A Guide to Designing Future Ground-based CMB Experiments

TL;DR

This study forecasts the potential of a future Stage-IV ground-based CMB polarization experiment (CMB-S4) to probe fundamental physics via Fisher-matrix forecasts across a broad experimental grid of detectors, resolution, and sky coverage. By integrating CMB lensing with DESI BAO and a 1% H0 prior, it quantifies constraints on neutrino properties (Mν, N_eff), dark energy (w0, wa), dark matter annihilation (p_ann), and inflationary parameters (Ω_K, n_s, α_s, r, n_t), highlighting when constraints become sample-variance limited. The results indicate sub-20 meV sensitivity to the total neutrino mass with BAO, σ(N_eff) ≈ 0.016, a DETF FoM up to ~303 for dark energy, and σ(r) around 9×10^-5 in favorable configurations, with r detection strongly aided by delensing. These findings inform design choices for CMB-S4 and underscore the powerful synergy between CMB polarization, galaxy surveys, and local H0 measurements for constraining new physics.

Abstract

In this follow-up work to the High Energy Physics Community Summer Study 2013 (HEP CSS 2013, a.k.a. Snowmass), we explore the scientific capabilities of a future Stage-IV Cosmic Microwave Background polarization experiment (CMB-S4) under various assumptions on detector count, resolution, and sky coverage. We use the Fisher matrix technique to calculate the expected uncertainties in cosmological parameters in $νΛ$CDM that are especially relevant to the physics of fundamental interactions, including neutrino masses, effective number of relativistic species, dark-energy equation of state, dark-matter annihilation, and inflationary parameters. To further chart the landscape of future cosmology probes, we include forecasted results from the Baryon Acoustic Oscillation (BAO) signal as measured by DESI to constrain parameters that would benefit from low redshift information. We find the following best 1-sigma constraints: $σ$ constraints: $σ(M_ν)= 15$ meV, $σ(N_{\rm eff})= 0.0156$, Dark energy Figure of Merit = 303, $σ(p_{ann})= 0.00588\times3\times10^{-26}$ cm$^3$/s/GeV, $σ(Ω_K)= 0.00074$, $σ(n_s)= 0.00110$, $σ(α_s)= 0.00145$, and $σ(r)= 0.00009$. We also detail the dependences of the parameter constraints on detector count, resolution, and sky coverage.

Figures (13)

• Figure 1: $N_{\ell}^{dd}$ for three $N_{\rm det}$ at $f_{sky}=0.75$ and $1'$ beam size. The deflection angle spectrum $C_{\ell}^{dd}$ is shown in black as reference. $C_{\ell}^{dd}$ is related to the lensing power spectrum $C_{\ell}^{\phi\phi}$ by $C_{\ell}^{dd} = \ell(\ell+1)C_{\ell}^{\phi\phi}$.
• Figure 2: Ratio of the lensing potential power spectrum for different total neutrino masses to a massless neutrino case: the heavier the neutrinos, the more suppressed the potential is.
• Figure 3: Constraints for $\sigma(N_{\rm{eff}})$ as a function of the number of detectors and observing sky fraction for $1'$ to $4'$ beam sizes.
• Figure 4: 1-$\sigma$ constraints on the total neutrino mass, for various detector numbers and observed sky fraction in units of meV. The top two panels show constraints from CMB for $1' - 4'$ beams. The bottom panels show constraints from CMB + BAO with the same beams in the CMB experiments. "CMB" includes lensing. We see that data from BAO pushes the constraints from CMB with lensing to a lower floor and a wide range of experimental configurations can obtain sub-20meV constraints on the sum of neutrino masses.
• Figure 5: Constraints on total neutrino mass across all sky fractions comparing CMB without lensing to CMB with lensing for experiments having $10^5$ detectors and $1'$ or $4'$ beams. The constraints on $M_{\nu}$ are greatly improved by CMB lensing. This general trend is observed across all experimental configurations when the lensing power spectrum is added.