings, emerge during phase transitions, resulting from the spontaneous breaking of symmetry in field theories. These defects, whether arising from global or gauge symmetries, strongly impact the dynamics of spacetime. In this paper, we investigate the relativistic orbits and shadow properties within the Schwarzschild black hole spacetime pierced by an infinitely thin cosmic string. By deriving a fully relativistic orbit equation, we analyze the dynamics of massive particles in this curved spacetime. Our analysis reveals significant differences in the relativistic orbits, particularly in the periastron shift induced by the deficit angle due to the presence of the cosmic string. Through simulations and ray-tracing formalism, we assess the alterations in the shadow cast by these spacetimes, dependent on the deficit angle parameter beta. We compare our results with those of standard spherically symmetric and static Schwarzschild black hole spacetime. This work has led to an interesting conclusion; it is quite difficult to tell apart black holes which are pierced by cosmic strings from those which are not solely based on observational features. This result holds true only for the range of string tensions allowed by observational data from Planck Satellite, Pulsar Timing Array (PTA), LIGO-VIRGO and even future estimates from LISA, for black holes based in a general relativistic background.