Possible interaction between baryons and dark-matter particles revealed by the first stars (1803.06698v1)

Abstract: The cosmic radio-frequency spectrum is expected to show a strong absorption signal corresponding to the 21-centimetre-wavelength transition of atomic hydrogen around redshift 20, which arises from Lyman-alpha radiation from some of the earliest stars. By observing this 21-centimetre signal - either its sky-averaged spectrum or maps of its fluctuations, obtained using radio interferometers - we can obtain information about cosmic dawn, the era when the first astrophysical sources of light were formed. The recent detection of the global 21-centimetre spectrum reveals a stronger absorption than the maximum predicted by existing models, at a confidence level of 3.8 standard deviations. Here we report that this absorption can be explained by the combination of radiation from the first stars and excess cooling of the cosmic gas induced by its interaction with dark matter. Our analysis indicates that the spatial fluctuations of the 21-centimetre signal at cosmic dawn could be an order of magnitude larger than previously expected and that the dark-matter particle is no heavier than several proton masses, well below the commonly predicted mass of weakly interacting massive particles. Our analysis also confirms that dark matter is highly non-relativistic and at least moderately cold, and primordial velocities predicted by models of warm dark matter are potentially detectable. These results indicate that 21-centimetre cosmology can be used as a dark-matter probe.

• The paper introduces 21-cm observations to probe baryon-dark matter scattering, indicating DM masses below a few GeV.
• It analyzes the unexpected 21-cm absorption signal from EDGES to derive a minimum b-DM scattering cross-section over 10⁻²¹ cm².
• The study urges further low-frequency experiments like HERA and SKA to refine dark matter constraints and disentangle astrophysical effects.

Analysis of the Paper "Cosmic Dawn as a Dark Matter Detector"

The paper "Cosmic dawn as a dark matter detector" by Rennan Barkana presents a novel approach to probing dark matter (DM) properties through the paper of 21-cm cosmology during cosmic dawn. The research explores the potential to detect DM interactions with baryons through the observation of a stronger-than-expected 21-cm absorption signal, potentially indicative of baryon-dark matter (b-DM) scattering processes.

Overview of Key Findings

The main assertion of the paper is the possibility of using the 21-cm spectral line as a dark matter probe. The 21-cm line arises from hyperfine transitions in neutral hydrogen and provides valuable insights into the conditions of the early universe. The paper underscores that the observed 21-cm absorption signal, reported by the EDGES experiment, exceeds the maximum absorption expected through conventional astrophysics. This anomaly is argued to be explicable by b-DM scattering, implying that dark matter could be cooler than previously assumed.

Numerical Constraints and Implications

The observed signal constrains the properties of dark matter:

• DM particle mass is inferred to be below a few GeV with considerable sensitivity to light particle masses, challenging the standard Weakly Interacting Massive Particle (WIMP) scenarios.
• The b-DM scattering cross-section must surpass a minimum threshold of $10^{-21}$ cm$^2$, described by a velocity model $\sigma(v) \propto v^{-4}$.

The paper elucidates that these observations facilitate a unique scrutiny of DM properties, including the potential existence of ultralight dark matter particles. Moreover, it highlights that alternative explanations for the excess absorption signal, within the existing astrophysical or cosmic frameworks, remain untenable.

Interaction with Astrophysical Models

Interpreting the 21-cm absorption signal necessitates considering the interplay between early heating mechanisms, such as Lyman-$\alpha$ coupling and X-ray heating, and cosmic gas cooling through b-DM interactions. This complex interaction indicates an intricate dependence on both DM and astrophysical parameters, which future observations must disentangle.

Future Prospects in Dark Matter Research

Moving forward, refinement in the measurement of 21-cm fluctuations, particularly through experiments like HERA and the SKA, is anticipated to further elucidate b-DM interactions. Such experiments will exploit the large spatial coherence of the intensity pattern of the 21-cm signal during cosmic dawn. The potential existence of baryon acoustic oscillation (BAO) signatures within the 21-cm power spectrum presents exciting avenues for testing fundamental cosmological parameters.

Additionally, the prospect of conducting low-frequency observations extends the potential for early-universe DM probes. Innovations in experimental setups, capable of reducing ionospheric noise, are critical for future advancements.

Conclusion

This paper contributes a significant theoretical framework to the field of dark matter research by proposing the use of cosmological signals from the cosmic dawn to detect and constrain DM interactions. While this approach necessitates future empirical confirmations, it underscores an exciting paradigm shift wherein cosmological observations entwine with particle physics to unravel the elusive nature of dark matter. The paper accentuates the necessity for integrated astrophysical, cosmological, and particle physics research to fully realize this approach's potential.

In summary, the insights from 21-cm cosmology could redefine boundaries in the search for dark matter, posing invaluable questions for theory and experimentation, and fostering a new dimension in understanding the universe's deepest secrets.