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From a very amateur astronomer seeking enlightenment: I often read that "red shift" proves the universe is expanding and that the farther away the source, the more red-shifted and therefore the faster it's moving away from us. The analogy with the Doppler effect is striking. But the logical consequences seem heavy and absurd, so another solution would be attractive.

Enter the "dark matter" theory. We seem to know next to nothing about DM, other than it's ubiquitous and massive in total while the individual particles are too tiny and/or aloof to be visible.
Considering how little we know about DM, how do we know it does not occasionally absorb a little bit of blue from a passing photon?

The word "absorb" may be misleading here; perhaps the process is more like reflection. In that case, would it not be logical for red shifting to be more pronounced the further away the source was, but with no implication that anything is expanding?
Can somebody explain why this theory is impossible? And if it's even remotely possible, wouldn't Occam's Razor prefer it to the popular idea of the expanding universe?

HolgerFiedler
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John Malcolm
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how do we know it does not occasionally absorb a little bit of blue from a passing photon?

Redshift means that any spectral feature, such as absorption and emission lines, will be shifted towards the red end of the spectrum. An example is the image below (taken from Wikpedia), showing absorption lines in the visible spectrum of a supercluster of galaxies about 1 billion light years away from us. This is how astronomers accurately measure redshift of distant galaxies. Absorption of part of the spectrum by dark matter (or any other form of matter) cannot produce such a shift of spectral lines.

Absorption lines in the visible spectrum of a supercluster of distant galaxies

J. Delaney
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the logical consequences seem heavy and absurd, so another solution would be attractive

This is a purely aesthetic judgement. One that I don’t share. Since different people have different tastes, there is no way that the universe can conform to everyone’s tastes. Nor is it required to conform to anyone’s taste. All we can do is observe and understand.

Considering how little we know about DM, how do we know it does not occasionally absorb a little bit of blue from a passing photon?

If it does, it still wouldn’t explain observations. DM is clumped around most galaxies. So it would not affect the cosmic background radiation that we observe between galaxies.

Because redshift is observed both for galaxies and the background radiation, it would have to be a new matter field. One that would replace dark energy rather than dark matter.

wouldn't Occam's Razor prefer it to the popular idea of the expanding universe?

No. Occham’s razor is not a justification for any random thing that someone finds aesthetically appealing. Occham’s razor favors models that are simpler, not more appealing.

Simplicity is judged by the number of free parameters in the model. Dark energy has a single free parameter. So it would be difficult for a new matter field to match that simplicity. At a minimum I think you would need a density parameter and an interaction strength parameter. Matter is complicated

Dale
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You are describing "tired light". The idea is that for some reason, there are interactions with an intergalactic medium - normal or dark matter (Note that dark matter does not interact with light otherwise it wouldn't be dark) and removes/scatters energy.

A fairly obvious piece of experimental evidence against this, and in support of a cosmological expansion interpretation of redshift, is that distant, time variable phenomena - supernovae, gamma ray bursts, quasar variability - have durations that are stretched by the same redshift factor. See Tired light red shift hypothesis for a summary.

Tired light would not affect the apparent duration of these events.

ProfRob
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Given our understanding of gravity, the universe has to be dynamical -- either expanding, contracting, or just momentarily stationary. A universe that was not expanding would begin to collapse under its own gravity. Thus, the simplest "Occam's Razor" explanation for the cosmological redshift is that the universe is expanding, and attempting to explain a static universe would require more complicated models.

Sten
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