# 21 cm radiation: A new probe of fundamental physics

## TL;DR

Future low-frequency radio telescopes observing 21-cm radiation during the dark ages can provide powerful constraints on variations of fundamental constants, especially the fine structure constant, surpassing current methods.

## Contribution

This paper demonstrates the potential of 21-cm observations to constrain fundamental physics, particularly variations in the fine structure constant, at high redshifts.

## Key findings

- 21-cm radiation is highly sensitive to variations in the fine structure constant.
- Future telescopes with large collecting areas can improve constraints on fundamental constants.
- 21-cm absorption offers unique insights into the universe between recombination and reionization.

## Abstract

New low frequency radio telescopes currently being built open up the possibility of observing the 21-cm radiation before the Epoch of Reionization in the future, in particular at redshifts 200 > z > 30, also known as the dark ages. At these high redshifts, Cosmic Microwave Background (CMB) radiation is absorbed by neutral hydrogen at its 21-cm hyperfine transition. This redshifted 21-cm signal thus carries information about the state of the early Universe and can be used to test fundamental physics. We study the constraints these observations can put on the variation of fundamental constants. We show that the 21-cm radiation is very sensitive to the variations in the fine structure constant and can in principle place constraints comparable to or better than the other astrophysical experiments (fractional change < 10^ {-5}). Making such observations will require radio telescopes of collecting area 10 - 10^6 sq. km compared to 1 sq. km of current telescopes. These observations will thus provide independent constraints on the fine structure constant at high redshifts, observations of quasars being the only alternative. More importantly the 21-cm absorption of CMB is the only way to probe the redshift range between recombination and reionization.