new work on the Hubble "constant" tension

[notFritzArgelander]

I'm going to come back to translate later. Need to do errands now.

https://arxiv.org/abs/1910.02978?fbclid ... -HlJpCKGQI

The current Hubble constant tension is usually presented by comparing constraints on H0 only. However, the post-recombination background cosmic evolution is determined by two parameters in the standard ΛCDM model, the Hubble constant (H0) and today's matter energy fraction (Ωm). If we therefore compare all constraints individually in the H0-Ωm plane, (1) various constraints can be treated as independently as possible, (2) single-sided constraints are easier to consider, (3) compatibility among different constraints can be viewed in a more robust way, and (4) whether or not a nonstandard model is able to reconcile all constraints in tension can be seen more effectively. We perform a systematic comparison of independent constraints in the H0-Ωm space based on a flat ΛCDM model. Constraints along different degeneracy directions consistently overlap in one region of the space, with the exception of the local measurement from Cepheid variable-calibrated supernovae. Due to the different responses of individual constraints to a modified model, it is difficult for nonstandard models with modifications at high-, mid- or low-redshifts to reconcile all constraints if none of them have unaccounted-for systematic effects. Based on our analysis, the local measurement is the most outlying and therefore drives the bulk of the tension. This may suggest that the most likely solution to the tension is an alteration to the local result, either due to some previously unseen feature in our local cosmic environment, or some other unknown systematic effect.

But the short answer is that (as I intuited) the local result based on Cepheids and standard candles is most likely just wrong.

[notFritzArgelander]

OK, what the folks did here is actually pretty deeply good.

The authors note that usual treatments of the H0, Hubble 'constant', problem consists of only looking at the cosmological constraints on H0. This is relatively simple and easy to do. But it neglects the deeper point that the FLRW metric and cold dark matter model constrains all the parameters of the cosmological model together. There is cross talk and correlation among the variables as solutions of the GR equations require. So they assess the credibility of measurements of H0 by assessing how it constrains the cosmological model as a whole.

The first paragraph of the paper https://arxiv.org/pdf/1910.02978.pdf gives such a clear picture of the historical background that I can't help but quote it.

Cosmology used to be called “a search for two numbers,” referring to the Hubble constant and the deceleration parameter (Sandage 1970). While the former describes today’s cosmic expansion rate and, when first discovered, caused Ein- stein to abandon his idea of the cosmological constant Λ, the latter turned out to be negative and prompted physicists to bring Λ back. Dubbed (flat) “ΛCDM,” the simplest cosmological model fully determines the dynamics of the homogeneous universe with another combination of two numbers, this time pairing the Hubble constant H0 with today’s matter energy fraction Ωm. Together with its description of large-scale inhomogeneities, this model has successfully explained various cosmological and astronomical observations. Its simplicity and (at least overall) concordance has made it the standard cosmological model, even while named after its two most mysterious aspects. The two numbers, H0 and Ωm, are the focus of this work.

The authors then select the best measures of H0 and Ωm for further analysis. These include measurements of the cosmic microwave background, time delays in strong gravitational lensing, changes is relative brightness of supernovae as a function of redshift (this is different from and independent of the Cepheid calibrated SN Ia luminosity scale), gamma ray attenuation measurements, yr 1 of the dark energy survey results, differential ages of passively evolving galaxies at two nearby redshifts, baryon acoustic oscillations (at low and high redshift), the cosmic age from stellar evolution models, and the Cepheid calibrated SN Ia luminosity scale.

Of these 9 different measures to constrain H0 and Ωm only 1 ONE, yes that's right one is discordant. That is the Cepheid calibrated SN Ia luminosity scale.

[chasmanian]

so this study says the Standard Cosmological Model seems really solid, right nFA?

[notFritzArgelander]

so this study says the Standard Cosmological Model seems really solid, right nFA?

Indirectly, yes. The study assumes that the flat ΛCDM model is correct, though. It directly shows that in a choice between the standard cosmology being wrong and the H0 value from Cepheids and SNIa luminosity being wrong that it's most likely the luminosity distance measures that are off. It's pretty suspicious that 8 of 9 measures for H0 and Ωm all agree and only the Cepheid-SN Ia value for H0 is an outlier.

I would have preferred to see a Bayesian statistical analysis though. If one doesn't appear soon....

[chasmanian]

thank you very much nFA.

[GCoyote]

so this study says the Standard Cosmological Model seems really solid, right nFA?

Indirectly, yes. The study assumes that the flat ΛCDM model is correct, though. It directly shows that in a choice between the standard cosmology being wrong and the H0 value from Cepheids and SNIa luminosity being wrong that it's most likely the luminosity distance measures that are off. It's pretty suspicious that 8 of 9 measures for H0 and Ωm all agree and only the Cepheid-SN Ia value for H0 is an outlier.

I would have preferred to see a Bayesian statistical analysis though. If one doesn't appear soon....

So if you drop off the net for a little while we'll know why. :Think: