Is cosmic distance-duality violated?
Abstract
In general relativity gravity is described by a metric with light travelling on null geodesics of that metric. Any such metric theory of gravity, when coupled with conservation of photon number in vacuum, leads to the reciprocity relation or distance-duality which states that distances measured using apparent luminosities essentially coincide with distances measured using apparent sizes. We show that the latest cosmological data sets are in disagreement with the reciprocity relation at 2-sigma, a result stable to change of data set and primarily due to sources at z > 0.5. When interpreted in terms of violation of photon number conservation the best-fit has a dimensionless scattering amplitude about 50 times larger than that associated with Compton scattering from the free-electrons in the intergalactic medium but corresponds to brightening of the SN-Ia. This may be the signal of new physics or simply evidence that current data sets suffer from additional systematic effects. It does however, disfavour axion-photon mixing and extinction of SN-Ia light as alternatives to cosmic acceleration. We discuss how future surveys, using gravitational wave standard candles, will be able to distinguish between violation of photon number and deviations of gravity from metric theory predictions.Keywords
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