Exploring the Universe with the Hydrogen 21 cm Line

Dear Colleagues,

Electrons and protons in neutral hydrogen can have their magnetic moments aligned or anti-aligned to each other. When the transition from the higher energy state to the lower energy state occurs, a photon is emitted with a wavelength of 21 cm at a corresponding frequency of 1420 MHz. Since neutral hydrogen is one of the most common elements in the Universe and was present in abundance before the formation of the first stars, it is an extremely useful tool to probe the “dark age” of the Universe, whose epoch lies between the recombination and reionization. Even in more recent periods, where hydrogen is mostly found in its ionized form, small dense regions within galaxies host a large amount of neutral hydrogen, which can in turn trace the location of late-time galaxies and clusters.

Given its prospect to probe cosmology and astrophysics, several telescopes have been planned and built, aiming to map the intensity of the hydrogen 21-cm line. These observations will lead to further exploration of dark energy, dark matter, and alternative models of gravity, merging several interdisciplinary research areas, such as radio astronomy, galaxy formation, and particle physics. We wish to invite both original and review papers to this Special Issue, in a wide perspective, exploring all facets that are inherent to the 21-cm line of hydrogen.

Dr. Ricardo G. Landim
Dr. Filipe B. Abdalla
Dr. Larissa Santos
Guest Editors

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• dark energy
• dark matter
• structure formation
• radio astronomy
• modified gravity
• observational cosmology