On cosmic acceleration without dark energy

Abstract

We elaborate on the proposal that the observed acceleration of the Universe\nis the result of the backreaction of cosmological perturbations, rather than\nthe effect of a negative-pressure dark-energy fluid or a modification of\ngeneral relativity. Through the effective Friedmann equations describing an\ninhomogeneous Universe after smoothing, we demonstrate that acceleration in our\nlocal Hubble patch is possible even if fluid elements do not individually\nundergo accelerated expansion. This invalidates the no-go theorem that there\ncan be no acceleration in our local Hubble patch if the Universe only contains\nirrotational dust. We then study perturbatively the time behavior of\ngeneral-relativistic cosmological perturbations, applying, where possible, the\nrenormalization group to regularize the dynamics. We show that an instability\noccurs in the perturbative expansion involving sub-Hubble modes. Whether this\nis an indication that acceleration in our Hubble patch originates from the\nbackreaction of cosmological perturbations on observable scales requires a\nfully non-perturbative approach.\n