String theory predicts an axiverse of ultralight axions whose effects on CMB polarization, matter power spectrum, and black hole superradiance can be probed by future astrophysical experiments.
Measuring the Small-Scale Power Spectrum of Cosmic Density Fluctuations Through 21 cm Tomography Prior to the Epoch of Structure Formation
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abstract
The thermal evolution of the cosmic gas decoupled from that of the cosmic microwave background (CMB) at a redshift z~200. Afterwards and before the first stars had formed, the cosmic neutral hydrogen absorbed the CMB flux at its resonant 21cm spin-flip transition. We calculate the evolution of the spin temperature for this transition and the resulting anisotropies that are imprinted on the CMB sky due to linear density fluctuations during this epoch. These anisotropies at an observed wavelength of 10.56[(1+z)/50] meters, contain an amount of information that is orders of magnitude larger than any other cosmological probe. Their detection, although challenging, could tightly constrain any possible running of the spectral index from inflation (as suggested by WMAP), small deviations from Gaussianity, or any significant contribution from neutrinos or warm dark matter to the cosmic mass budget.
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String Axiverse
String theory predicts an axiverse of ultralight axions whose effects on CMB polarization, matter power spectrum, and black hole superradiance can be probed by future astrophysical experiments.