Probing model-independent modified gravity and axion dark matter

• Main Author: Srinivasan, Sankarshana
• Format: Dissertation
• Language: English
• Published: University of Manchester 2023
• Subjects: Axions; Cosmology; Modified Gravity; Pulsars

The nature and dark matter and dark energy represents one of the biggest unsolved problems in fundamental physics over the next few decades. In this thesis, we discuss two subjects that attempt to explain one or both of dark matter and dark energy, modified gravity and axion dark matter. In chapter 2 we present the first model-independent cosmological N-body simulations with a time-dependent effective gravitational force parameterised by a single parameter, µ that is binned in redshift. We compute structure formation observables and show that the majority of the constraining power for future surveys is contained the non-linear regime. We find that the ReACT formalism is most accurately able to reproduce the non-linear matter power spectrum from our simulations. In chapter 3, we validate ReACT across a much larger portion of the modified gravity parameter space. We show that the accuracy of ReACT in the non-linear regime is dependent on the amplitude of the concentration-mass relation and that one can fit for this variation with knowledge of µ(z) and the linear growth factor D(z). We describe a plan for implementing ReACT into a Fisher pipeline in order to perform model-independent modified gravity forecasts for time-dependent binned µ. We review the axion as a well-motivated dark matter candidate and discuss the possibility detecting axion dark matter via its coupling to the electromagnetic sector with radio telescopes. In chapter 4, we discuss how one can optimise search strategies to detect the two-photon decay of axions to photons, with particular focus on the stimulated emission enhancement at the galactic centre. In chapters 5 and 6, we concentrate on the resonant conversion of axions to photons in neutronstar magnetospheres. In chapter 5, we compute the one-dimensional probability of conversion of axions to photons for general dispersion relations. We show that the spectral line signal is broadened due to the rotation of the magnetosphere. We also compute the full time-dependent signal profile via ray-tracing simulations that account for the lensing of the photons as they travel through the charge density of the magnetosphere and the gravitational field of the neutron star. We use these results to update the constraints on the axion-photon coupling using Very Large Array (VLA) data of the magnetar PSR J1745-2900. In chapter 6, we describe a new technique to mine radio pulsar data for the resonant signal using a matched filter. We show that the strength of this technique is in the fact that one can efficiently scan data across a wide range of frequencies for a time-dependent signal. We find that the time-dependence of the signal is suppressed in specific regions of the input parameter space, which one can constrain using a combination of modelling assumptions of the pulsar magnetosphere and measurements of the beam width. We discuss how one can derive a limit on the axion-photon coupling using this and test our pipeline on real data obtained from the Jodrell Bank catalogue on PSR B0834+06.