Day one of ESO H0 2020 Meeting: The H0 Tension

ESO has organized an important online conference (22-26 June 2020) on the current significant tension in measuring the expansion rate of the Universe, i.e. the Hubble constant H0. The video of each day’s talks and the panel discussion is uploaded on YouTube by the organizers and is publicly available.

Here is a summary of day one of the meeting with links to specific parts of each speaker’s talk mainly for future personal reference, but hopefully useful for others as well.

After Richard Anderson announced the start of the conference, the talks went on as follows:

Adam Riess, SH0ES:

1. Intro:
   Started with an intro to the prediction of the LCDM model for H0.
   Measuring H0 is the “Ultimate end-to-end test for LCDM“.
   A summary of 15 years of effort of the SH0ES project for direct measurement of H0 using Cepheids for calibrating distant SNeIa.
2. A three-step measurement:
   5 sources for geometric calibration of Cepheids, 19 galaxies for calibration of SNeIa using Cepheids, and 300 SNeIa for measuring H0.
3. The Hubble Constant “Problem”:
   Multiple methods for direct measurement of H0 find consistent results and removing any one of them does not solve the problem.
4. New recent studies:
   Including a new geometric calibration of MW Cepheids by Breuval et al. arXiv:2006.08763
5. Advantages of using Cepheids and efforts to control systematics
6. Crowding Bias, “the most tricky systematic for Cepheids”
   Mainly discussing a new approach for estimating crowding bias reported in Riess et al. arXiv:2005.02445
7. Discussing some FAQs
   E.g. are we in a “giant void”?
8. Near future of the SH0ES project:
   Cepheid calibration of 24 more SNIa host galaxies.
9. Final thoughts

Rachael Beaton, Chicago-Carnegie Hubble Project (CCHP)

1. Intro
   “A Cepheid independent path to the Hubble Constant”.
   Also a three-step approach: geometric anchor, calibrating nearby SNeIa using TRGB, and using distant SNeIa.
2. CCHP result for H0
   In the middle (within 2 sigma) of both Cepheids and CMB results, Freedman et al. (2019), using detached eclipsing binaries in LMC, 18 SNeIa hosts, and 100 distant SNeIa.
3. An intro to TRGB method:
4. Benefits of using TRGB method:
   E.g can use the low-stellar density environments, therefore avoiding crowding bias.
5. TRGB Measurement in Practice
   Comparing the TRGB in a nearby dwarf and a distant SNIa host.
6. TRGB Uncertainties:
   TRGB calibration from LMC and NGC4258 (hosting a megamaser and also used by SH0ES) are consistent, more results will come soon.
7. TRGB results for M101 (a close SNIa host galaxy)
8. Comparing TRGB and Cepheid distances
   There is an overall agreement for the 10 common galaxies of the two projects, but there are a few curious differences.
9. Distant SNeIa samples
   H0 from the two SNeIa samples used with TRGB method is found to be around 70 km/s.Mpc
10. Comparing the SNeIa absolute mag calibrations from TRGB and Cepheids.
    There is a ~0.1 mag difference, but only from 10 data points, hence no firm conclusion yet on this point.
11. CCHP Summary

Silvia Galli, Planck and the CMB

1. Intro
   Introducing the Planck mission: multi-frequency observation allowing separation of CMB from Galactic foreground. The 2018 data release improved the control of systematics.
2. H0 results from different data releases
   Consistently around 67 km/s.Mpc.
3. Six LCDM parameters from the CMB power spectrum
   Including the angular scale of the sound horizon used to measure H0
4. How H0 is indirectly measured from the CMB
   Angular diameter distance to the last scattering surface based on the measured angular scale and physical size of the sound horizon.
5. LCDM parameters are highly correlated
   In particular, H0 is highly correlated with Omega_m (the density parameter of dark matter), therefore, “it is wrong” to fix one and change another.
6. Can extensions to LCDM solve the H0 tension?
7. The future CMB observations
   

Lloyd Knox, The theoretical picture

1. Intro
   Introducing “The Hubble Hunter’s Guide”, Knox and Millea (2020).
   The theoretical picture and thinking about possible ways to change the cosmological model.
2. Searching for model X that “restores concordance”
3. Determining H0 from CMB data in three steps
   a) calibrating a standard ruler: the sound horizon
   b) using the ruler to infer distance
   c) using the angular diameter distance relation in LCDM as a function of redshift, adjusting Lambda, and obtaining H0.
4. Back to the search for model X
   It probably includes a lowering of the sound horizon.
5. Possible theoretical scenarios for solving H0 tension
6. Summary & Conclusions
   Still searching for model X.

The discussion session:

1. D. Huterer:
   Cosmic variance is 20 times smaller than H0 difference, hence cannot solve the tension.
2. B. Javanmardi:
   Questions from the SH0ES project about the analysis of the extragalactic Cepheids and mainly about the effect of uncertainties of optical measurements on the Wesenheit magnitude used for obtaining P-L relations.
3. E. McDonough:
   Towards a resolution of H0 tension: “early dark energy”.
4. W. Freedman:
   Calibration of TRGB.
   A list of systematics affecting different methods.
   Crowding bias is a problem for Cepheids for larger than ~20 Mpc distances.
   “A reminder to all of us … that maybe the worst systematics are the ones that we don’t even know about”.
5. J. Mould:
   “Three decadal goals for ESO for the local value of H0”, i.e. improving the distance ladder on all the three steps: SNe, nearby galaxies, and parallaxes.
   

End of day one.

The images showing the first slide of each talk are screenshots obtained from the public YouTube video published by ESO. The featured image at the top of the post is produced using the distance ladder image and the WMAP CMB map both obtained from the NASA website.