radio signals


Redshifted 21cm radiation:  

A global effort is brewing to detect 21cm radiation both from high redshift galaxies (z~1) and from the early universe (z~6-40).  Indeed, three instruments targeting the 21cm signal from reionization (z~6-12) have come online in the past few years (PAPER, MWA, LOFAR).  There is a lot of ongoing work to understand (1) what are the best instrument designs to observe this signal and (2) the requirements on the analysis pipelines to distill information from what will be gargantuan data sets.


In a study led by Aaron Parsons, we investigated array configurations that are the most sensitive to the cosmological 21cm signal (which turn out to be the ones with maximum baseline redundancy).  The PAPER 21cm array has used these configurations to set the most stringent bound on this limit.


The sky-averaged 21cm signal is the most promising observable in the near term of the cosmic dark ages (15<z<200).  It has the potential to teach us about the formation history of the first stars and black holes in the Universe.  With Gianni Bernardi and Lincoln Greenhill, we are currently finishing a paper that investigates how real world systematics inhibit the detection of the global (sky-averaged) HI 21cm signal.  This includes more realistic models for the foregrounds than the running power-laws most studies have assumed and also more realistic models for the instrumental response.


One of my first science papers (and my most highly cited) in detail calculated the sensitivity of different planned and hypothetical 21cm instruments to the 21cm power spectrum.

Dispersion Measure:  An electromagnetic wave (typically with frequencies in the radio) is delayed owing to the index of refraction of the intervening plasma.  The measurement of this delay (often called the dispersion measure) towards pulsars has been used to map the Galactic electron distribution.  However, until recently there were no indications that there was a class of extragalactic sources to which dispersion could be ascertained.  Recently there are hints of a new class of sources.  I wrote a paper on how cosmological dispersion could be used to learn about the distribution of cosmic gas at low redshifts.

Even more recently, in a study led by Chris Hirata, we showed that dispersion measure -- and hence electron column -- cannot be measured to time-steady sources, despite 3 papers in 2013 proposing schemes that they claimed could do do this.  Too bad!!!

Other extragalactic radio lines:  With Eric Switzer, we studied a new extragalactic observable that has the potential to teach us about Big Bang Nucleosynthesis and helium reionization, the 8.7GHz hyperfine absorption line of 3He+ (which is much more observable than one would naively guess, but still quite hard, likely requiring the SKA).  Aside from HI 21cm, this is the only other radio line from a primordial element that can in the foreseeable future be detected from the IGM.

the CMB: Most of my work on the CMB has involved the detectability of different signatures of cosmological reionization in the statistics of this radiation.  In particular, either the anisotropies created by reionization via the kinetic Sunyaev-Zel’dovich effect or the non-Gaussianities imprinted by sightline-to-sightline fluctuations in the Thomson optical depth created by patchy reionization. 

I have also thought about the WMAP haze and how the dark matter hypothesis for its origin can be distinguished from other, more mundane hypotheses.

I hope to think more about the thermal SZ and its potential to detect the WHIM and CGM.


I am interested in different cosmological radio signals, including...