Archive for Wendy Freedman

Hubble Tension – The Plot Thickens

Posted in The Universe and Stuff with tags , , , on July 2, 2021 by telescoper

One topic on this blog seems to be as perennial as the weeds in my garden: the so-called Hubble Tension. I just saw a review paper by Wendy Freedman, one of the acknowedged experts in this area, on arXiv here. I have abstracted the abstract here:

You can find the PDF here.

What’s particularly interesting about this discussion is that stellar distance indicators have typically produced higher values than the 69.8 ± 0.6 (stat) ± 1.6 (sys) km s-1 Mpc-1 quoted here, which is consistent with the lower value favoured by Planck. See the above graphic discussed here. So perhaps there’s no tension at all. Maybe.

Anyway, here’s that poll again! I wonder if this paper might change the voting.


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!

Cosmology at a Crossroads – Poll

Posted in The Universe and Stuff with tags , , , on June 13, 2017 by telescoper

A short comment piece by Wendy Freedman has appeared in Nature Astronomy; there’s a free version on the arXiv here. It gives a nice perspective on the current debate about the value of the Hubble constant from the point of view of an expert on cosmological distance scale measurements.

The abstract is here:

We are at an interesting juncture in cosmology. With new methods and technology, the accuracy in measurement of the Hubble constant has vastly improved, but a recent tension has arisen that is either signaling new physics or as-yet unrecognized uncertainties.

For the record, I’d go for `as-yet unrecognized uncertainties’, primarily because this field has a long history of drastically underestimated error-bars!

However, the publication of this piece gives me the excuse to resurrect the following poll, in which I invite you to participate: