21 cm radiation: A new probe of fundamental physics
  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.
