Relevant Degree Programs
Graduate Student Supervision
Doctoral Student Supervision (Jan 2008 - May 2019)
The prevailing model of modern cosmology stipulates the existence of exotic substances such as dark matter and dark energy and events such as inflation. However, their underlying nature is not currently known. In this thesis, we explore new models and measurement techniques that may be used to characterize their cosmological effects and shed light on their inner workings. A model of inflation driven by a substance that may be described macroscopically as a cosmological elastic solid is studied. The proper techniques for the quantization of perturbations within the elastic solid are presented. We find that a sufficiently rigid elastic solid with slowly varying sound speeds can produce an inflationary period. Interestingly, we find models where the elastic solid has an equation of state significantly greater than -1 that nevertheless produces nearly scale-invariant scalar and tensor spectra.The remaining chapters of this thesis concern the use of 21-cm radiation as a probe of the physics of dark matter and dark energy. The effects of warm dark matter on the highly-redshifted 21-cm signal is examined. If dark matter is warm instead of cold, its non-negligible velocities may inhibit the formation of low-mass halos, thereby delaying star-formation, which may delay the emission and absorption signals expected in the mean 21-cm signal. The effects of warm dark matter on both the mean 21-cm signal, as well as on its power spectrum, are described and degeneracies between the effects of warm dark matter and other astrophysical parameters are quantified.One of the primary goals of 21-cm radiation intensity mapping is to measure baryon acoustic oscillations over a wide range of redshifts to constrain the properties of dark energy from the expansion history of the late-time Universe. We forecast the constraining power of the CHIME radio telescope on the matter power spectrum and dark energy parameters. Lastly, we devise new calibration algorithms for the gains of an interferometric radio telescope such as CHIME.
Despite their phenomenological successes, the Standard Models (SMs) of particle physics and cosmology remain incomplete. Several theoretical and observational problems cannot be explained within this framework, including the hierarchy problem, dark matter (DM), and the baryon asymmetry of the Universe. The objective of this thesis is to investigate phenomenological and theoretical aspects of the solutions to these issues. We consider two kinds of phase transitions that can occur in the early or late Universe in extensions of the SM, that can be either responsible for dark matter and/or baryon asymmetry production or may be used to constrain possible models of new physics. In the first part we analyze string theory-inspired models where the Universe transitions from matter- to radiation-dominated evolution just before Big Bang Nucleosynthesis through out-of-equilibrium decays of a scalar modulus field. We employ these decays to produce DM and for baryogenesis. We study the phenomenology of these scenarios and its implications for high-scale physics. The second part of this thesis is dedicated to thermodynamic and quantum phase transitions in the early and late Universe, respectively. In the former case, we investigate the dynamics of the electroweak phase transition when the electroweak symmetry is broken down to electromagnetism in the Inert Doublet Model, a simple extension of the SM that can account for DM. Such transitions can generate the baryon asymmetry in a process called electroweak baryogenesis. Some extensions of the SM also predict similar transitions through quantum tunneling that break the colour and electromagnetic symmetries, indicating that our ground state is unstable. We use these arguments to put new constraints on the Minimal Supersymmetric Standard Model.
The Cosmic Microwave Background (CMB) radiation, photons free-streaming from their last scattering surface at redshift around 1090, is currently our main source of information about the origin and history of the Universe. The vast recent advancement in technology has led to new possibilities for gathering data especially detecting the CMB with high accuracy. The goal of the two projects studied in this thesis is to improve the cosmological perturbation theory to better test cosmology with the upcoming data.In chapter 4 we explore the effect of Rayleigh scattering on the CMB and cosmic structure. During and after recombination, in addition to Thomson scattering with free electrons, photons also coupled to neutral hydrogen and helium atoms through Rayleigh scattering. The frequency-dependence of the Rayleigh cross section breaks the thermal nature of CMB temperature and polarization anisotropies and effectively doubles the number of variables needed to describe CMB intensity and polarization statistics, while the additional atomic coupling changes the matter distribution and the lensing of the CMB. We introduce a new method to capture the effects of Rayleigh scattering on cosmological power spectra. We show the Rayleigh signal, especially the cross-spectra between the thermal (Rayleigh) E-polarization and Rayleigh (thermal) intensity signal, may be detectable with future CMB missions even in the presence of foregrounds, and how this new information might help to better constrain the cosmological parameters.In chapter 5 we study the Cosmic Neutrino Background (CNB). In addition to the CMB, the standard cosmological model also predicts that neutrinos were decoupled from the rest of the cosmic plasma when the age of the Universe was less than one second, far earlier than the photons. We study the anisotropy of the CNB and for the first time present the full CNB anisotropy power spectrum at large and small scales both for a massless and massive neutrinos. We also show that how presence of nonstandard neutrino self-interactions compatible with current cosmological data alters the CNB power spectrum.
