[Coding_Cat]

I doubt that is the case. As the article nores photon-photon scattering has been a predicted for a long time by QED. Even if it wasn't detected before it's impact on redshift could be calculated.

If you look at the abstract on the linked nature page the interaction has a cross-section of 70nb, which corresponds to a circle of radius 1.5e-9 nanometer[1]. It might occur a few times near a supernova or the swirl of a an accretion disk but I doubt it has a meaningful effect in deep-space.

A more exciting result would have been if they wouldn't have seen this effect. That would mean the theory was wrong and we had a new datapoint to look at.

[1]: these measurements are only for a specific energy range, they might vary dramatically with different energy levels but the key point is that this result agrees with theoretical predictions meaning that is should also be possible to calculate the contribution to redshift due to photon-photon scattering.

[madaxe_again]

but I doubt it has a meaningful effect in deep-space.

Don't underestimate how often unlikely events can happen in a big enough space...

As to photon-photon scattering, I might be wrong, but I don't believe it's considered in any models for expansion - similarly, if you do build light momentum transfer into your model, then expansion goes away - but because we "know" expansion to be true, those results aren't considered or published.

I think our givens are wrong. It'd hardly be the first time. I think I might be wrong. That'd hardly be the first time either.

But, as a betting physicist, my money is on expansion (at the very least acceleration) being bunkum.

[Coding_Cat]

>Don't underestimate how often unlikely events can happen in a big enough space...

That's true but the density of photons (from a given source) also drops of cubically as you move away from the source.

> imilarly, if you do build light momentum transfer into your model, then expansion goes away

I'm not really a cosmologist, but I know some physics (QFT in particular). If you have a derivation for a formula giving the impact of YY-scattering on redshift I'd love to read it.

> but because we "know" expansion to be true, those results aren't considered or published.

We know redshift to be true, and we have evidence for expansion based on measurements. Something that simply denies this will have a hard time being published. A theory that explains those findings without using expansion however would certainly make waves.

As and aside, since the cross-section of photon-photon scattering is energy dependent (and, as far as I can tell cosmological redshift is not), wouldn't that be a way to distinguish them? Scattering should occur more often at higher energies meaning that after a significant amount of scattering a bundle's spectrum should clump up more into the red.