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21 Centimeter Radiation
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21 centimeter radiation is radiation produced during a Hyperfine Transition of neutral hydrogen from the Triplet to the Singlet state. The radiation naturally has 21 cm wavelength, or a frequency of 1.4 GHz . The line has a very long lifetime of about ten million years in a vacuum, although it can be considerably shortened due to collisions with other hydrogen atoms and interaction with the Cosmic Microwave Background . It 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.


REFERENCES

  • P. Madau, A. Meiksin and M. J. Rees, "21-cm Tomography of the Intergalactic Medium at High Redshift", ''Astrophysical Journal'' 475, 429 (1997) .

  • M. Zaldarriaga, S. Furlanetto and L. Hernquist, "21 Centimeter Fluctuations from Cosmic Gas at High Redshifts", ''Astrophysical Journal'' 608, (2004) 608 .

  • X. Chen and J. Miralda-Escudé, "Observing the Reionization Epoch Through 21 Centimeter Radiation", ''Mon. Not. Roy. Astron. Soc.'' 347, 187 (2004) .

  • A. Loeb and M. Zaldarriaga, "Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation", ''Phys. Rev. Lett.'' 92, 211301 (2004) .

  • M. G. Santos, A. Cooray and L. Knox, "Multifrequency analysis of 21 cm fluctuations from the Era of Reionization", ''Astrophysical Journal'' 625, 575 (2005) .

  • R. Barkana and A. Loeb, "Detecting the Earliest Galaxies Through Two New Sources of 21cm Fluctuations", ''Astrophysical Journal'' 626, 1 (2005) .



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