Dark Matter

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.

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. This measurement would be a clean measurement of the matter power spectrum on these scales, free of the use of baryonic tracers. It would greatly limit the allowed models of dark matter and baryonic physics, shedding light on dark-matter particle properties and galaxy evolution.

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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. Neff , with a 1σ uncertainty of σ(Neff ) = 0.014.

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. Neff , with a 1σ uncertainty of σ(Neff ) = 0.014. This would cross the critical threshold of 0.027, which is the amount that any new particle species must change Neff away from its Standard Model value of 3.04. Such a measurement would rule out or find evidence for new light thermal particles with at least 95% confidence level. This is particularly important because many dark matter models predict new light thermal particles, and recent short-baseline neutrino experiments have found puzzling results possibly suggesting new neutrino species.

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

3. Constrain or discover axion-like particles by observing the resonant conversion of CMB photons into axions in the magnetic fields of galaxy clusters.  Nearly massless pseudoscalar bosons, often generically called axions, appear in many extensions of the standard model. A detection would have major implications both for particle physics and for cosmology, not least because axions are also a well-motivated dark matter candidate. CMB-HD has the opportunity to provide a world-leading probe of the electromagnetic interaction between axions and photons using the resonant conversion of CMB photons and axions in the magnetic field of galaxy clusters, independently of whether axions constitute the dark matter. CMB-HD would explore the mass range of 10−14 eV < ma < 2 × 10−12 eV and improve the constraint on the axion coupling constant by over 2 orders of magnitude over current particle physics constraints to gaγ < 0.1 × 10−12 GeV−1. These ranges are unexplored to date and complementary with other cosmological searches for the imprints of axion-like particles on the cosmic density field.

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Figure 1

Illustration of how CMB-HD lensing measurements probe the matter power spectrum. Shown in red are the forecasted errors. Indicated on the top x-axis is the corresponding mass scale today for the comoving wavenumbers on the bottom x-axis. CMB-HD lensing constraints span four orders of magnitude in wavenumber, 0.005 h/Mpc < k < 55 h/Mpc, and 12 orders of magnitude in mass scale, going down to 108M⊙. In the bottom panel, we show the error bars on the matter power spectrum as a fraction of the total power. CMB-HD achieves sub-percent fractional errors in the range 0.03 h/Mpc < k < 10 h/Mpc. Figure from arXiv:2405.12220, JCAP, 2025.

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Figure 2

Shows that CMB-HD can achieve σ(Neff ) = 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