Abstract
We present a case that current observations may already indicate new gravitational physicson cosmological scales. The excess of power seen in the Lyman-α forest and small-scale CMB experiments, the anomalously largebulk flows seen both in peculiar velocity surveys and in kinetic SZ, and the higher ISW cross-correlation all indicate thatstructure may be more evolved than expected from ΛCDM. While these anomalies may constitute a statistical fluke or may eventually disappear as systematic effects are better understood, we argue that they can be explained in models with infinite-volume (or, at least, cosmological-size) extra dimensions, where the graviton is a resonance with a tiny width.The longitudinal mode of the graviton mediates an extra scalar force which speeds up structure formation at late times,thereby accounting for the above anomalies. The required graviton Compton wavelength is relatively small compared to the present Hubble radius,of order 300-600 Mpc. Moreover, with certain assumptions about the behavior of the longitudinal mode on super-Hubble scales, our modified gravity framework canalso alleviate the tension with the low quadrupole and the peculiar vanishing of the CMB correlation function on large angular scales, seen both in COBE and WMAP. This relies on a novel mechanism that cancels a late-time ISW contribution against the primordial Sachs-Wolfe amplitude.
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