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   Research Interests:
The 21 cm Transition

The 21 cm transition of neutral hydrogen occurs when the spin of the electron flips relative to that of the nucleus. Although extremely weak (with a mean lifetime of 30 million years!) the enormous amount of hydrogen in the Universe makes it extremely useful for astrophysics. In particular, after the cosmic microwave background (CMB) last scattered (at z~1100) and before reionization, the Universe was full of neutral hydrogen. Mapping its properties across the sky and in frequency will allow us to generate a three-dimensional picture of the early stages of structure formation. The features in the map trace fluctuations in the gas density (the proto-cosmic web), the spin temperature of the 21 cm transition, and (most dramatically) the ionized fraction. The signal can be divided into four epochs:

  • The Dark Ages: Long before the first galaxies form, the IGM is dense and cold, and it absorbs 21 cm photons from the CMB. The absorption is modulated by the small density fluctuations already existing in the IGM (and their accompanying temperature fluctuations). Because this era precedes the appearance of luminous objects, these observations would allow us to make fundamental cosmological measurements of such things as the power spectrum, as well as constraining exotic physics like dark matter decay and annihilation.

  • First Light: Unfortunately, the 21 cm signal eventually fades as the gas density decreases at z~30. However, once the first luminous sources appear, they flood the Universe with photons and light it up in the 21 cm transition again. The patchwork of absorption and emission can teach us about the properties of the first sources of light.

  • The Heating Era: Eventually, galaxies and quasars produce enough X-ray photons to heat the IGM above the CMB temperature, transforming the absorption signal into emission. The timing and character of this transition can teach us about the first X-ray sources.

  • Reionization: Finally, once reionization begins in earnest, the character of the 21 cm fluctuations changes dramatically. The figure at the top of this page shows a sequence of simulated images of the 21 cm signal as ionized bubbles appear, grow, and merge until they fill the Universe completely. Observing the 21 cm signal during this phase will teach us a great deal about these generations of galaxies (see here).

    The first telescopes to observe this signal - which appears in the low-frequency radio band, at ~50-100 MHz - are now under construction. These instruments include:

  • The Mileura Widefield Array, in Australia, on which I am a collaborator
  • LOFAR, in the Netherlands
  • The Square Kilometer Array (location to be determined)

    With the MWA, we hope to characterize the 21 cm fluctuations from reionization statistically, and to image the largest ionized bubbles (probably surrounding quasars). The problem is difficult because of terrestrial interference (generated by TV stations and FM radios, among other things), distortions generated by the ionosphere, and other astronomical foregrounds - which are 10,000 times brighter than the 21 cm emission from high redshifts! Thus data analysis techniques to pull out as much information as possible, and with as much reliability as possible, are another active area of research.