Indirect Dark Matter Signatures in the Cosmic Dark Ages I. Generalizing the Bound on s-wave Dark Matter Annihilation from Planck (1506.03811v2)

Abstract: Recent measurements of the cosmic microwave background (CMB) anisotropies by Planck provide a sensitive probe of dark matter annihilation during the cosmic dark ages, and specifically constrain the annihilation parameter $f_\mathrm{eff} \langle \sigma v \rangle/m_\chi$. Using new results (Paper II) for the ionization produced by particles injected at arbitrary energies, we calculate and provide $f_\mathrm{eff}$ values for photons and $e^+e^-$ pairs injected at keV-TeV energies; the $f_\mathrm{eff}$ value for any dark matter model can be obtained straightforwardly by weighting these results by the spectrum of annihilation products. This result allows the sensitive and robust constraints on dark matter annihilation presented by the Planck Collaboration to be applied to arbitrary dark matter models with $s$-wave annihilation. We demonstrate the validity of this approach using principal component analysis. As an example, we integrate over the spectrum of annihilation products for a range of Standard Model final states to determine the CMB bounds on these models as a function of dark matter mass, and demonstrate that the new limits generically exclude models proposed to explain the observed high-energy rise in the cosmic ray positron fraction. We make our results publicly available at http://nebel.rc.fas.harvard.edu/epsilon.

• The paper establishes model-independent limits on s-wave dark matter annihilation by analyzing energy deposition effects on cosmic ionization using Planck data.
• It applies principal component analysis to reduce the complex impact of DM annihilation into a single normalization factor, ensuring robust constraints.
• The study maps effective energy deposition across keV-TeV scales, challenging DM models that require high annihilation cross-sections to explain astrophysical anomalies.

Overview of Indirect Dark Matter Signatures in the Cosmic Dark Ages from Planck Data

The paper titled "Indirect Dark Matter Signatures in the Cosmic Dark Ages I. Generalizing the Bound on $s$-wave Dark Matter Annihilation from Planck" offers a comprehensive examination of constraints on dark matter (DM) annihilation derived from the Planck satellite's precise measurements of the Cosmic Microwave Background (CMB). Authored by Tracy R. Slatyer, this research utilizes the strong constraints provided by CMB anisotropies to explore the potential annihilation behaviors of dark matter during the cosmic dark ages.

Key Contributions and Methodology

The paper explores the implications of energy injected into the universe by dark matter annihilation, emphasizing how such energy can ionize hydrogen gas and thus affect cosmic ionization histories. Specifically, it utilizes the annihilation parameter $f_\mathrm{eff} \langle \sigma v \rangle/m_\chi$ as constrained by Planck, where $f_\mathrm{eff}$ represents the effective energy deposition efficiency, $\langle \sigma v \rangle$ is the velocity-averaged cross-section of DM annihilation, and $m_\chi$ is the DM particle mass.

The paper builds upon new ionization results (presented in a companion paper, Paper II), offering $f_\mathrm{eff}$ values for particles in the keV-TeV energy range. These results enable the application of Planck's constraints to a wide variety of DM annihilation models with $s$-wave annihilation, moving beyond previous model-specific analyses to encompass a broader range of dark matter candidates.

Through the application of principal component analysis (PCA), the author demonstrates that the impact of DM annihilation on the CMB can be captured by a single normalization factor. This indicates that the constraints are largely model-independent, which significantly enhances the robustness of the derived limits against a spectrum of theoretical dark matter models.

Numerical Results and Implications

The numerical analysis reveals intriguing insights into the viability of several DM models. For instance, models attempting to explain observed cosmic ray positron excesses through high-energy DM annihilation are generally excluded, according to the derived limits. The robustness of these constraints poses challenges for models that require large annihilation cross-sections to match the thermally produced relic abundance or to explain astrophysical anomalies.

Moreover, the paper provides a detailed map of $f_\mathrm{eff}$ values over a wide energy range, consolidating and generalizing the potential constraints on many possible dark matter models. These findings extend to TeV scale masses and include subtle contributions from electroweak state radiation, thus accounting for multiple sources of uncertainty typically overlooked in similar analyses.

Potential for Future Developments

The methodology developed and the results obtained from this research pave the way for future studies seeking to further comprehend dark matter characteristics. By offering a generalized approach and publicly accessible results, Slatyer's work encourages further model-testing against observational data, solidifying the interaction framework between theoretical developments and empirical validation.

In subsequent research, one might speculate that adopting similar methodological frameworks could further extend these limits to even more massive or complex dark matter models—those that are characterized by intricate annihilation channels or unique interactions. That said, one corridor of advancement would explore improved precision in subsequent CMB measurements or other cosmological probes to fine-tune these constraints.

Conclusion

In sum, Tracy R. Slatyer's paper effectively marshals Planck data to set comprehensive, model-independent limits on $s$-wave DM annihilation. It excellently combines theoretical modeling with state-of-the-art observational data, reinforcing the utility of CMB as a probe for dark matter properties during the cosmic dark ages. Through novel insights on the generalized constraints applicable to diverse scenarios, the paper significantly advances our understanding of dark matter's cosmic role, offering a landmark extension beyond previous analyses.

This work thus stands as a crucial reference point for further explorations of potential dark matter signatures and is pivotal for framing future investigations within the broader context of particle cosmology.