Page "Physical cosmology" Paragraph 9

There is no definite or clear way to define the total energy of the universe due to the most widely accepted theory of gravity, general relativity.

As a result it remains controversial whether one can meaningfully say that total energy is conserved in an expanding universe.

For instance, each photon that travels through intergalactic space loses energy due to the redshift effect.

This energy is not obviously transferred to any other system, so seems to be permanently lost.

On the other hand, some cosmologists insist that energy is conserved in some sense ; this follows the law of conservation of energy.