The Cosmic Microwave Background (CMB) and especially its anisotropies have been a key source of information for cosmology and have played a major role in establishing the now-standard ΛCDM model. Upcoming experiments aim to extend CMB measurements even further, hoping to measure the sum of neutrino masses, search for primordial gravitational waves, further exploit secondary anisotropies, and constrain extended models more generally. In this thesis I present a study of the CMB temperature and polarization anisotropies, their power spectra, and cosmological parameter constraints using recent data from the Atacama Cosmology Telescope (ACT) and the Planck satellite. This includes extensive discussion of CMB analysis methods and the challenges in analysing these datasets individually and together, as well as presentation of new constraints on multiple cosmological models using these data.

We begin by presenting new angular power spectra and cosmological parameter constraints derived from the Planck PR4 (NPIPE) maps of the microwave sky. We conduct extensive internal checks for systematic errors, and compare these results to previous Planck products and external data. We find excellent consistency between NPIPE and the Planck 2018 maps at the parameter level, showing that the Planck cosmology is robust to substantial changes in the mapmaking. The lower noise of NPIPE leads to ∼10% tighter constraints on cosmological parameters, and we see both smaller error bars and a shift towards the standard ΛCDM values for beyond-ΛCDM parameters including the curvature of the Universe Ω_K and lensing amplitude A_L.

Next we continue to study Planck data, now in comparison with the ACT 2020 data release, DR4. Motivated by observed discrepancies between power and cross spectra from ACT DR4 and Planck 2018, particularly in the cross-correlation of temperature and E-mode polarization, we study challenges that may be encountered in the comparison of satellite and ground-based CMB data. In particular we focus here on the effects of Fourier-space filtering and masking involving bright point sources. We show that the filtering operation generates cross-shaped artefacts in the map that stretch far outside typical point source masks. If not corrected these artefacts can add bias or additional variance to cross-spectra, skewing results. However we find that the effect of this systematic is not large enough to explain the ACT-Planck differences presented with ACT DR4.

Finally, we combine the ACT and Planck likelihoods to study pre-recombination extensions to ΛCDM affecting the primordial helium fraction Y_P and effective number of light particles N_eff . We find that the small-scale ACT data can improve limits on these parameters by 10−20%, and that an ACT preference for low N_eff leads to tighter upper limits on this parameter.