Test of the Einstein equivalence principle with spectral distortions in the cosmic microwave background

TL;DR

This paper tests the Einstein Equivalence Principle by analyzing spectral distortions in the cosmic microwave background caused by energy-dependent redshift effects, setting stringent bounds on possible violations.

Contribution

It introduces an energy-dependent Friedmann-Robertson-Walker metric inspired by rainbow gravity to evaluate potential EEP violations through CMB spectral distortions.

Findings

Deviation from EEP is less than 10^{-9} at CMB energy scale.

Spectral distortions can serve as sensitive tests for EEP violations.

Energy-dependent redshift effects induce measurable CMB spectral distortions.

Abstract

The Einstein Equivalence Principle~(EEP) can be verified by the measurement of the spectral distortions of the Cosmic Microwave Background (CMB). The existence of energy-dependency in the cosmological redshift effect means the EEP violation. Introducing the energy-dependent Friedmann-Robertson-Walker metric motivated by rainbow gravity, we show that the energy-dependent redshift effect causes the CMB spectral distortions. Assuming the simple energy-dependent form of the metric, we evaluate the distortions. From the COBE/FIRAS bound, we find that the deviation degree from the EEP, which is comparable to the difference of the parameterized-post-Newtonian parameter "gamma" in energy, is less than 10^{-9} at the CMB energy scale.