Cosmic Dawn

21-cm calculations are sensitive to several delaying and heating mechanisms. The delaying mechanisms include the dark-matter/baryon relative velocity and Lyman-Werner (LW) star-formation feedback, and the heating mechanisms, in addition to the X-ray heating, include CMB and Lyman-α heating effects. I briefly describe these mechanisms. The baryon acoustic oscillations (BAO) that occur due to the interaction between the baryon and photon fluids before recombination generate supersonic relative velocities between dark matter and baryons just after recombination. This supersonic velocity prevents the formation of structures inside the mini halos (having masses in the range 10^5 − 10^7 M⊙) that form early in the hierarchical structure formation scenario. This delays the onset of the CD epoch and directly affects the evolution of the 21-cm signal. The LW photons emitted by each luminous source are absorbed by hydrogen atoms as soon as they redshift into one of the Lyman lines of the hydrogen atom. Along the way, whenever they hit an LW line they may cause the dissociation of molecular hydrogen. This, in turn, applies negative feedback on star formation, which regulates the process and delays CD. The Lyman-α heating mechanism is due to the resonant scattering between Lyman-α photons and the intergalactic medium (IGM) atoms, which in turn increases the IGM temperature. On the other hand, the CMB heating results from the energy transfer from the radio background (which is dominated by the CMB) into the IGM, mediated by the Lyman-α photons.

We have incorporated the CMB and Lyman-α heating in the 21cmFAST code. We have also created a direct interface between the public cosmic microwave background (CMB) Boltzmann code CLASS and 21cmFAST.

In the fuzzy dark matter (FDM) model, dark matter is composed of ultra-light particles with a de Broglie wavelength of ∼kpc, above which it behaves like cold dark matter (CDM). Due to this, FDM suppresses the growth of structure on small scales, which delays the onset of the cosmic dawn (CD) and the subsequent epoch of reionization (EoR). This leaves potential signatures in the sky averaged 21-cm signal (global), as well as in the 21-cm fluctuations, which can be sought for with ongoing and future 21-cm global and intensity mapping experiments.

The fluctuations in the dark matter-baryon relative velocity field are imprinted as Velocity acoustic oscillations (VAOs) in the 21-cm power spectrum during cosmic dawn (CD). VAOs keep the imprints of the comoving sound horizon scale and can be treated as a standard ruler to measure the cosmic expansion rate at high redshifts.