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

Understand

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.