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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.
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