21-cm Fluctuations from Charged Dark Matter (1804.01092v2)

Abstract: The epoch of the formation of the first stars, known as the cosmic dawn, has emerged as a new arena in the search for dark matter. In particular, the first claimed 21-cm detection exhibits a deeper global absorption feature than expected, which could be caused by a low baryonic temperature, and has been interpreted as a sign for electromagnetic interactions between baryons and dark matter. This hypothesis has a striking prediction: large temperature anisotropies sourced by the velocity-dependent cooling of the baryons. However, in order to remain consistent with the rest of cosmological observations, only part of the dark matter is allowed to be charged, and thus interactive. Here we compute, for the first time, the 21-cm fluctuations caused by a charged subcomponent of the dark matter, including both the pre- and post-recombination evolution of all fluids. We find that, for the same parameters that can explain the anomalous 21-cm absorption signal, any percent-level fraction of charged dark matter would source novel 21-cm fluctuations with a unique acoustic spectrum, and with an amplitude above any other known effects. These fluctuations are uncorrelated with the usual adiabatic anisotropies, and would be observable at high significance with interferometers such as LOFAR and HERA, thus providing a novel probe of dark matter at cosmic dawn.

Overview of 21-cm Fluctuations from Charged Dark Matter

The paper under consideration presents a novel investigation into the potential mechanisms underlying anomalous 21-cm signals detected during the epoch known as cosmic dawn, focusing specifically on the implications of charged dark matter. The authors propose that interactions between a tiny fraction of charged dark matter (CDM) and baryons may account for these unusual observations, notably a deeper absorption feature at a redshift of $z = 17$ observed by the EDGES collaboration.

Introduction to the Concept

During cosmic dawn, the first stars formed amidst the slowest relative motions between baryonic matter and dark matter—an epoch ideal for probing interactions mediated by electromagnetic fields. The EDGES signal suggests a lower baryonic temperature than anticipated, leading to the hypothesis that charged dark matter could partially thermalize with baryons, resulting in velocity-dependent cooling and significant temperature fluctuations.

Analytical Framework and Results

The authors explore the effects of CDM interactions on the 21-cm line fluctuation spectrum, incorporating both pre- and post-recombination evolutionary phases. They examine scenarios wherein only a subset of dark matter is charged and interactive, ensuring consistency with conventional cosmological observations.

Their analysis is built upon modifications to standard cosmological models, considering the interaction dynamics between charged dark matter and baryons. Utilizing the modified code {\tt CAMB} for their calculations, they demonstrate how a fraction of CDM could generate novel 21-cm fluctuations characterized by unique acoustic spectra.

Significantly, the findings indicate that any percent-level proportion of charged dark matter would lead to fluctuations with amplitudes exceeding those predicted by current models. These fluctuations are notably distinct and uncorrelated with the typical adiabatic anisotropies observed in cosmic microwave background data.

Implications for Observations and Future Research

The pronounced acoustic signature in the power spectrum of 21-cm fluctuations provides a compelling new probe into dark matter properties during cosmic dawn. These fluctuations are observable with forthcoming radio interferometer arrays such as LOFAR and HERA, both of which the paper anticipates will have the sensitivity required to detect or constrain the presence of interactive CDM fractions as diminutive as 1%.

The theoretical implications of this work suggest potential avenues for refining our understanding of dark matter interactions, especially in the context of low-frequency cosmological observations. Practically, this could lead to new methodologies for interrogating dark matter characteristics through observational astrophysics.

Conclusively, the research presented in the paper outlines a method to leverage 21-cm observations in the quest to uncover dark matter interactions, offering substantial contributions to the field of cosmology through probing previously inaccessible epochs. Future developments in observational technology and techniques may further support exploring such phenomena, potentially leading to revelations about the fundamental nature of dark matter and its effects throughout cosmic history.