I found a paper at http://lunar.colorado.edu/publicfiles/W ... gy_CFP.pdf that discusses the potential insights to be gained and what is involved in obtaining these measurements. While most of the technical discussion is beyond me, I am intrigued by what I think I do understand.http://en.wikipedia.org/wiki/Hydrogen_line
The line is of great interest in big bang cosmology because it is the only known way to probe the "dark ages" from recombination to reionization. Including the redshift, this line will be observed at frequencies from 200 MHz to about 9 MHz on Earth. It potentially has two applications. First, by mapping redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the matter power spectrum in the period after recombination. Second, it can provide a picture of how the universe was reionized, as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 centimeter background.
However, 21 centimeter experiments are very difficult. Ground based experiments to observe the faint signal are plagued by interference from television transmitters and the ionosphere, so they must be very secluded and careful about eliminating interference if they are to succeed. Space based experiments, even on the far side of the moon (which should not receive interference from terrestrial radio signals), have been proposed to compensate for this. Little is known about other effects, such as synchrotron emission and free-free emission on the galaxy. Despite these problems, 21 centimeter observations, along with space-based gravity wave observations, are generally viewed as the next great frontier in observational cosmology, after the cosmic microwave background polarization.
This paper suggests that the kind of knowledge this research could provide includes
- more about the neutrino mass
improved data re: the curvature of the Universe
insight into the nature of dark matter
insight into the evolution of baryonic intergalactic matter
probe processes relating to dark matter decay or annihilation, primordial black holes, and cosmic strings
The paper proposes three methods to deal with the difficulty in observation and interpretation
- robust modeling
ignoring astrophysical effects in some eras
take advantage of redshift-space distortions of the 21 cm background and viewing the radiation in very specific directions
Does the second point mean that the data for some redshifts would not be affected by other astrophysical occurrences?
From point 3 I am guessing that the combination of looking at specific redshifted data only in specific directions will help isolate the background radiation from all other 21 cm radiation. Or?
The paper also contains a synopsis of the future observational milestonesIn fact, the 21 cm background is the ideal probe of reionization, which imprints strong fluctuations in it . . . . Its weak oscillator strength (in comparison to Ly) allows us to penetrate even extremely high redshifts. We can also image it across the entire sky – instead of only rare, isolated Ly forest lines of sight. Moreover, unlike the CMB, it is a spectral line measurement, and we can distinguish different redshift slices to study the full history of the “dark ages” – extremely difficult even with a “perfect” CMB measurement [33]. Finally, it directly samples the 95% (or more) of the baryons that reside in the IGM.
- 1. First-Generation Arrays (A eff ∼ 104 m2): The arrays now operating or under construction will detect strong deviations from standard model, should these exist.
2. Second-Generation Arrays(Aeff ∼ 105 m2): Improvements by factors ranging from 1.4 to 2.7 for many quantities provided that astrophysical “contamination” can be cleaned.
3. Imaging Arrays(Aeff ∼ 106 m2): Better likelihood of separating out cosmological information. Improvement in constraints on inflationary parameters, curvature, neutrino mass by 1 - 2 orders of magnitude
What is an Imaging Array? (An even larger array plus processors to create detailed images from radio data? Or?)
Does anyone know how far off the Second-Generation Arrays and subsequent Imaging arrays are?
Even without answers to my questions, this is fascinating.
