On the detection of a cosmic dawn signal in the radio background (2112.06778v1)

Abstract: The astrophysics of cosmic dawn, when star formation commenced in the first collapsed objects, is predicted to be revealed as spectral and spatial signatures in the cosmic radio background at long wavelengths. The sky-averaged redshifted 21-cm absorption line of neutral hydrogen is a probe of cosmic dawn. The line profile is determined by the evolving thermal state of the gas, radiation background, Lyman-$\alpha$ radiation from stars scattering off cold primordial gas and the relative populations of the hyperfine spin levels in neutral hydrogen atoms. We report a radiometer measurement of the spectrum of the radio sky in the 55--85~MHz band, which shows that the profile found by Bowman et al. in data taken with the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) low-band instrument is not of astrophysical origin; their best-fitting profile is rejected with 95.3\% confidence. The profile was interpreted to be a signature of cosmic dawn; however, its amplitude was substantially higher than that predicted by standard cosmological models. Explanations for the amplitude of the profile included non-standard cosmology, additional mechanisms for cooling the baryons, perhaps via interactions with millicharged dark matter and an excess radio background at redshifts beyond 17. Our non-detection bears out earlier concerns and suggests that the profile found by Bowman et al. is not evidence for new astrophysics or non-standard cosmology.

Detection of Cosmic Dawn Signal in the Radio Background: Insights from SARAS 3

The paper "On the detection of a cosmic dawn signal in the radio background" presents an in-depth analysis conducted using the SARAS 3 radiometer to measure the radio sky spectrum in the 55-85 MHz band. The investigation primarily aims to address and validate the spectral profile detected by the EDGES low-band instrument, reported by Bowman et al., as indicative of the cosmic dawn period.

Overview and Methodology

The SARAS 3 experiment was engineered to measure the radio sky spectrum and obtain insights into the cosmic dawn period, which is presumed to manifest through the redshifted 21-cm absorption line of neutral hydrogen. In essence, the SARAS 3, with its innovative design and placement over water bodies, was conceptualized to offer a stable platform to observe the cosmic radio spectrum, leveraging a monocone antenna to minimize environmental influences and technological interference.

The paper challenges the previously reported detection by EDGES, which indicated a high-amplitude profile suggestive of cosmic dawn but was significantly higher than predictions from standard cosmological models. Several non-standard cosmogony explanations had been proposed to justify this anomaly, including millicharged dark matter interactions and an excess radio background from high redshifts.

Employing the SARAS 3 instrument, the authors conducted observations in selected low-RFI sites in Southern India. Calibration techniques were implemented rigorously to account for noise and ensure precision in signal interpretation, facilitating an accurate spectral measurement process.

Key Findings

1. Primary Finding: The SARAS 3 data analysis rejected the spectral profile reported by Bowman et al. with a confidence level of 95.3%, indicating that it is not of astrophysical origin. The paper found no significant excess variance supportive of the spectral distortions purported in the EDGES data.
2. Refined Analysis Techniques: The research utilized a $6^{th}$-order polynomial model to represent the foreground plus any calibration systematics. This approach allowed for an examination that is sensitive enough to detect or refute the presence of the reported profile in the SARAS 3 measurements.
3. Systematic Error Analysis: The rigorous calibration and error analysis undertaken emphasize the need for precision in such measurements. The paper underscores the absence of correlation between the residual distortions in EDGES and SARAS 3 spectra, which suggests that systematic errors may significantly contribute to the distortions observed by EDGES.
4. Confidence in Alternative Hypotheses: By employing MCMC and correlational analysis frameworks, the paper solidified the conclusion that the spectral feature observed by Bowman et al. could originate from instrument-related systematics rather than genuine cosmic signals.

Implications and Future Directions

The findings raise critical implications for interpreting the results from cosmic dawn investigations using radio astronomic observations. The discrepancy between SARAS 3 and EDGES highlights the potential for systematic errors significantly impacting observational cosmology, particularly concerning the early universe's electromagnetic environment.

Furthermore, this paper highlights the necessity for environments free from electromagnetic interference—conditioned for observations like those of SARAS 3 via deployment on water bodies—or potentially, in extraterrestrial settings, such as the lunar far side. These setups could offer an unperturbed ambiance to validly detect the redshifted 21-cm signal, marking genuine cosmic dawn events.

Future developments might envision utilizing instruments with further enhanced chromatic smoothness to reduce false spectrometric interference and better distinguish cosmological signals. Continued advancements in radio astronomy techniques and collaboration across instrumentation setups will be crucial to unravel the truth about the early cosmos and refine models of cosmic dawn. As the paper closes, it iterates the necessity of adopting alternative observational locales or innovating instrumental designs free from terrestrial and instrumental chromaticity perturbations to advance this line of research convincingly.