The correct way to handle the rest of the universe is to quantify forces.

For every action there is an equal and opposite reaction. This means, for example, that if a photon pushes an electron sideways, which indeed it does, then the electron equally pushes the photon sideways, and by the same amount. This is why momentum is conserved.

It is also why photons are always scattered at an angle. You can even calculate the angle given how hard the electron was pushed.

If there is any other particle in the universe getting some momentum or energy, then it does so by being pushed. You should quantify the forces, because the forces apply locally as well.

If either the photon or the electron pushes on anything else, then there is a corresponding energy and momentum transfer. But a real energy momentum analysis quantifies this and includes it in the balance. If this is not done, then you cannot claim to have an energy momentum analysis.

Another simple principle is that light particles get the lion's share of energy, and heavy ones get the lion's share of momentum. You can quantify this as well by using conservation rules.

Lyndon's paper gives the electron 0.00024% of the photon's energy. It is going to be tough to give the remaining energy to the rest of the universe! In real physics, the local particles take up nearly all the energy, and it is momentum picked up by other more massive objects. This applies, for example, in the Mossbauer effect. Momentum is taken up by many atoms in a rigid crystal lattice, while a locally excited atom, and subsequently a re-emitted photon, takes up almost exactly the energy of the original photon.

Cheers -- Sylas

PS. This is not an "attack" in some aggresive personal sense. It is just review of a theory. The response needs to address the actual criticism or defect being raised. In this case, the proper response is to go ahead and quantify the energy momentum involvement of all particles relevant to the alledged process. It is an simple thing to do, which can be done for any other particle interaction in physics, including those that involve "the rest of the universe" like the Mossbauer effect.

So don't take offence; just go ahead and give a quantified energy momentum balance, including both energy and momentum terms that add up and balance both before and after the interaction.