An absorption profile centred at 78 megahertz in the sky-averaged spectrum (1810.05912v1)

Abstract: After stars formed in the early Universe, their ultraviolet light is expected, eventually, to have penetrated the primordial hydrogen gas and altered the excitation state of its 21-centimetre hyperfine line. This alteration would cause the gas to absorb photons from the cosmic microwave background, producing a spectral distortion that should be observable today at radio frequencies of less than 200 megahertz. Here we report the detection of a flattened absorption profile in the sky-averaged radio spectrum, which is centred at a frequency of 78 megahertz and has a best-fitting full-width at half-maximum of 19 megahertz and an amplitude of 0.5 kelvin. The profile is largely consistent with expectations for the 21-centimetre signal induced by early stars, however, the best-fitting amplitude of the profile is more than a factor of two greater than the largest predictions. This discrepancy suggests that either the primordial gas was much colder than expected or the background radiation temperature was hotter than expected. Astrophysical phenomena (such as radiation from stars and stellar remnants) are unlikely to account for this discrepancy, of the proposed extensions to the standard model of cosmology and particle physics, only cooling of the gas as a result of interactions between dark matter and baryons seems to explain the observed amplitude. The low-frequency edge of the observed profile indicates that stars existed and had produced a background of Lyman-alpha photons by 180 million years after the Big Bang. The high-frequency edge indicates that the gas was heated to above the radiation temperature less than 100 million years later.

• The paper's main finding is the detection of a 78 MHz absorption profile with a 19 MHz width and 0.5 K amplitude that exceeds predictions.
• It employs rigorous EDGES instrument calibration and extensive two-year data collection to rule out artifacts, ensuring result validity.
• The measured anomaly implies potential dark matter–baryon interactions, challenging standard cosmology and prompting revised early Universe models.

Analyzing the Detection of an Absorption Profile at 78 MHz in the Early Universe's Radio Spectrum

This paper presents a detailed observational paper utilizing the Experiment to Detect the Global EoR Signature (EDGES) instrumentation, focusing on the detection of a 21-centimetre hydrogen line—a crucial probe of the early Universe. The comprehensive investigation reports the observation of an anomalously strong absorption feature centered at 78 MHz, with implications for cosmology beyond standard models.

The absorption profile is characterized by a full-width at half-maximum of 19 MHz and an amplitude of 0.5 Kelvin. Notably, this observed amplitude surpasses prior predictions by more than a factor of two. Such results suggest deviations from standard theoretical models, raising critical questions regarding either the temperature of the primordial gas or the cosmic microwave background's (CMB) radiation temperature at that epoch. Astrophysical phenomena intrinsic to early stellar and remnant radiative processes alone are deemed insufficient to account for this deviation.

This work posits that interactions between dark matter and baryons might explain the anomalously low gas temperature suggested by the absorption profile amplitude. Specifically, theorized cooling interactions would need to involve dark matter particles with masses below a few GeV and interaction cross-sections exceeding ~10^-21 cm².

The paper details the meticulous calibration and validation of the double-pronged EDGES instruments, stationed 150 meters apart, with data collection spanning over two years. The calibration process incorporated several techniques and verification trials to rule out instrumental and RFI artifacts, establishing the robustness of the observed profile. Moreover, systematic uncertainties and statistical validation were critically addressed through numerous trials.

In terms of implications, this work stimulates reconsideration of parameters affecting the early Universe's thermal history and hints at new physics, potentially involving dark matter. The robust detection of this feature benefits ongoing experiments, including LEDA, SCI-HI/PRIZM, and SARAS-2, which aim to verify and refine our understanding of the 21-cm signal during the Cosmic Dawn.

The paper's methodological rigor sets a benchmark for future measurements, highlighting the challenges and solutions related to instrument calibration, RFI mitigation, and systematic error management in cosmological observations. Going forward, this detection should further motivate the deployment of space-based observatories as they avoid Earth-based interference and achieve higher sensitivity to the 21-cm global signal, providing clearer windows into cosmic history.

This work enriches the theoretical landscape, as theorists will need to adapt models to either accommodate the unusually low early gas temperatures or explore non-standard cosmological scenarios, potentially reshaping the foundational understanding of the Universe's evolution following the Big Bang. The findings underscore the synergistic potential of observational cosmology and particle physics in uncovering new facets of cosmic evolution and the nature of dark matter.