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The redshifted 21-cm background is expected to be a powerful probe of theearly Universe, carrying both cosmological and astrophysical information from awide range of redshifts. In particular, the power spectrum of fluctuations inthe 21-cm brightness temperature is anisotropic due to the line-of-sightvelocity gradient, which in principle allows for a simple extraction of thisinformation in the limit of linear fluctuations. However, recent numericalstudies suggest that the 21-cm signal is actually rather complex, and itsanalysis likely depends on detailed model fitting. We present the firstrealistic simulation of the anisotropic 21-cm power spectrum over a wide periodof early cosmic history. We show that on observable scales, the anisotropy islarge and thus measurable at most redshifts, and its form tracks the evolutionof 21-cm fluctuations as they are produced early on by Lyman-a radiation fromstars, then switch to X-ray radiation from early heating sources, and finallyto ionizing radiation from stars. In particular, we predict a redshift windowduring cosmic heating (at z ~ 15), when the anisotropy is small, during whichthe shape of the 21-cm power spectrum on large scales is determined directly bythe average radial distribution of the flux from X-ray sources. This makespossible a model-independent reconstruction of the X-ray spectrum of theearliest sources of cosmic heating.

Reconstructing the nature of the first cosmic sources from the anisotropic 21-cm signal