There have been suggested alternatives, e.g., photons' losing momentum due to interactions with plasma. I just reviewed a paper which actually found that this mechanism accounted for the variations, as one varies one's line-of-sight, in the empirically observed value of Hubble's constant. I.e., the statistical fluctuations in the measurement of Hubble's constant vary by 20,000 parts per million, depending on which line-of-sight you adopt.

Here is a quote, "The all-sky maps of the observed variation of Hubble's constant can be reproduced from a theoretical point of view by introducing an intergalactic plasma with a varying density of electrons."

The paper, which I thought was reasonable and deserved publication, quotes other respectable papers in the literature, including a review paper by L. Marmet, L.~{Marmet}, {Survey of Redshift Relationships for the Proposed Mechanisms at the 2^{}$\{$nd$\}$ Crisis in Cosmology Conference}, in: {F.~Potter} (Ed.), \textit{Astronomical Society of the Pacific Conference Series}, Vol. 413, 2009, 315--335.

For more on the intergalactic plasma of electrons, see D.~L. {Mamas}, {An explanation for the cosmological redshift}, {\it Physics Essays} {\bf 23} (2010), 326.

None of the suggested alternatives have yet received a consensus of acceptance by those who favour the Doppler effect explanation, which is the consensus explanation accepted at present.

There have been suggested alternatives, e.g., photons' losing momentum due to interactions with plasma. I just reviewed a paper which actually found that this mechanism accounted for the variations, as one varies one's line-of-sight, in the empirically observed value of Hubble's constant. I.e., the statistical fluctuations in the measurement of Hubble's constant vary by 20,000 parts per million, depending on which line-of-sight you adopt.

Here is a quote, "The all-sky maps of the observed variation of Hubble's constant can be reproduced from a theoretical point of view by introducing an intergalactic plasma with a varying density of electrons."

The paper, which I thought was reasonable and deserved publication, quotes other respectable papers in the literature, including a review paper by L. Marmet, "Survey of Redshift Relationships for the Proposed Mechanisms at the 2nd Crisis in Cosmology Conference", in: F. Potter (Ed.), Astronomical Society of the Pacific Conference Series, Vol. 413, 2009, 315-335.

For more on the intergalactic plasma of electrons, see D. L. Mamas, "An explanation for the cosmological redshift", Physics Essays 23 (2010), 326.

None of the suggested alternatives have yet received a consensus of acceptance by those who favour the Doppler effect explanation, which is the consensus explanation accepted at present.