Measuring the speed of cosmological gravitational waves
  In general relativity gravitational waves propagate at the speed of light,
however in alternative theories of gravity that might not be the case. We study
the effects of a modified speed of gravity, $c_T^2$, on the B-modes of the
Cosmic Microwave Background (CMB) anisotropy in polarisation. We find that a
departure from the light speed value would leave a characteristic imprint on
the BB spectrum part induced by tensors, manifesting as a shift in the angular
scale of its peaks. We derive constraints by using the available {\it Planck}
and BICEP2 datasets showing how $c_T^2$ can be measured, albeit obtaining weak
constraints due to the overall poor accuracy of the current BB power spectrum
measurements. The present constraint corresponds to $c_T^2 = 1.30 \pm 0.79$ and
$c_T^2< 2.85$ at $95%$ C.L. by assuming a power law primordial tensor power
spectrum and $c_T^2<2.33$ at $95%$ C.L. if the running of the spectral index is
allowed. We derive forecasts for the next generation CMB satellites, which we
find capable of tightly constraining $c_T^2$ at percent level, comparable with
bounds from binary pulsar measurements, largely due to the absence of
degeneracy with other cosmological parameters.
