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21 CM COSMOLOGY

"The most incomprehensible thing about the universe is that it is comprehensible."

Albert Einstein.

The fact that our minds can come up with theories, tools, and experiments to study and understand the universe will always remain unfathomable to humanity. Unfortunately in most of the cases, we can not observe the cosmic realm directly especially in cosmology ( It is the study of shape, size, origin, formation, and fate of our universe ) instead we come up with tools that can be exploited to probe into the unknown.


This article is about an upcoming probe in the field of cosmology, which is going to enhance our current understanding immensely and new windows for the observations will open up.


This new era of upcoming precision cosmology is known as 21 cm cosmology.

When magnetic moments of electron and proton interacts in a hydrogen atom a hyperfine splitting line corresponding to a wavelength of 21.1 cm and a frequency of 1420 MHz is produced. This 21 cm line will allow us to study numerous current theories in more detail. Astrophysicists are using this unique feature of hydrogen atom extensively.


Let's take an existing example, Giant dark cloud or dust particles covering the active regions in our space extinct the most of visible wavelengths emitted making it hard to observe them, so researchers looking at these astrophysical processes like galaxy evolution and star formation uses 21 cm absorption spectroscopy as a probe. To use this probe in the understanding of cosmology instruments and experiments are being designed.


Low-frequency array LOFAR is a large radio telescope in the Netherland constraining the origin of cosmic rays. 21CMA is a ground-based meter-wave interferometric array intended to probe the 21cm radiation of neutral hydrogen from the cosmic dawn and the epoch of reionization. SKA, MWA, and PAPER are on the list too. A crucial gap in redshift observation

Z = 1 - 150 will be filled, and linear modes present at high redshifts will be studied spaning the large volume of the universe. The following are some measurements expected to improve in the upcoming era of precision cosmology structure formation, heating and reionization of the universe, primordial black hole evaporation, cosmological parameters, the mass of neutrinos, dark matter annihilation, and many other measurements will be constrained.

The long term goal of using 21 cm observation in cosmology is to target cosmic dark ages using many different redshifts.

The dark age is the early period in the history of our cosmos. As the name suggests, nothing was present in the universe in that age to produce visible light but now it will be possible to study the early universe by mapping in the upcoming era of cosmology.

Still, there's much to discover yet in this field, but studies show it has great potential to unlock the secrets of the cosmos.

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Further reading 1. Pritchard, Jonathan R., and Abraham Loeb. "21 cm cosmology in the 21st century." Reports on Progress in Physics 75.8 (2012): 086901. 2. Furlanetto, Steven R., S. Peng Oh, and Frank H. Briggs. "Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe." Physics reports 433.4-6 (2006): 181-301. 3. Morales, Miguel F., et al. "Understanding the diversity of 21 cm cosmology analyses." Monthly Notices of the Royal Astronomical Society 483.2 (2019): 2207-2216.


Written by -

Sharma Saurabh,

NIT Surat, Int-MSc Student, Bose.X Member, XCAR's Co-lead


Edited by -

Ishita Srivastava,

Bose.X Associate Member, Lead of Newsletter Team

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