Gravitational-wave background: detection and implications

Student thesis: Doctoral ThesisDoctor of Philosophy


The gravitational-wave background (GWB) is a superposition of many astrophysical and cosmological sources, such as unresolved compact binaries, cosmic strings, and phase transitions in the early universe. The first three observing runs by LIGO/Virgo/KAGRA (LVK) have not made a detection of the background. Nonetheless, the existing stringent upper limits can be used to constrain theoretical models and study the early Universe.

In this thesis we explore the various aspects of the GWB - from its detection to implications. We start with a careful treatment of correlated magnetic noise sources that could limit our intrinsic detector sensitivity, and even yield a false detection. In a separate study, we highlight the importance of source separation in the case of a detection. By separating the individual sources we may reveal remnants of early Universe processes. The gravitational radiation that decoupled soon after the Big Bang is a window into physics at energy scales inaccessible to particle colliders. We search for GWB from a first-order phase transition in the LVK frequency range, and in this way place constraints on couplings and masses of particles beyond the Standard Model. New physics could also present itself in the form of parity violation in the early Universe and we search for hints of a polarised GWB. Additionally, we study mergers of remnants formed by the hypothetical first stars in the Universe, and the GWB they create. We look ahead and investigate detection prospects with proposed detector upgrades, providing strong motivation for the next generation of detectors. Finally, we test general relativity using data on rotation curves of the
surrounding galaxies, and by measuring the speed of gravitational waves.
Date of Award1 Jun 2023
Original languageEnglish
Awarding Institution
  • King's College London
SupervisorMairi Sakellariadou (Supervisor) & Eugene Lim (Supervisor)

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