EDGES signal in presence of magnetic fields (2001.00194v3)

Abstract: We study the 21-cm differential brightness temperature in the presence of primordial helical magnetic fields for redshift $z=10-30$. We argue that the $\alpha$-effect that sets in at earlier time can be helpful in lowering the gas temperature to 3.2 degrees Kelvin at $z=17$. This effect can arise in the early Universe due to some parity violating high energy processes. Using the EDGES (Experiment to Detect the Global Epoch of Reionization Signature) results, we find the upper and lower limits on the primordial magnetic field to be $6\times 10^{-3}~{\rm nG} $ & $5\times 10^{-4}~{\rm nG}$ respectively. We also discuss the effect of Ly$\alpha$ background on the bounds. Our results do not require any new physics in terms of dark matter.

• The paper proposes that primordial helical magnetic fields and the alpha-effect can explain the strong EDGES 21-cm absorption signal by lowering intergalactic medium gas temperature.
• Incorporating the alpha-effect, the study finds gas temperature could be reduced to approximately 3.2 K at redshift 17 without needing exotic dark matter physics.
• Analyzing the EDGES observation with this model constrains primordial magnetic field strengths to between $5 imes 10^{-4}$ nG and $6 imes 10^{-3}$ nG.

Analysis of "EDGES Signal in Presence of Magnetic-Fields"

The paper by Natwariya and Bhatt investigates the intriguing phenomena associated with the 21-cm brightness temperature signal in the presence of primordial magnetic fields during the cosmic dawn era. The research is contextualized within the framework outlined by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), which detected an unexpected absorption signal at redshifts between approximately 15 and 20. This observation raises questions concerning the conventional $\Lambda$CDM cosmological model which fails to predict such a substantial absorption.

Key Contributions and Findings

1. Introduction of Helical Magnetic Fields: The authors propose that primordial helical magnetic fields may have a role in the enhanced 21-cm absorption. A notable theoretical construct in this work is the inclusion of the $\alpha$-effect, a concept from magnetohydrodynamics (MHD) involving the amplification of magnetic fields via turbulent interactions lacking mirror symmetry.
2. Gas Temperature Reduction: The paper posits that the $\alpha$-effect could lower the gas temperature to approximately 3.2 K at a redshift of 17 without introducing new dark matter physics. This temperature reduction is critical for aligning the observed absorption signal with standard models of the cosmic microwave background (CMB).
3. Magnetic Field Magnitude Constraints: By analyzing the EDGES observation with the proposed model, the paper constrains the strengths of primordial magnetic fields to between $5 \times 10^{-4}$ nG and $6 \times 10^{-3}$ nG. These constraints align with existing bounds from other cosmological investigations and demonstrate the non-requirement of additional energetic processes such as dark-matter baryon interactions.
4. X-ray Heating Considerations: Incorporating potential X-ray heating from early star formation post-redshift 30, the authors adjust the permissible magnetic field strengths to between $9 \times 10^{-4}$ nG and $3 \times 10^{-3}$ nG. This approach provides a more comprehensive view of plausible astrophysical influences during the timeframe of interest.

Implications and Speculative Extensions

The implications of incorporating helical magnetic fields in early universe modeling are significant, as they potentially reduce the need for exotic dark matter interactions to explain observed phenomena. Furthermore, the $\alpha$-effect offers a mechanism by which the thermal state of the intergalactic medium can be dynamically evolved due to intrinsic field properties.

This work opens avenues for further theoretical investigations into the properties and genesis of primordial magnetic fields and their implications for observable cosmological signals. The connection between early universe magnetic conditions and later cosmic structures offers a fertile ground for research, particularly in refining our understanding of magnetogenesis and large-scale cosmic magnetism.

Future Directions

Future research may focus on refining the interaction models that govern the $\alpha$-effect, exploring its consistency across various cosmological epochs, and integrating these findings with other observational data beyond EDGES. Multidisciplinary studies combining theoretical physics, astronomical observations, and computational cosmology will be crucial in elucidating the broader implications of these results in cosmological narratives.