Cosmology Talks: Adam Riess on Cepheid Crowding and the Hubble Tension

Here’s another example from the series of cosmology talks being curated by Shaun Hotchkiss. In this one, esteemed astronomer and Nobel Prize winner Adam Riess talks about what he and collaborators considered to be the leading candidate for a systematic error in the SHOES measurement of the expansion rate of the Universe. This is “Cepheid crowding”, the possibility that background sources change our interpretation of Cepheid brightness, ruining one step in the SHOES distance ladder. Riess and collaborators devise a nice way to test whether the crowding is correctly accounted for and find that it is, so crowding cannot be the “explanation” of an error in the distance ladder measurement of H0. Riess also stresses that both the early and late universe measurements of H0 are now backed up by multiple different measurements. Accordingly, if the resolution isn’t fundamental physics, then no single systematic can entirely solve the tension.

P. S. The paper that accompanies this talk can be found on the arXiv here.

4 Responses to “Cosmology Talks: Adam Riess on Cepheid Crowding and the Hubble Tension”

  1. Phillip Helbig Says:

    The greybeards here will remember the debate between Sandage with 55 +/- 7 and de Vaucouleurs with 85+/- 5 or whatever.

    In what sense is the present debate different? Hardly anyone was suggesting “new physics” back then.

    Did anyone ever definitively find out, in detail, what was wrong in that old debate? Systematic errors? Uncertainties underestimated? Both? On whose part?

    Those who don’t learn from history are doomed to repeate it. 😦

  2. James Dunlop Says:

    I remain more impressed by how similar the Hubble constant measurements are now, rather than how supposedly different they are. A tiny dose of that prior called “common sense” tells you they are the same number. In a similar vein I’m impressed by how similar the ages of the oldest stellar systems are to the age the Universe. One can get excited about tiny differences there as well, but the more impressive thing is how similar the age of the Universe is (as derived from Cosmology) to the age of the oldest stars or stellar systems (from nuclear physics).

    • telescoper Says:

      Agreed. The history of the Hubble constant is one of consistently underestimated error bars..

    • Phillip Helbig Says:

      The first is clear: they have to be similar, otherwise the whole standard-cosmology theory doesn’t work. With regard to the second, I agree that the agreement is impressive, but of course it is not the absolute difference but the diffrence in units of the error bars which is key. It is easy to say that error bars are underestimated, but much harder to come up with correct ones in a convincing matter.

      Interesting is that more than one local measurement gives a high value, so even if the error bars are all underestimated and the measurements are all compatble, there would still be this discrepancy to explain, at least at some level.

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