One explanation for the Cold Spot in the cosmic microwave background (CMB) is that dark matter is redshifting CMB photons; see Fermilab's article "Scientists move a step closer to understanding the “cold spot” in the cosmic microwave background". But this dark matter would affect the orbital speeds of and within galaxies. Can current telescopes detect anomalously high orbital speeds in the direction of the CMB Cold Spot?

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    $\begingroup$ Can you cite sources please? Galaxies hadn't yet formed when the CMB was produced. $\endgroup$
    – James K
    Commented Apr 16, 2023 at 10:50
  • $\begingroup$ @James K You are right that galaxies hadn't formed when the CMB was produced. There was an article I read that said that dark matter may have been able to account for the redshift. However, the discussion here news.fnal.gov/2022/01/…. show there is too little dark matter $\endgroup$
    – user50623
    Commented Apr 16, 2023 at 14:12

1 Answer 1


Can the CMB Cold Spot be explained by dark matter redshifting photons?

More specifically, the explanation discussed by the linked article is that the CMB Cold Spot is (at least partially) explained by matter redshifting photons as they enter a cosmic void, the Eridanus Supervoid, that is relatively nearby (~2.5 billion light years away). The basis for this explanation is that a cosmic void is a region of below-average matter density, so the gravitational potential is high there. Photons are gravitationally redshifted when they enter and blueshifted when they leave. Over time, cosmic expansion means that voids expand, an effect that weakens the gravitational potential. Their density also drops over time relative to the average, due to a void's weak gravitational attraction compared to that of its surroundings, and this effect strengthens the gravitational potential. However, at recent times, the influence of dark energy causes the first effect (expansion) to win, so the gravitational potential weakens over time. Photons are thus redshifted when they enter the void and blueshifted not as much when they leave at a later time. This effect, known as the integrated Sachs-Wolfe effect, is why a nearby cosmic void can contribute to the appearance of a cold (red) spot in the CMB.

Can current telescopes detect anomalously high orbital speeds in the direction of the CMB Cold Spot?

Cosmic voids have less of all matter, not just dark matter. They're places where you expect to find fewer galaxies, not places where you expect galaxies to be dark matter-deficient. Indeed we normally identify voids in the first place as regions where there are fewer galaxies than average.

How do we know that voids are equally underdense in all matter? In general, large-scale variations in the density of the universe are known to be the same in dark and ordinary matter, at least to within a few percent. See the literature on "compensated isocurvature perturbations", e.g. this article, which searches for how such perturbations would gravitationally lens the CMB.

For the CMB Cold Spot in particular, this research article (which is the one associated with the Fermilab outreach publication linked in the question) notes that the Eridanus Supervoid actually appears to have slightly too much dark matter compared to the number of visible galaxies, where the amount of dark matter is inferred from gravitational lensing of more distant galaxies. Note that this is the opposite of what is needed to explain the CMB Cold Spot. However, the excess is not very statistically significant, and they also discuss possible systematic errors that could be responsible for the discrepancy.


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