21-cm radiation: a new probe of variation in the fine structure constant

Authors | Rishi Khatri, Benjamin Wandelt

Summary of Paper
Arxiv Preprint
Published paper in Physical Review Letters

Links

Background information on the fine-structure constant
Article in New Scientist
Article at Physicsweb.org

News

• March 24, 2007
  Physicsweb.org has published an article about this paper.
• March 14, 2007
  Published in the 16 March 2007 issue of Physical Review Letters (Vol.98, No.11)
• March 8, 2007
  This paper will be the subject of a New Scientist article next week!

Summary

One of the fundamental problems in physics is to explain why the fundamental "constants" like the fine structure constant α speed of light (c), Gravitational constant (G), electron to proton mass ratio μ etc have the values they have and whether or not they are constant in time and space. We propose that the 21-cm signal from the dark ages can be a sensitive probe of variations in the fine structure constant. During the time after recombination and before the first stars light up the universe, 1000 > z > 30 , the universe consists mostly of neutral hydrogen and helium. There is however CMB present which is absorbed by the neutral hydrogen due to its 21-cm spin flip transition during 500 > z > 30. This 21-cm absorption signal is sensitive to the variations in the fine structure constant α because the Einstein coefficients for the absorption of the background radiation (A ∝ α¹³) and the energy difference between the two states of hydrogen atom (v₂₁ ∝ α⁴) are very sensitive to α. In addition the H-H collision cross sections for the spin flip (which affects the spin temperature ) and the ionization fraction of the gas also depend on α. One observable is the global or mean 21-cm signal averaged over the whole sky. Although the upcoming telescopes have the sensitivity to observe this signal, the foreground removal may prove to be very challenging. There are also fluctuations in the 21-cm signal due to the fluctuations in the baryon density, temperature and spin temperature (the fluctuations in ionization fraction will also contribute but will be negligible compared to above). Thus we can measure the spatial angular power spectrum of 21-cm radiation and that too will depend on the value of α during the dark ages (Fig 1 and 2). The foreground removal though still challenging may be more feasible in this case utilizing on the differences in the statistical properties of the foregrounds and the 21-cm signal. The cosmic 21-cm signal probes different redshifts than BBN or CMB, which are also sensitive to variations in α, and is thus complimentary to these probes.

Figure 1

Angular power spectrum compared at actual redshift (z). Upper panel shows the angular power spectrum
√ l ( l + 1 ) C_l / 2π at several redshifts. Bottom panel shows ( √ C_l ( δ α = – 2%) – √ C_l ) / √ C_l for the same redshifts.

Figure 2

Angular power spectrum compared at observed redshift, z_obs ≡ – 1. Upper panel shows the angular power spectrum √ l ( l + 1 ) C_l / 2π at several redshifts. Bottom panel shows ( √ C_l ( δ α = – 2%) – √ C_l ) / √ C_l for the same redshifts.


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Rishi Khatri
2007.03.04

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