We measure the speed of light with current observations, such as Type Ia Supernova, galaxy
ages, radial BAO mode, as well as simulations of forthcoming redshift surveys and gravitational
waves as standard sirens. By means of a Gaussian Process reconstruction, we find that the
precision of such measurements can be improved from roughly 6% and to about 2–2.5% when the
gravitational wave simulations are considered, and to 1.5–2% when redshift survey are
included in the analysis as well. This result demonstrates that we will be able to perform a
cosmological measurement of a fundamental physical constant with significantly improved precision,
which will help us underpinning if its value is truly consistent with local measurements, as
predicted by the standard model of Cosmology.