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Figure 1: Cosmic chronology in 21-cm with two zoom-ins illustrating the impact from alternative dark matter models.

The first galaxies appeared during the first half billion years of our universe, and ionized and heated the hydrogen gas left over from the Big Bang. The atomic hydrogen at this time emits radiation at 21cm, and statistical fluctuations in this signal can be measured today by radio interferometers, providing otherwise unobtainable information about dark matter in the universe and the formation of the first galaxies. This project is motivated by these upcoming experiments.

Dark matter remains theoretical to this day, as experiments of direct detection, relying on low-energy electronic-recoil from baryon-dark matter particle interaction, are extremely challenging and have not produced well-established results yet. The alternative approach to study its particle physics is by searching for corresponding baryons, either as products when dark matter particles annihilate or decay, or as by-products after successfully manufacturing them in a collider. However, as all currently available evidence for dark matter's existence is based on astrophysical dynamics, there is limited information on how to detect their footprints through visible particles in the local group. This has caused great difficulties in developing search strategies, as the prior probability distribution of the dark matter model parameters is painfully broad and largely unconstrained by earlier studies.

The Cosmic Dawn (CD) could be our breakthrough. Heralding the formation of the very first galaxies, the CD offers a window into how primordial structures form and interact in the early universe, through the 21-cm hydrogen line. This line corresponds to the spin-flip transition of neutral atoms that occupy every corner of our universe during its infancy. It is not only sensitive to the thermal and ionizing state of intergalactic hydrogen gas but also influenced by physical cosmology, including properties of dark matter and dark energy. In addition, since the intergalactic medium (IGM) actively interacts with high-energy photons, any radiative footprints of the first galaxies, dark matter, and even dark energy are also encoded in the large-scale 21-cm tomography. Because interferences from complex baryonic processes remain confined to small scales during the early times, the CD is a unique laboratory for probing the nature and imprints of dark matter from a cosmological perspective, which is expected to provide new insights into studying their detailed particle physics.

My recently funded DECRA aims to quantify how to extract information on physical cosmology and dark matter properties from upcoming 21-cm experiments. As part of this project, you can choose to work on:

1. Establishing a suite of novel 21-cm simulations that co-vary astrophysical properties and the nature of dark matter, sampling 4D light-cones of our universe on the fly. This will be further assisted by developing sophisticated models of first galaxy formation, offering prior knowledge that is essential for exploring the early universe.
2. Exploiting the fast-growing AI industry to develop machine-learning algorithms and generative models to emulate our computationally expensive simulations. The goal is to alleviate modeling bottlenecks in terms of the dynamical range and generate a portable database of the early-time 21-cm signal.

Dr Yuxiang Qin is the project supervisor.