Measuring the speed of cosmological gravitational waves

Abstract

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.

5 pages, 3 figure. Minor edits to match the version submitted to the journal

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• Higher-Curvature Corrections and Tensor Modes
• The effective theory of fluids at NLO and implications for dark energy
• Oscillating modulation to B-mode polarization from varying propagating speed of primordial gravitational waves
• Probing the speed of gravity with LVK, LISA, and joint observations
• A cosmological exclusion plot: Towards model-independent constraints on modified gravity from current and future growth rate data
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• Quantization of the nonstandard propagating gravitational waves in the cosmological background
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