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Cosmic acceleration is widely believed to require either a source of negativepressure (i.e., dark energy), or a modification of gravity, which necessarilyimplies new degrees of freedom beyond those of Einstein gravity. In this paperwe present a third possibility, using only dark matter and ordinary matter. Themechanism relies on the coupling between dark matter and ordinary matterthrough an effective metric. Dark matter couples to an Einstein-frame metric,and experiences a matter-dominated, decelerating cosmology up to the presenttime. Ordinary matter couples to an effective metric that depends also on theDM density, in such a way that it experiences late-time acceleration. Lineardensity perturbations are stable and propagate with arbitrarily small soundspeed, at least in the case of `pressure' coupling. Assuming a simpleparametrization of the effective metric, we show that our model cansuccessfully match a set of basic cosmological observables, includingluminosity distance, BAO measurements, angular-diameter distance to lastscattering {\it etc.} For the growth history of density perturbations, we findan intriguing connection between the growth factor and the Hubble constant. Toget a growth history similar to the $\Lambda$CDM prediction, our model predictsa higher $H_0$, closer to the value preferred by direct estimates. On the flipside, we tend to overpredict the growth of structures whenever $H_0$ iscomparable to the Planck preferred value. The model also tends to predictlarger redshift-space distortions at low redshift than $\Lambda$CDM.

A Universe Without Dark Energy: Cosmic Acceleration from Dark Matter-Baryon Interactions