Modern cosmology has an interesting approach to the question of where the energy lost by light goes as that light becomes redshifted over large cosmological distances. There seem to be several, not entirely coherent ways of approaching the question. Probably the most conventional approach is this, where Ethan Siegel seems to say, a “universal” expansion is causing the energy loss, but at the same time, the energy lost is driving the expansion of the “Universe” :

The photons have an energy, given by a wavelength, and as the Universe expands, that photon wavelength gets stretched. Sure, the photons are losing energy, but there is work being done on the Universe itself by everything with an outward, positive pressure inside of it!

It is all well and good, to claim that mathematically either view is correct, however, physically speaking, either claim is nonsense. One view posits a bounded, constrained, “Universe” against which the contents are doing work by pushing the boundary outward. There is no evidence for this peculiar viewpoint other than you can arrive at it by assuming, without question, certain things the standard model assumes – without question. It is a mathematicist argument with no basis in the physics of observed phenomena.

The other possibility from the mathematicist’s perspective, is that an otherwise imperceptible, but somehow substantive, spacetime is expanding and so stretching the wavelength of the photon. Again, this has no basis in observed phenomena; it is just an appeal to a purely theoretical construct, one which only serves to obscure whatever physics is actually taking place.

So, how can you account for the energy loss of a cosmologically redshifted photon without veering off into the metaphysical nerd-land presented by mathematicism? Simply put, you have to model light emitted from distant sources (galaxies on the cosmological scale) as consisting of expanding spherical wavefronts. Those wavefronts can be thought of as hyperspheres which are related to the mathematical-geometrical concept of a light cone.

The expanding spherical wavefront view of cosmological radiation rests only on the realistic assumption that galaxies radiate omnidirectionally. This is, in terms of modern cosmology, a simplifying assumption, one which eliminates the need for an expanding “Universe”. It is this expanding spherical wavefront phenomenon that has been misinterpreted in the standard model, to imply a “universal” expansion. The only things undergoing expansion in the Cosmos are the expanding spherical wavefronts of light emitted by galaxies.

The theoretical, light cone concept, closely parallels the physical structure of an expanding spherical wavefront considered 4-dimensionally. In the theory of light cones the surface of the light cone is the aggregate of all the hyperspheres which are themselves, sections of the light cone. All of the points on the light cone have no spatial or temporal separation. The 4-dimensional surface of the light cone constitutes a simultaneity – in 4-dimensions.

Deploying this mathematical model in the context of the observed expanding wavefronts suggests that when a 3-dimensional observer interacts with the light cone, at a specific 3-spatial + 1-temporal, dimensional location, the observer is observing the simultaneous state of the entire expanding wavefront, which, in the model, is the state of the hypersphere of the light cone that intersects with the observer’s 3+1-dimensional “now”.

A 3D observer cannot observe the entirety of the 4D light cone, only the portion of hypersphere (spherical wavefront) with which it directly interacts. However since all the points on the hypersphere are identical, information about the state of the hypersphere is available at the point of observation.

At large cosmological distances that wavefront will be observed to be redshifted, reflecting a net energy loss to the spherical wavefront. This energy loss is caused by the wavefront’s encounter with and partial absorption by, all the intervening galaxies (and other matter) encountered over the course of its expansion.

This energy loss can be crudely estimated using the standard General Relativity equation for gravitational redshifting. That, in turn, suggests the possibility that all observed gravitational effects are a consequence of the interaction between 3-dimensional matter and 4-dimensional electromagnetic radiation.

This post is based on a recent Quora answer.