Is the Expansion of the Universe Isotropic?

There’s a new paper out that has been making a few waves in cosmology. Here’s the title and abstract:

It’s published in Astronomy & Astrophysics but you can find it on the arXiv here.

Here’s a gratuitous pretty picture showing the distribution of the X-ray clusters used in the analysis.

The discussion in the paper focuses on two possibilities: (i) that the clusters are participating in a large-scale correlated motion; and (ii) that the Expansion of the Universe is not occurring isotropically. The latter option is the one that has attracted the most media attention (presumably because it has the most far-reaching implications). This seems to me to be a very unlikely explanation, however, because anisotropic expansion of the magnitude implied would leave a ~10% signal in the Cosmic Microwave Background which is not observed.

There is, however, a third possibility (admittedly duller than the other two) which is that there is some unknown systematic error in the observations…

4 Responses to “Is the Expansion of the Universe Isotropic?”

  1. Phillip Helbig Says:

    “This seems to me to be a very unlikely explanation, however, because anisotropic expansion of the magnitude implied would leave a ~10% signal in the Cosmic Microwave Background which is not observed.”

    Why do people continue to get this wrong? If there are 100 pieces of evidence in favour of hypothesis A, and one piece of evidence which makes hypothesis B more likely, it is not “time to rewrite the textbooks” nor for any of the other buzzwords and exaggerated headlines.

    • telescoper Says:

      People are under pressure to promote their work, I suppose, and the media like a good story. To be fair the paper itself is far more sensible than the press coverage.

      • Phillip Helbig Says:

        That’s often the case. Still, I think that authors should have to pass off expositions of their work in which they were involved in some way, and veto over-the-top stuff.

      • flopacaud Says:

        As a co-author of the work, I must say that my explanation of choice is some local anisotropy or bulk flow, despite the media picking up on the other one. The effect is more strongly observed among low-z clusters and seems to fade away in more distant sub-samples. I must also say that the paper spends most of the time dealing with the third explanation mentioned in this blog but, so far, we could not rule out the effect from calibration or data processing issues.

        Bottom line is: we are running a much more ambitious project to check for this effect with the first release of the eROSITA all-sky survey. This should provide much clearer results.

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