Measuring the speed of cosmological gravitational waves

Understand

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.