The standard cosmological model that has emerged in the last decades describes an acceleratingly expanding universe where the familiar baryonic matter accounts for a very small fraction of the overall energy budget. The vast majority of the energy content of the Universe appears to belong to an elusive dark sector made up of dark matter and dark energy. In this thesis, we explore the cosmological consequences of new physics that could govern this unknown dark sector. We first consider a model where dark matter can annihilate to Standard-Model particles through a Breit-Wigner resonance. We show in this case that the energy released by dark matter annihilating in the first proto-halos is likely substantial. We determine that the bounds on the allowed energy injection into the primordial gas and the energy density of the diffuse gamma-ray background strongly constrain the magnitude of the resonantly-enhanced annihilation cross-section.We then perform a thorough analysis of a dark sector made of atom-like bound states. This so-called Atomic Dark-Matter model predicts novel dark-matter properties on small scales but retains the success of cold dark matter on cosmological scales. We revisit the atomic physics necessary to capture the thermal history of the dark atoms and discuss the required improvements over the hydrogen calculation. To solve the perturbation equations, we develop a second-order tight-coupling approximation and further discuss its implications for the baryon-photon case. We compute the matter power spectrum in this model and show that it displays strong dark-matter acoustic oscillations and a cutoff on small scales. Interestingly, we also identify key cosmic microwave background signatures that distinguish the atomic dark-matter scenario from other dark-matter theories. We determine that astrophysical constraints on this model generally favour dark atoms that are both more massive and have higher binding energies than standard atomic hydrogen. We finally consider how oscillations in the bispectrum of primordial fluctuations affects the clustering of dark-matter halos. We discover that features in the inflaton potential such as oscillations and bumps become imprinted in the mass dependence of the non-Gaussian halo bias. This finding opens the possibility of characterizing the inflationary potential with large-scale-structure surveys.
Master's Student Supervision (2010 - 2018)
The nature of dark matter continues to be one of the most elusive mysteries in physics. The astrophysical and cosmological support for dark matter seems overwhelming, but all of the current observational evidence is from only the gravitational influence on baryonic matter. According to the standard cosmology, dark matter is five times as prevalent as baryonic matter, where, taking the contribution from dark energy in to account, only 5% of our universe is made of baryonic matter. Ongoing experimental searches for particle dark matter have provided only constraints without direct detection. As such, alternative theories to dark matter need to be explored. One such alternative idea is an emergent gravity theory. Gravity, no longer a fundamental interaction, emerges from thermodynamic principles in the form of an entropic force. When this theory is applied to cosmology, the gravitational effect that we observe and attribute to dark matter is rather a memory effect from the emergence of space; it is an intrinsic property of the spacetime itself. As it is unclear how to proceed from this theory in general, a proper framework is required so that we can eventually make testable predictions. We propose that the addition of a slow, gravity-like force to general relativity is such a framework. We establish that the Yukawa interaction is gravity-like in certain limits, from both a particle physics and a general relativity perspective, where the massless Yukawa field has infinite range. Exploring spherical collapse in Einstein-de Sitter cosmology, we show that the addition of the Yukawa interaction does not affect the overall evolution of the density contrast, except to decrease the time to collapse. We consider the equations of motion for a massive scalar field coupled to a massless scalar Yukawa field, and plot the solutions as functions of the scale factor. The resulting plots have distinct behaviour before and after the scale factor is of the same magnitude as the coupling. Finally, we consider the effects of a slow, gravity-like force and derive the Lagrangian density for a slow, massless scalar field.
We study the evolution of a warm dark matter and perfect fluid system to determine its behaviour in the linear regime. Comparative analysis is performed between cold dark matter, hot dark matter and warm dark matter approximating each case. Numerical issues causes differences between thewarm dark matter approximations and the respective case. Numerical issues that we have been unable to solve prevent the calculation of sufficient k-spacemodes to study interesting scales. Analytic methods to obtain the real space perturbations and distribution functions are derived.