Cosmic dawn, as the name would suggest, is an early period of the universe's history when the first luminous objects came into being. It is theorised that the first stars ignited from the collapse of molecular hydrogen clouds around 100 million years after the Big Bang. The end of this cosmic dawn would mark the beginning of reionization. As these stars became more and more energetic, they began to release ultra-violet (UV) photons into their surrounding medium. These UV photons ionized the surrounding neutral hydrogen left over from recombination in the intergalactic medium (IGM). This epoch of reionization (EoR) is poorly constrained and understood, yet remains fundamental to our understanding of the evolution of our universe. 

Photo credit: Scientific American

A number of lines are being targeted by current and upcoming experiments including, Lyman-alpha, H-alpha, OII/III, CO, and [CII], but perhaps the most powerful probe of cosmic dawn and the EoR is the 21 cm line of neutral hydrogen (HI). The 21 cm signal allows one to trace neutral hydrogen in the intergalactic medium (IGM) as a function of redshift as well as position on the sky. In addition, since hydrogen is a primordial element, one can, in principle, make unprecedentedly large maps of the universe in HI spanning the cosmic dark ages, through cosmic dawn and the EoR, all the way to the present day. Hydrogen intensity mapping allows us to directly observe the process of reionization, where we see HI emission from the neutral IGM, and as stars begin to ionize hydrogen atoms, neutral bubbles start to grow. The details of our reionization history is dependent on the properties of early galaxies, and hydrogen intensity mapping will allows us to probe the astrophysics of this first galactic population. In addition, 21 cm observations can independently probe cosmological parameters which remains poorly constrained by CMB measurements.

Photo credit: Patrick Breysse

In the beginning, our universe was a hot dense primordial soup of particles where photons, electrons, and atomic nuclei oscillated together like waves in the ocean. As the universe expanded and cooled, the electrons were able to cling to their neighbouring nuclei for the first time, creating the original atoms. This allowed the photons in this fluid to be set free. We see this light today as the cosmic microwaved background (CMB). These photons, however, have not travelled unimpeded. Weak gravitational lensing of the CMB arises when photons from the Big Bang are deflected by the gravitational potentials they encounter on their way to us, resulting in distortions to the statistics of the CMB. By studying these distortions, one can reconstruct the total mass distribution along the line of sight (LOS). This is an incredible probe of the matter density field since it is measured directly, without the use of a luminous tracer. 

Photo credit: ESA and the Planck Collaboration

Fast radio bursts (FRBs), are short, bright bursts of electromagnetic radiation, the source of which remains unknown. Despite uncertainties about their progenitors, FRBs can be used as cosmological tools for studying the ionization fraction in the IGM as well as the baryon density of the universe. This electromagnetic radiation gets dispersed on its way to us, meaning that every time an FRB photon encounters an electron along its path, the photon’s travel time gets delayed. This means that FRBs have encoded in them information about the medium through which they traveled, namely the density of electron there are along the line of sight. The same way turning on a flashlight in a dusty room allows you to see all the particles that were secretly floating around in the air, FRBs are cosmic flashlights. By studying their dispersion measure, we can learn about how much stuff is between us and the source of the burst. Since the process of reionization introduces electrons into the IGM, FRBs that are emitted during reionization should be able to help constrain our reionization history. We are only now entering the early days of FRB cosmology and I am excited to see what this new probe will tell us about our universe.

Photo credit: Danielle Futselaar