# Using Astronomical Telescopes to Study Unseen Matter

### Key CMB-HD Science Objectives

*1. Measure the small-scale matter power spectrum from weak gravitational lensing using the CMB as a backlight; with this, CMB-HD aims to distinguish between a matter power spectrum predicted by models that can explain observational puzzles of small-scale structure, and that predicted by vanilla cold dark matter (CDM), with a significance of at least 8σ. *

*2. Measure the number of light particle species that were in thermal equilibrium with the known standard-model particles at any time in the early Universe, i.e. N _{eff} , with a 1σ uncertainty of σ(Neff ) = 0.014. *

*3. Constrain or discover axion-like particles by observing the resonant conversion of CMB photons into axions in the magnetic fields of galaxy clusters. *

## Figure 1

CMB lensing power spectrum for an m ~ 10^{-22} eV FDM model and a CDM model. The error bars correspond to observations with 0.5*µ*K-arcmin CMB noise in temperature and 15 arcsecond resolution over 50% of the sky. Reionization kSZ has also been included as a foreground here. **Given these errors, one can ****distinguish between CDM and a suppression ****of structure below 10 ^{9}M⊙ with a significance **

**of about**8σ.

## Figure 2

Shows that CMB-HD can achieve σ(N_{eff} ) = 0.014, which would cross the critical threshold of 0.027. This would potentially rule out or find evidence for new light thermal particles with 95% (2σ) confidence level. (Original figure by Benjamin Wallisch in arXiv:1903.04763 and arXiv:1810.02800; modified with addition of CMB-HD limit. Used with permission.)

### References (see these works and references therein)

Sehgal, N et al, CMB-HD:

Astro2020 RFI Response, Feb 2020, https://arxiv.org/abs/2002.12714

Sehgal, N et al, CMB-HD:

An Ultra-Deep, High-Resolution Millimeter-Wave Survey Over Half the Sky, September 2019,

https://arxiv.org/pdf/1906.10134.pdf