## Gravity with two metrics

Like all fundamental forces, gravity is carried (or mediated) by a field. While the electromagnetic force is mediated by a vector field, the photon, gravity is mediated by a tensor field, called the metric since it also provides the geometric features of the space-time. Just like the photon, the metric is also a massless field. Recent work, however, has shown that a gravity theory with two metrics is also possible, provided one of them (or a combination of both) is massive. While the geometric properties of space-time as seen by particles are still determined by one metric only, the gravitational properties depend on both. In particular, the massive metric can be responsible for the cosmic accelerated expansion. So far, however, it was not clear whether these models were stable in the past. In a recent **paper** we have shown, quite interestingly, that there is only one possible choice of parameters that make the theory stable at all times and, at the same time, provide for a valid accelerated regime in agreement with observations.

## A cosmological exclusion plot

Laboratory tests of gravity often produce an exclusion plot, i.e. a plot that shows the region of parameter space that is excluded by the experiment. Typically the parameters are chosen to be the range and strength of the Yukawa force, the simplest modification of gravity. In a recent **paper** (with L. Taddei) we have realized a cosmological exclusion plot which shows the region that a future cosmological galaxy survey can exclude. Analogously to laboratory experiment we constrain the range and strength of a Yukawa force, but now the range is measured in Megaparsecs rather than in meters like on Earth.

## Gravitational waves and early-time modified gravity

My latest paper is Friction in Gravitational Waves: a test for early-time modified gravity (with V. Pettorino), which follows a recent paper on the same topic, Effects of modified gravity on B-mode polarization (with G. Ballesteros and V. Pettorino). Here we show that by measuring the B mode polarization in the cosmic microwave background one can obtain information on how gravitational waves propagated in the early universe. This, in turn, allows one to see whether gravity was different from standard Einstein’s General Relativity.