Measured redshift invariance of photon velocity

Abstract

We report the first direct photon velocity measurements for extragalactic objects. A fiber-optic, photon time-of-flight instrument, optimized for relatively dim sources ($m 12$), is used to measure the velocity of visible photons emanating from galaxies and quasars. Lightspeed is found to be $3.00\pm0.03\times10^{8} \mathrm{m s}^{-1}$, and is invariant, within experimental error, over the range of redshifts measured ($0\leq z\leq1.33$). This measurement provides additional validation of Einstein's theory of General Relativity (GR) and is consistent with the Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW) metricl, as well as several alternative cosmological models, notably the hyperbolic anti-de Sitter metric, though not with the pseudo-Euclidean de Sitter metric.