Gravitational Waves and the CMB

The discovery of gravitational waves by the LIGO experiment has led to considerable excitement. The Cosmic Microwave Background (CMB) is capable of detecting gravitational waves of much lower frequency. Primordial gravitational waves, for example, have been well studied and constrained by the large angle polarisation of the Planck and BICEP2 experiments. However, a gravitational wave background with intermediate frequency (in the range ~10^-15 to 10^-10 Hz) is also detectable by the CMB. This signal is expected to act like massless particles in the short wavelength approximation, although will depend on the frequency and initial conditions (e.g. if the gravitational waves are produced by cosmic strings or some other phase transition, cyclic models, reheating of the inflation). The first part of this project will explore the signatures of these different processes in the CMB, and will obtain a limit of the energy density of gravitational waves as a function of frequency. The second part of the project will relate these bounds to parameters describing a network of cosmic strings. Cosmic strings are expected to form in the early Universe, and will lead to a stochastic background (as well as a transient signal) of gravitational waves. The aim here is to study how the modelling of the string network influences the gravitational wave signature.