  Observations of the 21-cm line from primordial hydrogen promise to be one of
the best tools to study the early epochs of the Universe: the Dark Ages, the
Cosmic Dawn, and the subsequent Epoch of Reionization. In 2018, the EDGES
experiment caught the attention of the cosmology community with a potential
detection of an absorption feature in the sky-averaged radio spectrum centred
at 78 MHz. The feature is deeper than expected, and, if confirmed, would call
for new physics. However, different groups have re-analyzed the EDGES data and
questioned the reliability of the signal. The Radio Experiment for the Analysis
of Cosmic Hydrogen (REACH) is a sky-averaged 21-cm experiment aiming at
improving the current observations by tackling the issues faced by current
instruments related to residual systematic signals in the data. The novel
experimental approach focuses on detecting and jointly explaining these
systematics together with the foregrounds and the cosmological signal using
Bayesian statistics. To achieve this, REACH features simultaneous observations
with two different antennas, an ultra wideband system (redshift range 7.5 to
28), and a receiver calibrator based on in-field measurements. Simulated
observations forecast percent-level constraints on astrophysical parameters,
potentially opening up a new window to the infant Universe.
