Archive for arXiv:1907.05922

The Hubble Constant from the Tip of the Red Giant Branch

Posted in The Universe and Stuff with tags , , , , on July 16, 2019 by telescoper

At the risk of boring everyone again with Hubble constant news there’s yet another paper on the arXiv about the Hubble constant. This one is another `local’ measurement, in that it uses properties of nearby stars,  time based on a new calibration of the Red Giant Branch. This one is by Wendy Freedman et al. and its abstract reads:

We present a new and independent determination of the local value of the Hubble constant based on a calibration of the Tip of the Red Giant Branch (TRGB) applied to Type Ia supernovae (SNeIa). We find a value of Ho = 69.8 +/- 0.8 (+/-1.1\% stat) +/- 1.7 (+/-2.4\% sys) km/sec/Mpc. The TRGB method is both precise and accurate, and is parallel to, but independent of the Cepheid distance scale. Our value sits midway in the range defined by the current Hubble tension. It agrees at the 1.2-sigma level with that of the Planck 2018 estimate, and at the 1.7-sigma level with the SHoES measurement of Ho based on the Cepheid distance scale. The TRGB distances have been measured using deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of galaxy halos. The zero point of the TRGB calibration is set with a distance modulus to the Large Magellanic Cloud of 18.477 +/- 0.004 (stat) +/-0.020 (sys) mag, based on measurement of 20 late-type detached eclipsing binary (DEB) stars, combined with an HST parallax calibration of a 3.6 micron Cepheid Leavitt law based on Spitzer observations. We anchor the TRGB distances to galaxies that extend our measurement into the Hubble flow using the recently completed Carnegie Supernova Project I sample containing about 100 well-observed SNeIa. There are several advantages of halo TRGB distance measurements relative to Cepheid variables: these include low halo reddening, minimal effects of crowding or blending of the photometry, only a shallow (calibrated) sensitivity to metallicity in the I-band, and no need for multiple epochs of observations or concerns of different slopes with period. In addition, the host masses of our TRGB host-galaxy sample are higher on average than the Cepheid sample, better matching the range of host-galaxy masses in the CSP distant sample, and reducing potential systematic effects in the SNeIa measurements.

You can download a PDF of the paper here.

Note that the value obtained ising the TRGB here lies in between the two determinations using the cosmic microwave background and the Cepheid distance scale I discussed, for example, here. This is illustrated nicely by the following couple of Figures:

I know that this result – around 70 km s-1 Mpc-1 – has made some people a bit more relaxed about the apparent tension between the previous measurements, but what do you think? Here’s a poll so you can express your opinion.

My own opinion is that if there isn’t any tension at all at the one-sigma level then you should consider the possibility that you got sigma wrong!

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