21 cm Cosmology Observations

The birth of the Universe in the Big Bang produced a hot soup of radiation and matter. After some 300,000 years, this soup had cooled sufficiently that free electrons and protons could combine to form neutral hydrogen atoms. Thus began the Cosmological Dark Age, an epoch when there were no stars, no quasars, no luminous sources. Compared to today, the universe was dark and cold.

Over time, that action of gravity fragmented the 'sea' of Hydrogen, and condensations collapsed to form the first generations of stars (see figure). As their numbers grew, galaxies formed and reionized the universe, leaving behind today's largely transparent intergalactic medium dotted with quasars and clusters of galaxies.

There are few data with which to constrain models of what the Universe looked like during the Dark Age and later Reionization epochs. Quasars and galaxies at the tail end of Reionization are just now being detected with infrared telescopes, but numbers are small, and other means are needed for broad study and certainly to look back to the Dark Age.

Targeting the radio emission from the Hydrogen gas rich intergalactic medium is a new area of research. This is often referred to as '21cm cosmology' after the natural wavelength of Hydrogen's most pervasive atomic transition and the one that astronomers can most readily detect at very high redshift.

Astronomers in the RG Division are building the Large Aperture Experiment to Detect the Dark Age (LEDA) with an eye to answering fundamental questions such as when was the 'Cosmic Dawn,' when the first generations of stars formed; how soon thereafter did black holes (the remnants of dead stars) begin to heat the universe via their X-ray emissions; and were there exotic unexpected sources of heat in the early universe, e.g., radiation from Dark Matter. RG Division members have also led in design of the recently commissioned Murchison Widefield Array in Australia, which will target the 21 cm signal from the later Reionization epoch, hoping to answer long standing questions about the astrophysics of early galaxy formation.