[Paper Review] CMB spectral distortions as a novel way to probe the small-scale structure problems

James A.D. Diacoumis, Yvonne Y. Y. Wong|arXiv (Cornell University)|Oct 9, 2017

Cosmology and Gravitation Theories3 citations

TL;DR

This paper proposes that cosmic microwave background (CMB) spectral distortions—specifically the μ-parameter—can distinguish between dark matter microphysics scenarios like Warm Dark Matter (WDM) and Late Kinetic Decoupling (LKD). By modeling DM–neutrino and DM–photon elastic scattering, the study shows that future experiments like PRISM could detect deviations in spectral distortions, with sensitivity thresholds of ~4.8×10⁻³² cm² for time-independent DM–neutrino cross sections and ~1.1×10⁻³⁰ cm² for DM–photon scattering.

ABSTRACT

In this work we consider CMB spectral distortions as a probe of dark matter microphysics in the early universe. We demonstrate that future experiments such as PRISM have the potential to distinguish between scenarios which offer solutions to the small-scale problems of CDM cosmology.

Motivation & Objective

• To investigate whether CMB spectral distortions can distinguish between competing solutions to the small-scale structure problems in CDM cosmology.
• To examine how DM–neutrino and DM–photon elastic scattering alter the evolution of photon-baryon perturbations and subsequent spectral distortions.
• To assess the sensitivity of future experiments like PRISM to detect these distortions and constrain dark matter interaction cross sections.
• To quantify the impact of diffusion damping and heat conduction on the μ-parameter in interacting dark matter models.
• To compare the constraining power of CMB spectral distortions with existing astrophysical limits from Planck, Lyman-α forest, and Milky Way satellites.

Proposed method

• Modeling the photon temperature transfer function with modified damping and acoustic oscillation amplitudes due to DM–neutrino and DM–photon coupling.
• Deriving an analytical expression for the diffusion damping scale in the presence of DM–photon scattering, accounting for weak coupling during the μ-era.
• Calculating the heating rate of CMB photons via heat conduction, which exhibits a 'burst' shape during weak coupling phases.
• Using the μ-parameter as the primary observable to characterize spectral distortions from energy injection during the μ-era.
• Simulating the evolution of perturbations under different scattering cross sections and coupling regimes (tight vs. weak coupling).
• Comparing predicted μ-distortions in interacting dark matter models against ΛCDM predictions and existing observational constraints.

Experimental results

Research questions

• RQ1 Can CMB spectral distortions distinguish between WDM and LKD scenarios, which are indistinguishable in large-scale structure?
• RQ2 How do DM–neutrino and DM–photon elastic scattering alter the amplitude and damping of photon-baryon acoustic oscillations?
• RQ3 What is the role of heat conduction and diffusion damping in shaping the μ-parameter in interacting dark matter models?
• RQ4 How sensitive are future experiments like PRISM to DM–neutrino and DM–photon scattering cross sections?
• RQ5 Can CMB spectral distortions provide stronger constraints on DM interactions than current astrophysical probes like the Lyman-α forest?

Key findings

• DM–neutrino scattering enhances the μ-parameter by up to 20% in the tight-coupling limit due to reduced anisotropic stress and increased perturbation amplitude.
• For time-independent DM–neutrino cross sections, PRISM could detect interactions with σ_DM−ν ≥ 4.8×10⁻³² (m_DM/GeV) cm².
• For temperature-proportional DM–neutrino scattering (σ ∝ T²), detection requires σ⁰_DM−ν ≥ 2.5×10⁻⁴⁷ (m_DM/GeV) cm², potentially exceeding current Lyman-α forest limits.
• DM–photon scattering leads to a transient 'burst' in photon heating due to weak coupling, with heat conduction becoming a dominant dissipation mechanism.
• PRISM sensitivity to DM–photon scattering requires σ_DM−γ ≥ 1.1×10⁻³⁰ (m_DM/GeV) cm² for time-independent cross sections and σ⁰_DM−γ ≥ 1.8×10⁻⁴⁰ (m_DM/GeV) cm² for T²-dependent cross sections.
• The competing effects of damping and localized heating in DM–photon models reduce constraining power compared to DM–neutrino scattering, but still enable detectable signatures.