Adam Riess - Cepheid crowding is not the cause of the Hubble tension.

Adam tells us 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.

They 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.

He also stresses that both the early and late universe measurements of H0 are now backed up by multiple different measurements. Therefore, if the resolution isn't fundamental physics, then no single systematic can entirely solve the tension.

We also discuss a few topics around the H0 tension, including what resolution of the tension he would pick as most likely if forced to gamble (answer: a deviation from vanilla ΛCDM in the early universe 😲).

[talks_curator]

*Index to Key Parts of the Talk:*
[00:02] Shaun's intro to the topic and Adam's work
[00:57] Q&A: Adam's brief summary of the paper: Cepheid crowding can be ruled out at greater than 5σ as an issue causing the Hubble tension
[02:58] Q&A: what are two simple things you'd want viewers to remember about this talk?
[04:21] Q&A: what is the motivation for this work? Of all the potential systematics to check, why is crowding the one that you chose?
[06:09] First slide: the paper and the SH0ES colleagues involved
[06:49] Background on the Hubble tension problem
[08:38] The distance ladder method for measuring the Hubble constant: late universe method
[09:31] Representative early universe methods to determine the Hubble constant: early vs late universe determinations differ by 5-6σ (*see Adam's later comment at [37:23] on his rule of thumb to clarify what differentiates early vs late universe methods)
[10:54] Why do we use Cepheid variable stars?: includes brief comments on other calibration methods besides Cepheids
[12:38] Importance of Cepheids since there aren't enough supernovae to build the distance ladder just from SNe
[13:14] Cepheids and Crowding - the non-trivial challenge: idealized situation vs reality for accurate photometric measurements; cannot resolve the non-uniform background under Cepheids but that must be accounted for to measure accurate photometry; image showing what crowding is; how to figure out how much crowding there is; measuring the offset or bias that results on average from the other field stars that are crowding into the resolution element; extra wrinkles; possibly the most significant challenge for Cepheid measurements beyond 20 Mpc is crowding and blending from redder stars particularly in near-infrared observations (Wendy Freedman quote)
[15:44] Amplitudes Provide _Direct_ Measure of Cepheid Crowding: crowded flux is bigger fractional contribution when Cepheid at minimum, which compresses the light curve amplitude; mathematical relationship
[18:09] Three Examples of Different Crowding Levels: NGC 1559: illustrates the amplitude compression effect from crowding
[19:45] Step 1: Calibrate Amplitude-period Relation with Milky Way Cepheids: Cepheid amplitudes depend on period; expected amplitude as a function of period and crowding
[21:01] Step 2: 224 NIR amplitudes Measured in 4 hosts, compare to Milky Way (benchmark): empirically measured amplitudes match prediction (with no free parameters) from crowding assessed in the conventional way from local (Milky Way) regions, confirming their accuracy for estimating the background at the (extragalactic) Cepheid locations
[23:21] Step 3: Constrain "unrecognized" crowding, compare to Hubble tension: "unrecognized" crowding needed to explain Hubble tension ruled out at >5σ
[25:10] So what could explain the Hubble tension?
[32:56] Future Prospects: Improvements coming in the next couple years
[34:50] Q&A: If you had to guess right now, what is the most probable explanation on either the theory or observational side?
[37:23] *Adam's rule of thumb to clarify what differentiates late universe vs early universe methods
[39:13] Q&A: could a breakdown in the FRW metric at late times be an explanation?
[41:10] Q&A: outside of your own research, what do you think is the most interesting thing in cosmology at the moment?