[astro-ph/0505173] Measuring the Primordial Deuterium Abundance During the Cosmic Dark Ages

Authors:  Kris Sigurdson, Steven R. Furlanetto (Caltech)
Abstract:  We discuss how measurements of fluctuations in the absorption of cosmic microwave background (CMB) photons by neutral gas during the cosmic dark ages, at redshifts z ~ 7--200, could reveal the primordial deuterium abundance of the Universe. The strength of the cross-correlation of brightness-temperature fluctuations due to resonant absorption of CMB photons in the 21-cm line of neutral hydrogen with those due to resonant absorption of CMB photons in the 92-cm line of neutral deuterium is proportional to the fossil deuterium to hydrogen ratio [D/H] fixed during big bang nucleosynthesis (BBN). Although technically challenging, this measurement could provide the cleanest possible determination of [D/H], free from contamination by structure formation processes at lower redshifts, and has the potential to improve BBN constraints to the baryon density of the Universe \Omega_{b} h^2. We also present our results for the thermal spin-change cross-section for deuterium-hydrogen scattering, which may be useful in a more general context than we describe here.
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Anze Slosar
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[astro-ph/0505173] Measuring the Primordial Deuterium Abunda

Post by Anze Slosar » May 12 2005

Very interesting paper.

Well, there is contamination from large scale modes, which authors claim to be negligible contribution (which sounds plausible). In any case, you can deal with this one way or another, but what really worries me is a population of low brightness radio sources at moderate redshifts, which you see in both frequencies (mind you it's only a factor of 4 in frequency, which is not that much) and can really mess up your correlation. Do people have any idea about these contaminants? Surely some work must have been done on this for the standard 21cm stuff?

Christopher M. Hirata
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[astro-ph/0505173] Measuring the Primordial Deuterium Abunda

Post by Christopher M. Hirata » May 13 2005

If the issue is synchrotron/free-free foregrounds, then while it is true that the foreground maps should be highly correlated at 21 vs. 92 cm, I would also expect that the usual trick of looking at modes with high wavenumbers in the radial direction (i.e. that oscillate rapidly in frequency space) should get rid of the correlation, at least in principle. Given the large number of radio recombination lines, there are undoubtedly some pairs of lines near the 92/21 frequency ratio (has anyone calculated how much of a correlation one should expect?). But there are lots of such lines so if this is a problem I would expect to see weird correlations at lots of frequency ratios, not just 92/21 -- so at the very least we will know if we are being fooled :)

Aside from radio recombination line issues, the proposal here is certainly a very difficult experiment (we have yet to measure the signal from H, and the D signal is much smaller and sits under a much brighter foreground). But I suppose it can pay to think big.

Kris Sigurdson
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[astro-ph/0505173] Measuring the Primordial Deuterium Abunda

Post by Kris Sigurdson » May 13 2005

Contamination by foreground radio sources (not only extragalactic point sources but also the Galactic synchrotron background) is indeed a major concern for all of these low-frequency experiments. For redshifted 21 cm observations, as Chris mentioned, removing it relies on noting that all the known foregrounds have smooth spectra (either synchrotron or free-free), while the signal varies rapidly in frequency because it is a line transition. Thus one essentially fits a smooth curve to the spectrum of each pixel and attributes the residuals to the 21 cm signal. While the actual precision of this removal is uncertain (because we know little about the underlying source populations), several theoretical estimates show that it can be quite effective (see, e.g., Zaldarriaga et al. 2004 astro-ph/0311514, Wang et al. 2005 astro-ph/0501081).

In the H-D case, the task is actually easier, because one is interested only in a specific frequency separation, nu_H-nu_D. Presumably the correlation from the foregrounds will be smooth across the interval nu_H-(0.9*nu_D) and nu_H-(1.1*nu_D), while the H-D correlation only shows up at nu_H-nu_D. So one can look for a boost in the correlation at the specific frequency of interest. Naively, we require the fit to the spectrum in each sky pixel to have ~1% error (which would, when averaged over the whole sky, reduce the total error to <[D/H]). Again, a precise estimate requires knowledge of the source population, the observing strategy, and the instrumental characteristics, but this seems very reasonable for the kinds of future telescopes necessary to detect the signal. On the other hand, if anyone ever thinks seriously about observing this correlation, these issues will no doubt require a lot more careful thought!

Kris and Steve

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