Early-Universe Constraints on Dark Matter-Baryon Scattering and their Implications for a Global 21cm Signal (1803.09734v1)

Abstract: We present and compare several cosmological constraints on the cross section for elastic scattering between dark matter (DM) and baryons, for cross sections with a range of power-law dependences on the DM-baryon relative velocity $v$, especially focusing on the case of $\sigma \propto v^{-4}$. We study constraints spanning a wide range of epochs in cosmological history, from pre-recombination distortions to the blackbody spectrum and anisotropies of the cosmic microwave background (CMB), to modifications to the intergalactic medium temperature and the resulting 21cm signal, and discuss the allowed signals in the latter channels given the constraints from the former. We improve previous constraints on DM-baryon scattering from the CMB anisotropies, demonstrate via principal component analysis that the effect on the CMB can be written as a simple function of DM mass, and map out the redshifts dominating this signal. We show that given high-redshift constraints on DM-baryon scattering, a $v^{-4}$ scaling of the cross section for light DM would be sufficient to explain the deep 21cm absorption trough recently claimed by the EDGES experiment, if 100% of the DM scatters with baryons. For millicharged DM models proposed to explain the observation, where only a small fraction of the DM interacts, we estimate that a PIXIE-like future experiment measuring CMB spectral distortion could test the relevant parameter space.

• The paper demonstrates that DM-baryon scattering with a v⁻⁴ scaling critically alters CMB and 21cm signals, offering refined constraints on interaction models.
• It employs principal component analysis and Fisher forecasting to assess how varying velocity dependencies shape cosmological observables.
• The findings imply that a fully interacting DM component could explain the EDGES 21cm absorption while millicharged DM models remain testable with future experiments.

Overview of Early-Universe Constraints on Dark Matter-Baryon Scattering

The paper by Slatyer and Wu explores the interplay between dark matter (DM) and baryon interactions in the early universe, focusing on the influence of these interactions on observable cosmological phenomena such as the cosmic microwave background (CMB) and the 21cm signal. By examining a variety of velocity dependencies for the scattering cross-section, the authors uncover constraints that help inform our understanding of dark matter models and their viability in light of recent astrophysical observations.

Key Findings and Methodological Approaches

The paper explores cosmological constraints on the DM-baryon scattering cross-section, parameterized as $\sigma \propto v^n$, where $v$ represents the relative velocity. The research intensely scrutinizes $n = -4$ as well as other power-law dependencies, spanning epochs from the pre-recombination era through the cosmic dark ages. These phases include assessments from indicators such as distortions in the CMB blackbody spectrum, its anisotropies, and the thermal evolution of the intergalactic medium (IGM) as it relates to the 21cm signal.

Enhanced CMB Constraints with High-$\ell$ Data: By incorporating high-$\ell$ data from additional sources like ACT and SPT, the authors observe a refined constraint for $n = -4$, while observing a consistently tighter limit for $n = 0$ as compared to previous works. Employing principal component analysis, the paper suggests that modifications to the CMB spectrum from DM-baryon scattering can be succinctly encapsulated in terms of simple functions indexed by DM mass, offering a clear pathway to understanding the redshift domains crucial to CMB signals.

Implications on EDGES 21cm Signal: The paper posits that if 100% of DM interacts with baryons and follows the $v^{-4}$ scaling, it might be sufficiently influential to explain the pronounced 21cm absorption trough noted by the EDGES experiment from redshifts $z\sim 15-20$. Yet, in millicharged DM models where only a fractional part of DM interacts, projections indicate that future experiments such as PIXIE could potentially test these parameter spaces through spectral distortions of the CMB.

Implications and Future Directions

The research provides key insights into the constraints on cross-section scaling laws, reinforcing the negligible effect of $n = 0$ interactions on low-redshift gas temperatures while establishing the potential significance of $n = -4$ at redshifts relevant to 21cm observations. This suggests that the 21cm signal can serve as an effective probe for discerning subtle DM-baryon interactions which escape other cosmological constraints. Given these insights, the paper prompts further quantitative evaluation of scenarios involving small millicharged DM components, particularly as future CMB observations become more precise.

The employment of both principal component analysis and Fisher forecast methodology exemplifies robust approaches to predicting theoretical constraints from observational data and lends itself to further exploration in studies of spectral distortions in potential upcoming missions beyond PIXIE or similar setups.

Conclusion

The paper by Slatyer and Wu serves as a substantial contribution to the field of early-universe cosmology, offering intricate analyses on how dark matter might influence and be constrained by the thermal history and observational signals of the early universe. The exploration of novel parameter spaces for DM-baryon interactions, coupled with thoughtful statistical analyses, continues to shape the frontier of understanding dark matter's elusive nature through cosmological imprints.