Hydrogen (which constitutes 92% of all atoms in the Universe) emits at a fairly long wavelength of 21 centimeters, as least if it is atomic.  During and before hydrogen reionization, much of the hydrogen in the Universe was atomic and, thus, emitting copiously at this wavelength.  (Okay, copious may be an overstatement as the timescale for an excited atom to emit via this transition is an excruciating 10 million years!)  Efforts across the globe are gearing up to detect 21 cm emission from the first billion years in the Universe.  This observable has the potential to reveal unprecedented information about reionization, and it is really the only signal we can detect from when the Universe was between 5 and 300 million years old (during the Universe’s ``adolescence’’).   I have been involved in quantifying the sensitivity of potential instruments to this signal, in exploring design optimizations for these observatories, and in understanding the physics that 21cm observations can constrain.  I have been or currently am involved on the theoretical side with several of these efforts: the Murchison Widefield Array (MWA) in western Australia, the Precision Array for Probing the Epoch of Reionization (PAPER) in South Africa, as well as a newly commissioned instrument, the Large-aperture Experiment to Detect the Dark Ages (LEDA), in New Mexico.


It has recently been realized by Tseliakhovich & Hirata (2010) that the baryons were moving supersonically with respect to the dark matter after recombination and prior to when the Universe was reheated by astrophysical sources.  The size of this velocity difference varies spatially in the Universe, with it being supersonic in most of the volume and reaching Mach numbers as high as several in select locations at 20<~z<200.  If the fluctuations in the 21cm background coupled to this velocity difference, even in a very weak manner, the result could be much larger and more distinctive spatial correlations than in previous models, which had ignored streaming.  Interestingly, we found that this coupling could generate an order unity enhancement to the 21cm signal (although at the most observable redshifts and length scales probably a smaller enhancement than this). 

The above figure shows how a typical difference in this relative velocity impacts the gas in a 3x106 Msun halo -- a halo that is likely to form stars --, from a cosmological simulation run with the Enzo code.  The panel on the left does not have any relative velocity whereas in the panel on the right the baryons are moving with Mach number of 1.8 relative to the dark matter (the RMS Mach number in our universe), but otherwise the same initial conditions.  The gas in this halo is significantly impacted by the relative velocity.  For movies illustrating the impact of the relative velocity click here. These simulations use cosmological initial conditions generated with the code available here.