[2005.02445] The Accuracy of the Hubble Constant Measurement Verified through Cepheid Amplitudes

Authors:  Adam G. Riess, Wenlong Yuan, Stefano Casertano, Lucas M. Macri, Dan Scolnic
Abstract:  The accuracy of the Hubble constant measured with extragalactic Cepheids depends on robust photometry and background estimation in the presence of stellar crowding. The conventional approach accounts for crowding by sampling backgrounds near Cepheids and assuming they match those at their positions. We show a direct consequence of crowding by unresolved sources at Cepheid sites is a reduction in the fractional amplitudes of their light curves. We use a simple analytical expression to infer crowding directly from the light curve amplitudes of >200 Cepheids in 3 SNe~Ia hosts and NGC 4258 as observed by HST -- the first near-infrared amplitudes measured beyond the Magellanic Clouds. Where local crowding is minimal, we find near-infrared amplitudes match Milky Way Cepheids at the same periods. At greater stellar densities we find that the empirically measured amplitudes match the values predicted (with no free parameters) from crowding assessed in the conventional way from local regions, confirming their accuracy for estimating the background at the Cepheid locations. Extragalactic Cepheid amplitudes would need to be ~20% smaller than measured to indicate additional, unrecognized crowding as a primary source of the present discrepancy in H_0. Rather we find the amplitude data constrains a systematic mis-estimate of Cepheid backgrounds to be 0.029 +/- 0.037 mag, more than 5x smaller than the size of the present ~0.2 mag tension in H_0. We conclude that systematic errors in Cepheid backgrounds do not provide a plausible resolution to the Hubble tension.
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Shaun Hotchkiss
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[2005.02445] The Accuracy of the Hubble Constant Measurement Verified through Cepheid Amplitudes

Post by Shaun Hotchkiss » May 07 2020

A reminder that I've been making technical cosmology videos. The format is me interviewing authors of papers I found interesting. The most recent is on this paper and is now online here. The idea is for this to fill the same niche as a departmental seminar or plenary conference talk, so it is technical, but not meant to be only for those already working on the specific topic. Hopefully it will be of some (potential) benefit to all people at the level of a Cosmology PhD student or above, whatever their own research is on.

I also hope the interview technique makes the talk slightly more fun to watch than a dry webinar. The feedback from speakers so far is that they certainly prefer an interview instead of speaking at their computer without feedback for 50 minutes.

The paper itself is interesting because it rules out what was before this perhaps the leading candidate for a possible systematic error in the distance ladder measurements of H0. Of course, there are now multiple measurements of H0 in the local universe, so if the resolution is systematic errors it would need to be multiple errors in more than one method. But it is still important science to hunt down all possible systematics and make sure they're small, so this is a useful few pages of the full H0 detective novel.

Feedback is definitely welcomed! As are requests to be interviewed and requests for videos on specific papers.

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