Title: Hydrogen spin oscillations in a background of axions and the 21-cm brightness temperature

Speaker: Prof. Subhendra Mohanty, Physical Research Laboratory

Abstract: The 21-cm line signal arising from the hyperfine interaction in hydrogen has an important role in cosmology and provides a unique method for probing of the universe prior to the star formation era. The first observation of the 21-cm line from the cosmic dawn by EDGES experiment shows an anomalously large dip which indicates that the spin temperature of Hydrogen is lower than predicted by astrophysics models. I will discuss the idea that the spin flip of Hydrogen by the coherent emission/absorption of axions causes a lowering of their spin temperature and can explain the stronger than expected absorption of 21-cm light reported by the EDGES collaboration. Axion interaction with the two level HI is analogous to the Jaynes-Cummings model of a two level atom in a cavity and we derive the spin flip frequency in this formalism and I will show that the coherent oscillations frequency $\Omega \propto 1/f_a$ in contrast with the incoherent transitions between the HI hyperfine levels where the transition rates $\propto 1/f_a^2$. I will show that the axion process has a discernible effect on the spin temperature and goes in the right direction for explaining the EDGES observation.