A method for separating the physics from the astrophysics of high-redshift 21 centimeter fluctuations

Fluctuations in the 21 cm brightness from cosmic hydrogen at redshifts z greater than or similar to 6 have their source in the primordial density perturbations from inflation, as well as the radiation from galaxies. We propose a method to separate these components based on the angular dependence of the 21 cm fluctuation power spectrum. Peculiar velocities increase the power spectrum by a factor of similar to 2 compared with density fluctuations alone and introduce an angular dependence in Fourier space. The resulting angular structure relative to the line of sight facilitates a simple separation of the power spectrum into several components, permitting an unambiguous determination of the primordial power spectrum of density fluctuations, and of the detailed properties of all astrophysical sources of 21 cm fluctuations. We also demonstrate that angular multipoles are not useful for measuring large-scale power, and thus, theoretical predictions should focus on the three-dimensional power spectrum of 21 cm fluctuations.