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 ?-effect that sets in at earlier time can be helpful in lowering the gas temperature to 3.2 K 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 � 10?3 and 5 � 10?4 nG, respectively. We also discuss the effect of Ly?? background on the bounds. Our results do not require any new physics in terms of dark matter.