RG Research: Epoch of Reionization

The birth of the Universe in the Big Bang produced an extremely 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 (HI). Thus began the Cosmological Dark Ages, a time when there were no stars, no quasars, no luminous sources. The universe was dark. Over time, self-gravity fragmented the matter and condensations collapsed to form the "first" stars, black holes, and quasars (see figure). As their numbers grew, these luminous objects reionized the universe, leaving behind a largely transparent universe dotted with quasars and clusters of galaxies.

There are few data to constrain models of what the universe looked like before and during Reionization. The best available today would be optical quasar spectra that probe individual lines of sight from the present epoch back to reionization. Lyman alpha absorption by intergalactic Hydrogen creates a forest of dark bands seen against quasar continua. For objects at redshifts beyond the end of reionization, these bands blend together, blocking the quasar emission almost entirely. This is the so-called Gunn-Peterson trough.

This quasar data is limited to just a few specific lines of sight. A more general means of study would involve directly mapping atomic Hydrogen gas, which after all was the dominant component of the universe at the time. The natural frequency of atomic Hydrogen has a wavelength of 21 cm. However, because reionization occurred so long ago (i.e., at such high redshift), the wavelength of the emission we might receive is 1.5-3 meters, about the same as that of radio and television.

Astronomers in the RG Division are currently involved in an innovative project to develop powerful new capabilities for radio astronomy below about 1.6 GHz, optimized for the wide fields of view and unprecedented sensitivity required for observations of the Dark Ages. The initial goal of the so-called Mileura Widefield Array project is to create two complementary and co-located but substantially independent scientifically-capable demonstration instruments, one in the 80-300 MHz frequency range and the other in the 800-1600 MHz range.

Project Links

Mileura Widefield Array


Lincoln Greenhill, Justin Kasper, Daniel Mitchell,

On-going collaborators outside the CfA

Robert Sault, Miguel Morales

  Dark Matter simulation

A simulation by Springel & Hernquist (2003) of the collapse of matter into the earliest stars and galaxies near the end of the Dark Ages. Image Source: Springel, Volker; Hernquist, Lars (2003), The history of star formation Λ in a cold dark matter universe, Monthly Notice of the Royal Astronomical Society, Volume 339, Issue 2, pp. 312-334.


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