Cosmological Parameter Forecasts for a CMB-HD Survey

TL;DR

This paper forecasts the capabilities of a CMB-HD survey to precisely measure cosmological parameters, improving constraints on inflation, light particles, and baryonic effects, with implications for fundamental physics.

Contribution

It provides detailed parameter forecasts for a CMB-HD survey, including improvements over previous surveys and methods to mitigate biases from baryonic effects.

Findings

Achieves a factor of two improvement in inflation probe via $n_s$.

Can rule out light thermal particles like the QCD axion.

Delensing and BAO data significantly tighten parameter constraints.

Abstract

We present forecasts on cosmological parameters for a CMB-HD survey. For a $\Lambda$CDM + $N_{eff}$ + $\sum m_\nu$ model, we find $\sigma(n_s) = 0.0013$ and $\sigma(N_{eff}) = 0.014$ using CMB and CMB lensing multipoles in the range of $\ell \in [30, 20000]$, after adding anticipated residual foregrounds, delensing the acoustic peaks, and adding DESI BAO data. This is about a factor of two improvement in ability to probe inflation via $n_s$ compared to precursor CMB surveys. The $N_{eff}$ constraint can rule out light thermal particles back to the end of inflation with 95% CL; for example, it can rule out the QCD axion in a model-independent way assuming the Universe's reheating temperature was high enough that the axion thermalized. We find that delensing the acoustic peaks and adding DESI BAO tightens parameter constraints. We also find that baryonic effects can bias parameters if not marginalized over, and that uncertainties in baryonic effects can increase parameter error bars; however, the latter can be mitigated by including information about baryonic effects from kinetic and thermal Sunyaev-Zel'dovich measurements by CMB-HD. The CMB-HD likelihood and Fisher estimation codes used here are publicly available; the likelihood is integrated with Cobaya to facilitate parameter forecasting.