## Gravitational Lensing, Cosmological Distances and the Hubble Constant

To continue the ongoing Hubble constant theme, there is an interesting paper on the arXiv by Shajib et al. about determining a distance to a gravitational lens system; I grabbed the above pretty picture from the paper.

The abstract is:

You can click on this to make it bigger. You will see that this approach gives a `high’ value of H_{0} ≈ 74.2, consistent with local stellar distances measures, rather than with the `cosmological’ value which comes in around H_{0} ≈ 67 or so. It’s also consistent with the value derived from other gravitational lens studies discussed here.

Here’s my ongoing poll on the Hubble constant, with

Follow @telescoper

October 18, 2019 at 3:16 pm

“You will see that this approach gives a `high’ value of H0 ≈ 74.2, consistent with local stellar distances measures, rather than with the `cosmological’ value which comes in around H0 ≈ 67 or so. It’s also consistent with the value derived from other gravitational lens studies discussed here.”Perhaps not surprising, as, compared to the CMB, such measurements are “local”.

October 18, 2019 at 3:26 pm

From some related discussion (my emphasis):

> The preprint is 1909.06712

Two additional preprints are at

https://arxiv.org/abs/1907.04869 and

https://arxiv.org/abs/1910.06306

These report direct measurements of gravitational lens distances rather than a recalibration of the standard distance ladder.

The lead author Shajib of 06306 spoke here today and showed an updated version of Fig 12 of the 04869 preprint. The upshot is that the discrepancy between the local and the CMB measurements of H_0 is between 4 and 5.7 sigma, depending on how conservative one wants to be about assumptions. The impression I got is that either there’s a systematic error somewhere or there’s new physics. The local H_0 is based on two independent methods — distance ladder and lensing — so big systematic errors in local H_0 seem unlikely. The CMB H_0 is based on Planck with WMAP having given an H_0 value more consistent with the local one. “New physics” could be something as simple as time-varying dark energy, but for now it’s too soon to say much.

One other note from the talk:

. Shajib and others are trying to automate the modeling, but until that’s done, measuring a large sample of lenses will be labor-intensive. Even then, it will be cpu-intensive.it takes an expert modeler about 8 months to a year to model a single lens system, and about 40 lenses areShahib mentioned 1 million cpu-hours for his model of DES J0408-53545354needed to give the desired precision of local H_0.

October 18, 2019 at 3:27 pm

From some related discussion (my emphasis, and slightly edited so that there is only one link):

> The preprint is 1909.06712

Two additional preprints are at

1907.04869 and

1910.06306

These report direct measurements of gravitational lens distances rather than a recalibration of the standard distance ladder.

The lead author Shajib of 06306 spoke here today and showed an updated version of Fig 12 of the 04869 preprint. The upshot is that the discrepancy between the local and the CMB measurements of H_0 is between 4 and 5.7 sigma, depending on how conservative one wants to be about assumptions. The impression I got is that either there’s a systematic error somewhere or there’s new physics. The local H_0 is based on two independent methods — distance ladder and lensing — so big systematic errors in local H_0 seem unlikely. The CMB H_0 is based on Planck with WMAP having given an H_0 value more consistent with the local one. “New physics” could be something as simple as time-varying dark energy, but for now it’s too soon to say much.

One other note from the talk:

. Shajib and others are trying to automate the modeling, but until that’s done, measuring a large sample of lenses will be labor-intensive. Even then, it will be cpu-intensive.it takes an expert modeler about 8 months to a year to model a single lens system, and about 40 lenses areShahib mentioned 1 million cpu-hours for his model of DES J0408-53545354needed to give the desired precision of local H_0.