Such an explanation for observed redshift apparently comes up again and again, but thus far hasn't held up to experiment... it even had a name: "tired light". https://en.wikipedia.org/wiki/Tired_light

It'll be interesting to see if this sparks new debate on the subject.

'Tired light' appears to be an umbrella term for various disparate theories that are only related in their attempt to provide an alternative explanation for red shift. If this particular hypothesis, which is not explicitly cosmological, is validated in terrestrial experiments, I think its cosmological implications would be unavoidable.

> is validated in terrestrial experiments

They didn't run any experiments. They only have some a theoretical model and simulations in the computer.

The main part of the article, that says that the momentum of the light is split in the photon and in the density wave looks good. (It almost looks obvious, but it's not my expertise area https://xkcd.com/793/ .)

The relation with the Hubble constant is totally unexpected. It is not in the abstract of the peer review article. (I can't read the full text.) So it's probably only a declaration for the press release. Take it with a grain of salt.

The article like nearly every physical article has its preprint available on the arxiv.

https://arxiv.org/abs/1603.07224

note: "on the arxiv" and not "on arxiv". The name puns on x => greek letter chi.

If it's proven true, though, would that mean the "expanding universe" theory would be disproved as well?

Not necessarily. It might be that this effect is real (this was a computational simulation so an experimental verification is needed.) and needs to be accounted when doing cosmology. But it might very well account just a part of the redshift, and the remainder then needs to be accounted by something else, the default explanation being the expanding universe.

Edit: A clarification: I'm not an expert.

Without an expanding universe, one problem is "How does everything form, given that gravity exists?"

No expansion = everything falls together.

not if the universe is flat and simply connected (i.e, infinite): https://physics.stackexchange.com/questions/109063/how-come-...

The reckless child in me wants to jump for joy at the absurdity of having a long-held tenent of my understanding of the universe shattered. The quiet pragmatist is begging for the reserved "Well let's wait and see". I didn't expect an article on recent developments in computational physics to render such an emotional reaction. Either way, a very thought provoking article!

No, there are too many observations that are consistent with an expanding universe. For example, primordial nucleosynthesis [1] would either not have occurred in a non-expanding universe, or would not have stopped so soon. In fact, it can be used to show that it is consistent with having only 3 families of light neutrino species.

[1] https://en.wikipedia.org/wiki/Big_Bang_nucleosynthesis

Some relatively small effects of so-called "tired-light" scenario could likely be accommodated, but it would not change the main picture.

But if we subtract this (presumably minor) effect from the observed redshift of distant galaxies, we're necessarily left with lower expansion speeds. In turn this means there's less need for gravity to hold them together in the various structures we observe from the galactic level up. This has the implication (I presume) of affecting inferred requirements of dark matter and dark energy.

However, what observations (besides assumptions about the source of background microwave radiation) are consistent with the pejoratively-named BigBang theory? We don't even know how big the universe is. It mat be far larger than the observable universe. And all the pocket universes are technically part of the same multiverse -- what makes us think there was for sure a big bang?

All that the "Big Bang" refer to is 1) the fact that the (visible) universe was once much hotter and denser than it is now and 2) if we extrapolate back in time, using the theories that we have, there was a point where the density was infinite.

Of course, we know that our theoretical model break down before that (since we don't have an experimetally verified theory of gravity compatible with quantum mechanics). Physicist that work in the field (and I used to be one of them) are well aware that we cannot extrapolate past the regions of validity of our theories and no one claim that there was indeed a singular point ("Big Bang") as mentioned in descriptions aimed at a non-expert audience.

I am pretty sure it's not true that "no one" claims there was not a singularity at the beginning.

http://www.hawking.org.uk/the-beginning-of-time.html

I even saw that recently Stephen Hawking started theorizing that the "Big Bang" was like a black hole. And other physicists weigh in:

http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/...

Look at this wikipedia article. Someone must be writing it?

https://en.m.wikipedia.org/wiki/Initial_singularity

In fact there are some contradictions between Quantum Mechanics and an actual big bang:

https://m.phys.org/news/2015-02-big-quantum-equation-univers...

But only if interstellar mass is evenly distributed... which it isnt. If this is to replace hubble we should see differences in redshift where the light passes through more matter. There should be differences between redshift distance measurements and other non-redshift distances. That shouldnt be too hard to detect (or not).

It could be extremely hard. We don't have many good ways to measure how far away objects are in space. In fact the main way we have been estimating distance so far is by measuring red shift. We also have very limited means for measuring the density of the intergalactic medium.

I just don't think we have any good ways to distinguish whether a high redshift is due to the object being very distant or the intergalactic medium in that direction being particularly dense, for most objects.

I guess the geometry of galactic filaments may be examined for distortions/textures relating to non-standard red shifting. We may see the filaments are bent from our viewpoint in accordance with the presence of voids or other filaments along the way.

There are some very good ways. Thats how redshifting was proven in the first place. Cephiads (sp), pulsing stars, were used to prove that galaxies were a thing. They give very accurate distances at ranges where redshift is detectable.

There are type Ia supernovas that can give precise distances, no?

They give very precise distances by red-shift calculations.

There's no direct way to measure the distance of very far objects. There's only red-shift and Hubble's Law.

Wasn't it the other way around? A type of supernovas ("standard candles") were discovered before the expansion and used to discover the expansion? The formula for distance only uses absolute and apparent magnitudes to calculate distance?

Yes, my comment above is wrong. It was the other way around.

Yeah they calibrated red shift from standard candles.

They use luminosity for SNe Ia.

Isn't the CMBR remarkably uniform in all directions? Does that matter?

I believe that emission and absorption lines are also shifted. Wavelength-dependent absorption by intervening gas wouldn't do that.

Indeed, I recall that redshift measurement is performed by comparing the shift on emission spectra (which is absolute), not some bizarre ”degree of red dimming” which would be hard to quantify.

I don't see how it is pertinent, as space is not "a transparent medium", it is void. Pretty much all the photons emitted by a star that we can detect cross only the star's gas surrounding, then the emptiness of space, then our atmosphere. So, it does not vary with the distance of the star, thus the author's argument does not hold (anyway, he was being super-speculative here).

The space is not void as there is non-zero density of matter that we see in absorbtion lines of quasars [1]. This implies that the space has to be treated as transparent medium with non-trivial optical properties.

[1] - https://ned.ipac.caltech.edu/level5/Madau6/Madau_contents.ht...

Ok, now I agree with you: https://ned.ipac.caltech.edu/level5/Madau6/Madau2.html

> it is void

Space is not a complete vacuum.

Well, actually it seems you are true. There are absorption rays for the inter-galactic medium, as shown in: https://ned.ipac.caltech.edu/level5/Madau6/Madau2.html So, please ignore my previous reply ↓

Yeah, but sufficiently so that most photons are not encountering any other things on their way.

Not clear - have you taken virtual particles into account?

Yeah this seems to be happening more and more often. I guess this is it.