21-cm cosmology is a field that aims to explore the universe during its early stages, specifically the first hundreds of millions of years. This is achieved by observing the redshifted 21 cm line of atomic hydrogen, which is emitted due to spin-flip transitions.

This field of study has the potential to transform our understanding of the formation of the first stars and galaxies, as well as the thermal history of the universe. It can provide insights into dark energy, dark matter, neutrino masses, and inflation.

A new generation of radio telescopes is being constructed for this purpose, with the first results starting to trickle in. The 21 cm radiation 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.

The hope is to infer details about the re-ionization processes and history. This method of observation is still in its infancy, with a few first telescopes in operation.

The Global 21-cm spectrum uniquely probes the cosmological model during the Dark Ages plus the evolving astrophysics during Cosmic Dawn, yielding constraints on the first stars, on accreting black holes, and on exotic physics such as dark matter-baryon interactions.

A single low frequency radio telescope can measure the Global spectrum between ~10-110 MHz because of the ubiquity of neutral hydrogen. Precise characterizations of the telescope and its surroundings are required to detect this weak, isotropic emission of hydrogen amidst the bright foreground Galactic radiation.

Technology challenges include advanced calibration techniques to disentangle covariances between a bright foreground and a weak 21-cm signal, using techniques similar to those for the CMB, thermal management for temperature swings of >250C, and efficient power to allow operations through a two-week lunar night. This simple telescope sets the stage for a lunar farside interferometric array to measure the Dark Ages power spectrum.