Optimal CMB Lensing

Ge et al. (SPT-3G Collaboration)

The deflection of CMB photons by the intervening gravitational potential is called CMB lensing. This is an important tool to map the mass distribution of the Universe. The reconstructed CMB lensing potential can also be used to undo the deflections, thereby cleaning out the lensing-induced B-mode signal that otherwise contaminates the primordial B-mode polarization signal. I am a junior member of South Pole Telescope (SPT) collaboration, working on applying MUSE to optimally reconstruct the CMB lensing potential and estimate unlensed bandpowers simultaneously. This new optimal method will be crucial for the analysis of future low-noise CMB maps, where the traditional Quadratic Estimator is less optimal.

"Cool" Atomic Dark Matter

Hughes, Ge et al.

The "Atomic" Dark Matter (ADM) model consists of two massive dark fermions (a "dark proton'' and a "dark electron'') interacting with a massless dark photon. The massive dark fermions can be part or all of the dark matter, and the dark photon contributes to radiation in the Universe. In this work, we found that the current BAO and Planck CMB data are still consistent with an ADM model prediction with BBN-predicted primordial helium abundance, which contains about 10% of the total DM being ADM and has the dark photon at low temperature. The main constraining power on ADM models is from CMB lensing. Future CMB surveys, like SPT, Simons Observatory (SO) and CMB-S4, will be able to test the ADM model, if it is the cause of the S8 tension.

Constraints on Amount of Light Relics from CMB Observations

Ge, Cyr-Racine, Knox, 2023

We analytically discuss constraints on light relics from cosmic microwave background maps, using a family of scaling transformations that scale key rates in the evolution equations. Besides the previously noticed causes of changing the photon scattering rate and the free-streaming species fraction, we identify two novel causes of changing the fraction of matter with pressure support and the recombination rates. We use the Mirror World Dark Sector model as a reference model that admits a scaling transformation that increases light relic energy density while avoiding all of these causes. Constraints on light relics in a given model can then be understood as due to the differences between the given model and the reference model.

Symmetry of Cosmological Observables, and a High Hubble Constant as an Indicator of a Mirror World Dark Sector

Cyr-Racine, Ge, Knox, 2021

We find that a uniform scaling of the gravitational free-fall rates and photon-electron scattering rate leaves most dimensionless cosmological observables nearly invariant. A "mirror world" dark sector can exploit this invariance and resolve the Hubble tension, although only with a primordial helium abundance inconsistent with the amount inferred from the observations.

Atomic Dark Matter and Concordance of Cosmological Probes

A Symmetry of Cosmological Observables, a Mirror World Dark Sector and the Hubble Constant Tension