3. Thermal Equilibrium is reached at the cosmic microwave background (CMB) temperature, represented by a near perfect omnidirectional black-body spectrum at 2.726K.
Light undergoing gradual redshift over time essentially decreases in temperature towards an equilibrium at the CMB temperature. TRH predicts that the full spectrum of light slowly approaches a more perfect black-body temperature curve as the light reaches and stays at the CMB temperature.
Thermal equilibrium requires both source/high energy (light) and sink/low energy (unknown), and this is an active area of research with respect to TRH. Some possible energy sinks include dark matter, cold neutrinos and light itself, which should all be investigated. Another possibility could be an aether, if the properties of this aether are consistent with observations confirming Einstein’s Relativity. TRH predicts that a relativistic aether is the ‘sink’ and light is the ‘source’ for the dynamic thermal equilibrium that is responsible for both the redshift of light, and the presence of the CMB. See Relativistic Ether for more info.