Princeton University Gravity Group Lunch Seminar

ADDED - Cosmic acceleration is widely believed to require either a source of negative pressure (i.e., dark energy), or a modification of gravity, which necessarily implies new degrees of freedom beyond those of Einstein gravity. In this talk I will present a third possibility, using only dark matter and ordinary matter. The mechanism relies on the coupling between dark matter and ordinary matter through an effective metric. Dark matter couples to an Einstein-frame metric, and experiences a matter-dominated, decelerating cosmology up to the present time. Ordinary matter couples to an effective metric that depends also on the DM density, in such a way that it experiences late-time acceleration. Assuming a simple parametrization of the effective metric, we show that our model can successfully match a set of basic cosmological observables. To get a growth history similar to the ΛCDM prediction, our model predicts a higher H0, closer to the value preferred by direct estimates. On the flip side, we tend to overpredict the growth of structure whenever H0 is comparable to the Planck preferred value. The model also tends to predict larger redshift-space distortions at low redshift than ΛCDM.