In the Dark

Last week when I wrote about the 2nd Data Release from Gaia, somebody emailed me to ask whether the new results said anything about the cosmological distance ladder and hence the Hubble Constant. As far as I could see, no scientific papers were released on this topic at the time and I thought there probably wasn’t anything definitive at this stage. However, it turns out that there is a paper now, by Riess et al., which focuses on the likely impact of Gaia on the Cepheid distance scale. Here is the abstract:

We present HST photometry of a selected sample of 50 long-period, low-extinction Milky Way Cepheids measured on the same WFC3 F555W, F814W, and F160W-band photometric system as extragalactic Cepheids in SN Ia hosts. These bright Cepheids were observed with the WFC3 spatial scanning mode in the optical and near-infrared to mitigate saturation and reduce pixel-to-pixel calibration errors to reach a mean photometric error of 5 millimags per observation. We use the new Gaia DR2 parallaxes and HST photometry to simultaneously constrain the cosmic distance scale and to measure the DR2 parallax zeropoint offset appropriate for Cepheids. We find a value for the zeropoint offset of -46 +/- 13 muas or +/- 6 muas for a fixed distance scale, higher than found from quasars, as expected, for these brighter and redder sources. The precision of the distance scale from DR2 has been reduced by a factor of 2.5 due to the need to independently determine the parallax offset. The best fit distance scale is 1.006 +/- 0.033, relative to the scale from Riess et al 2016 with H0=73.24 km/s/Mpc used to predict the parallaxes photometrically, and is inconsistent with the scale needed to match the Planck 2016 CMB data combined with LCDM at the 2.9 sigma confidence level (99.6%). At 96.5% confidence we find that the formal DR2 errors may be underestimated as indicated. We identify additional error associated with the use of augmented Cepheid samples utilizing ground-based photometry and discuss their likely origins. Including the DR2 parallaxes with all prior distance ladder data raises the current tension between the late and early Universe route to the Hubble constant to 3.8 sigma (99.99 %). With the final expected precision from Gaia, the sample of 50 Cepheids with HST photometry will limit to 0.5% the contribution of the first rung of the distance ladder to the uncertainty in the Hubble constant.

So, nothing definitive yet but potentially very interesting in the future and this group, led by Adam Riess, is now claiming a 3.8σ tension between measurements of the Hubble constant from cosmic microwave background measurements and from traditional `distance ladder’ approaches, though to my mind this is based on some rather subjective judgements.

The appearance of that paper reminded me that I forgot to post about a paper by Bernal & Peacock that appeared a couple of months ago. Here is the abstract of that one:

When combining data sets to perform parameter inference, the results will be unreliable if there are unknown systematics in data or models. Here we introduce a flexible methodology, BACCUS: BAyesian Conservative Constraints and Unknown Systematics, which deals in a conservative way with the problem of data combination, for any degree of tension between experiments. We introduce hyperparameters that describe a bias in each model parameter for each class of experiments. A conservative posterior for the model parameters is then obtained by marginalization both over these unknown shifts and over the width of their prior. We contrast this approach with an existing hyperparameter method in which each individual likelihood is scaled, comparing the performance of each approach and their combination in application to some idealized models. Using only these rescaling hyperparameters is not a suitable approach for the current observational situation, in which internal null tests of the errors are passed, and yet different experiments prefer models that are in poor agreement. The possible existence of large shift systematics cannot be constrained with a small number of data sets, leading to extended tails on the conservative posterior distributions. We illustrate our method with the case of the H₀ tension between results from the cosmic distance ladder and physical measurements that rely on the standard cosmological model.

This paper addresses the long-running issue of apparent tension in different measurements of the Hubble constant that I’ve blogged about before (e.g. here) by putting the treatment of possible systematic errors into a more rigorus and consistent (i.e. Bayesian) form. It says what I think most people in the community privately think about this issue, i.e. that it’s probably down to some sort of unidentified systematic rather than exotic physics.

The title of the paper includes the phrase `Conservative Cosmology’, but I think that’s a bit of a misnomer. I think `Sensible Cosmology’. Current events suggest `conservative’ and `sensible’ have opposite meanings. You can find a popular account of it here, from which I have stolen this illustration of the tension:

A chart showing the two differing results for the Hubble constant – The expansion rate of the universe (in km/s/Mpc)
Result 1: 67.8 ± 0.9 Cosmic microwave background
Result 2: 73.52 ± 1.62 Cosmic distance ladder

Anyway, I have a poll that has been going on for some time about whether this tension is anything to be excited about, so why not use this opportunity cast your vote?