Prof. David Alonso

Abstract:

The Simons Observatory (SO), due to start full science operations in early 2025, aims to set tight constraints on inflationary physics by inferring the tensor-to-scalar ratio 𝑟 from measurements of cosmic microwave background (CMB) polarization 𝐵-modes. Its nominal design including three small-aperture telescopes (SATs) targets a precision 𝜎⁡(𝑟=0)≤0.003 without delensing. Achieving this goal and further reducing uncertainties requires a thorough understanding and mitigation of other large-scale 𝐵-mode sources such as Galactic foregrounds and weak gravitational lensing. We present an analysis pipeline aiming to estimate 𝑟 by including delensing within a cross-spectral likelihood, and demonstrate it for the first time on SO-like simulations accounting for various levels of foreground complexity, inhomogeneous noise and partial sky coverage. As introduced in an earlier SO delensing paper, lensing 𝐵-modes are synthesized using internal CMB lensing reconstructions as well as Planck-like cosmic infrared background maps and LSST-like galaxy density maps. We then extend SO’s power-spectrum-based foreground-cleaning algorithm to include all auto- and cross-spectra between the lensing template and the SAT 𝐵-modes in the likelihood function. This allows us to constrain 𝑟 and the parameters of our foreground model simultaneously. Within this framework, we demonstrate the equivalence of map-based and cross-spectral delensing and use it to motivate an optimized pixel-weighting scheme for power spectrum estimation. We start by validating our pipeline in the simplistic case of uniform foreground spectral energy distributions. In the absence of primordial 𝐵-modes, we find that the 1⁢𝜎 statistical uncertainty on 𝑟, 𝜎⁡(𝑟), decreases by 37% as a result of delensing. Tensor modes at the level of 𝑟=0.01 are successfully detected by our pipeline. Even when using more realistic foreground models including spatial variations in the dust and synchrotron spectral properties, we obtain unbiased estimates of 𝑟 both with and without delensing by employing the moment-expansion method. In this case, uncertainties are increased due to the higher number of model parameters, and delensing-related improvements range between 27% and 31%. These results constitute the first realistic assessment of the delensing performance at SO’s nominal sensitivity level.

Abstract:

We report the first detection, at very high significance (23σ), of the cross-correlation between cosmic shear and the diffuse x-ray background, using data from the Dark Energy Survey and the ROSAT satellite. The x-ray cross-correlation signal is sensitive to the distribution of the surrounding gas in dark matter halos. This allows us to use our measurements to place constraints on key physical parameters that determine the impact of baryonic effects in the matter power spectrum. In particular, we determine the mass of halos in which feedback has expelled half of their gas content on average to be log_{10}(M_{c}/M_{⊙})=13.643_{-0.12}^{+0.081} and the polytropic index of the gas to be Γ=1.231_{-0.011}^{+0.015}. This represents a first step in the direct use of x-ray cross-correlations to obtain improved constraints on cosmology and the physics of the intergalactic gas.