CONSTRAINING THE GRAVITATIONAL WAVE BACKGROUND OF COSMIC STRINGS USING PULSAR TIMING ARRAYS

Abstract

The University of ManchesterABSTRACT OF THESIS submitted by Sotirios Asimaki Sanidasfor the Degree of Doctor of Philosophy and entitled"Constraining the Gravitational Wave Background of Cosmic Strings using Pulsar Timing Arrays", April 2012 The existence of cosmic strings was proposed in the mid-seventies as a by-product of the various phase transitions that occured in the early Universe. Cosmic strings are one-dimensional topological defects; structures of extremely high energy density with infinitesimal widths and lengths of cosmological size. After they were proposed, cosmic strings with GUT energy scales became very popular as a potential source for galaxy formation, but after CMB observations ruled out this possibility, they stopped attracting much scientific attention. The whole field was revived as part of superstring theory, where the formation of cosmic (super)string networks is a very common characteristic of brane inflation models, allowing them to acquire energies over a much more extended range. Attempts to detect cosmic strings centers on the three most basic observational signatures they create: CMB anisotropies, gravitational lensing events and the stochastic gravitational wave background they are expected to have created. So far, no detection of cosmic strings has been achieved. Their non-detection has inevitably led to setting constraints on their most important characteristic; their lineal energy density (or tension) which describes their energy scale. The topic of this thesis is how to use pulsar timing arrays (PTAs) in order to set constraints on the string tension. The limits PTAs can set on the amplitude of the stochastic gravitational wave background at ~nHz frequencies can be used to set constraints on the string tension. Such an effort is much more complicated than CMB or gravitational lensing investigations due to the large number of unknown cosmic string model parameters which are involved and for which, not only we do not have any observational evidence for their value, but moreover, they can acquire values over very wide ranges. So far, previous investigations were based on assumptions about these parameters and on the specific gravitational wave emission mechanism from cosmic string loops. In this work we have constructed a new code to reproduce the gravitational wave background from a cosmic string network, based on the widely accepted one scale model. Using this, we have performed numerous simulations to study the effects on the gravitational wave spectrum for each cosmic string model parameter, covering the whole parameter space of interest for each of them. Moreover, we have also extended the application of our code in order to describe cosmic string networks which create loops on more than one scale, models of which have recently appeared in the literature. In particular, we have investigated cosmic string networks which create loops at two distinct scales and loops with scales described by a log-normal distribution After studying the properties of the gravitational wave spectrum from cosmic strings, we combined our simulations with the most stringent limit so far on the stochastic gravitational wave background imposed by the EPTA. This limit is provided as a function of the slope of the gravitational wave background and we have also used this information for the first time to acquire even more accurate results. In our approach, we did not make any assumption about the values of the cosmic string model parameters, investigating all possibilities and we managed to compute a conservative and completely general constraint on the cosmic string tension, G mu

Date of Award	31 Dec 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Richard Battye (Supervisor) & Benjamin Stappers (Supervisor)