[m1el]

> I'm keeping my money on expansion and dark matter being hooey brought about by incomplete of incorrect understanding of light.

That's a very interesting hypothesis. Unfortunately, it's easy to verify that photon-photon scattering doesn't explain the expansion of the universe.

1) photon-photon scattering is elastic, so no energy is lost, and no redshift is occuring

2) if it isn't elastic, the scattering is a random process. So different photons will lose a different amount of energy, which means that measured spectra are going to be blurred.

3) rather than seeing redshift, due to photon-photon scattering, you'd see fog, which gets cloudier and cloudier with distance.

[madaxe_again]

Those are all solid points, so I retract - but the momentum paradox bit stands!

I wish I had someone else to chat physics with - isolation leads to screwy notions which can easily be quashed by the right counterpoints.

Edit: a thought. Please (genuinely!) tell me where I'm wrong. As it's elastic, could we not end up with groups of lower energy photons with the same vector as an original high energy photon, which would similarly explain redshift? Doesn't address your point re: fog, however, unless they're universally tightly grouped.

[triclops200]

It looks like from what we know, 2 high energy (it's much less common with low energy) photons collide and change direction: from what I understand, they don't split into low energy photons. It'd be basically impossible for all the high energy photons to scatter directly away from earth (why would the earth be special?) leaving only low energy ones towards us.