Cosmic Dawn

The Cosmic Dawn is the era during which the the first stars and primordial galaxies formed in the Universe, approximately half a billion years after the Big Bang. It is directly followed by the Epoch of Reionization, during which these first sources ionize the Universe.

Illustration of the time history of reionization in the CoDa II simulation that shows the time evolution from redshift z= 150 (left) to z= 5.8 (right). Blue regions are photo-heated.

The 21 cm signal of neutral hydrogen is a tracer of this epoch, and contains valuable information regarding the nature and formation of the first sources. This signal stems from a transition between the two hyperfine levels around the ground state of the hydrogen atom, and its characteristics depend on the distribution and physical properties of the neutral hydrogen patches in the Universe, which are affected by the emissions of the first sources. Indeed, the Lyman-alpha radiation couples the spin temperature of HI to the temperature of the gas, which cause the signal to be seen in absorption against the CMB (z~15 to ~8). At a later stage, the gas is heated by X-rays, and the spin temperature can exceed the CMB temperature, making the signal appear in emission (z~8) . At the end of Reionization, the signal vanishes with the last neutral clumps in the Universe (z~6).

Average brightness temperatures of the 21 cm signal in each of the 45 LICORICE simulations of the 21SSD database.

The NenuFAR collaboration has put significant efforts to improving the modelling of the Cosmic Dawn, be it with fast semi-analytical codes (see for instance Fialkov et al. 2019) or costly and complex numerical simulations, such as LICORICE (Semelin et al. 2018) or CoDa (Orvick et al. 2020) .