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Synopsis

Modern cosmology has entered an era of unprecedented precision, where complementary probes test the cosmological model with increasing accuracy. While the ΛCDM model successfully describes a broad range of observations, its fundamental components remain poorly understood, and several persistent tensions have raised the possibility of new physics beyond this model.

In this thesis, we first study the validity of the quasi-static approximation (QSA) in scalartensor theories, focusing on the symmetron model as a well-motivated example of late-time dark energy with screening mechanism. We consider both dark-matter-only and universally (all matter) coupled cases, and derive a criterion for the validity of the QSA. We also present a new version of Modified Growth with CAMB (MGCAMB), developed for current and upcoming cosmological surveys, and use it to constrain the parameters of the CDM-only coupled symmetron model.

Next, we investigate the extent to which modifying the ionization history at cosmological recombination can relieve the Hubble tension. Using both a flexible cubic-spline parameterization of the ionization history and a physically motivated four-parameter phenomenological model, we show that modified recombination can reduce the Hubble tension to below the 2σ level while improving the fit to the current CMB and BAO data and reducing the S8 tension.

We then show that modified recombination provides a more compelling resolution to the minor tension between BAO and CMB than dynamical dark energy (DDE). While both frameworks improve the overall fit to DESI and Planck data, modified recombination also predicts a higher Hubble constant H0, thereby partially alleviating the Hubble tension.

Finally, we present a sound-horizon-agnostic determination of the Hubble constant that treats the sound horizon at drag-epoch rd as a free parameter, avoiding assumptions about early-Universe physics. By combining DESI DR2 BAO with CMB lensing, supernovae, and DES Y3 galaxy weak lensing and clustering data, we break the rd−H0 degeneracy and obtain competitive constraints on both H0 and the neutrino mass. Our generated forecasts predict sub-percent precision on H0, demonstrating the promise of a model-agnostic framework for testing the cosmological model around the epoch of recombination.

For Zoom link info, please contact Lindiwe Coyne at physgrad@sfu.ca.