Observational Cosmology Seminar

Talk 1: Covariance Computation of CMB Lensing Power Spectrum (10 minutes + 2 minutes Q&A)

Speaker: Gordon Leung

The cosmic microwave background (CMB) provides a powerful probe of both the physics of the early Universe and the growth of large-scale structure. In particular, weak lensing by intervening matter remaps the primordial CMB temperature anisotropies and polarization, inducing statistical mode coupling that can be used to reconstruct the lensing field. The resulting CMB lensing power spectra can therefore serve as robust probes of cosmological parameters. Parameter inference using a Gaussian likelihood requires accurate modelling of the covariance matrix, including both auto-covariances and cross-experiments covariances; neglecting correlations between experiments could lead to unrealistically tight cosmological constraints. The autocovariance includes both cosmic variance of the lensing signal and reconstruction noise. Cross-covariance arises from overlapping sky coverage, where lensing reconstructions are correlated between experiments. We have computed the analytical covariance between lensing power spectra reconstructed with quadratic estimators from Planck, SPT-3G, and ACT-DR6. Our preliminary results for the SPT-3G winter field show that the maximum total correlation with Planck is 14%, with ACT is 21%. We have extended our analysis to the SPT-3G Ext-10K survey and found that the SPT Wide Field has a maximum correlation of 26% with Planck. We will further validate these results using simulations.

Talk 2: Pitfalls for Models of Large-Scale Hydrogen Distributions

Speaker: Calvin Osinga (University of Maryland) (30 minutes + 10 minutes Q&A)

Large-scale HI surveys and their cross-correlations with galaxy distributions have immense potential as cosmological probes. Interpreting these measurements requires theoretical models that must incorporate redshift-space distortions (RSDs), such as the Kaiser and fingers-of-God (FoG) effect, and differences in the tracer and matter distributions via the tracer bias. These effects are commonly approximated with assumptions that should be tested on simulated distributions. For this purpose, we use the hydrodynamical simulation suite IllustrisTNG to assess the performance of models of z <= 1 HI auto and HI-galaxy cross-power spectra, finding that the models employed by recent observations introduce errors comparable to or exceeding their measurement uncertainties. In particular, neglecting FoG causes ≳10% deviations between the modeled and simulated power spectra at k≳0.1 h / Mpc. Even with perfect knowledge of each model ingredient, models can still err by ∼10% on relevant observational scales. These remaining errors arise from multiple RSD damping sources on HI clustering, which are not sufficiently described with a single FoG term. Overall, our results highlight the need for an improved understanding of RSDs to harness the capabilities of future measurements of HI distributions.