Probing The Early Universe With The Hydrogen 21 cm Line

Cosmic timeline illustration. Image credits: NASA/CXC/M.Weiss

The Universe was created with a Big Bang, that created a hot soup of radiation and matter. After some 300,000 years, this hot plasma had cooled sufficiently, such that free electrons and protons could combine to form neutral hydrogen atoms.

This marked the beginning of the Cosmological Dark Age. An epoch when there were no stars, no quasars, no luminous sources. An era of darkness before the formation of stars (hence the name ‘Dark Ages’).

The universe then was dark and cold.

As expansion cooled the Universe, Hydrogen went through a phase transition, rapidly becoming neutral and releasing the cosmic microwave background light at a redshift of about z=1,089. High-energy photons produced by the first stars and quasars later reionized the Hydrogen in the intergalactic medium (IGM), forcing the Universe back through a second extended and patchy phase transition referred to as the epoch of reionization (EoR).

As their numbers grew, galaxies formed and reionized the universe, leaving behind today’s largely transparent intergalactic medium dotted with quasars and clusters of galaxies.

Role of hydrogen in cosmic creation

 21 cm LINE

There was a catch! First of all how do we know what was happening? If we remember correctly, the unionized material does not emit light.

All thanks to a quantum-mechanical transition between hyperfine energy levels in hydrogen (called spin-flip transition). Due to this microwave radio radiation with a wavelength of approximately 21 cm was emitted. (A more technical explanation about this important transition can be found at Caltech).

Overlay of Chandra X-ray image with radio continuum and 21 cm line emission. Image: Chandra/Harvard

This allows observation of this epoch in the Universe, spanning the time from shortly after the Universe became transparent and neutral at an age of around 380,000 years, through the start of reionization of hydrogen in stars at around 400 million years, up to full reionization at an age of around 1 billion years. As the radiation reaches Earth, the 21 cm line is observed at frequencies from about 200 MHz to 10 MHz.

The 21 cm line from the local hydrogen abundance was first observed by Ewen and Purcell in 1951, and thereby kick-started the field of radio astronomy.

Radio astronomy reveals the hidden universe!


There are two of them. One can be the mapping of the redshifted 21 cm radiation, from which we can infer the statistical distribution of matter over time. This in turn directly gives information about dark energy, dark matter, neutrino masses, and inflation, which can be thought of as the cosmic microwave background measured at many different points in time.

The second explores the “holes” in the 21 cm radiation due to neutral hydrogen which becomes re-ionized. This is mainly by the radiation from the first stars gradually switching on during this epoch. The hope is here to infer the details of the re-ionization processes and history from 21 cm cosmology.

Quasars and galaxies at the tail end of reionization are just now being detected with infrared telescopes, and other means are needed for broad study and certainly to look back to the Dark Age.

HERA-Hydrogen Epoch of Reionization Array,
HERA radio telescope. Image credits: SKA Africa

Targeting the radio emission from the Hydrogen gas-rich intergalactic medium is a new area of research. This is often referred to as ’21 cm cosmology’, after the natural wavelength of Hydrogen’s most pervasive atomic transition, probably one that astronomers can most readily detect at very high redshift.

The study of EoR provides an opportunity to study the properties of the first galaxies and stars. The ionization and temperature of the medium play a significant regulatory role in galaxy formation

South African MeerKAT radio telescope. Image credits: SKA Africa

Recent advances in radio instrumentation ( like HERA and SKA) and techniques make it possible to observe the redshifted 21-cm emission of the neutral hydrogen (HI) for these redshifts (z > 8), like a probe into the past, telling us about how it all began.

Here is the 3-minute video explaining the cosmic timeline in some detail!

Video on the evolution of the Universe:


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