Dan Scolnic

When the model includes the actual selection functions and a realistic MW disk prior, the forward model reproduces the observed distributions (heavy black) and yields the same calibration as SH0ES. Bonus insight shown in the paper: because the Cepheid PL relation is very tight, Bayesian and frequentist approaches agree as long as the prior isn’t in strong conflict with the data.

The issue is visible in Fig. 1. The dashed curves show the parallax distribution expected if MW Cepheids followed a single uniform-in-volume (spherical) prior without selection. But the observed Cepheids (two samples, red and green, with different selection functions) clearly don’t look like that, they live in the Galactic disk and are shaped by survey selection (closer, less extinction). If the prior + selection don’t reproduce the observed distributions, the inference gets biased.

Really impressive H0-related paper by Richard Stiskalek et al. arxiv.org/pdf/2603.09880 to make full Bayesian forward model of the Milky Way Gaia+HST Cepheid sample—periods, parallaxes, magnitudes, MW disk geometry, and survey selection all modeled together. Key: Bayesian model must reproduce the data. A recent reanalysis (HM26) that lowered H₀ modeled the MW disk as a sphere and ignored selection, pushing Cepheids farther away. With a realistic disk + selection, the Cepheid calibration and the Hubble tension remain. Thread.

Was such an honor to host @ValerieFoushee and hear about all her work on the Space & Aeronautics Subcommittee!

Duke’s @michaeltroxel received NASA’s Exceptional Public Achievement Medal for leading the OpenUniverse 2024 simulations--one of the most detailed synthetic views of the cosmos ever made. Congratulations!🔭✨ Read more: trinity.duke.edu/news/michael-t…

I love Bayesian analyses — but they’re only as good as their priors. There’s no need to rush to “solve” the Hubble tension without modeling real selection functions.

Similar issues arise on the 2nd and 3rd rungs: Cepheid distances are not volume-distributed due to selection (see Stiskalek+25), and SN calibrator brightness selection offsets a realistic host-distance prior. On the 3rd rung, the SN BBC correction already accounts for volume selection. In this new paper, they point to Desmond+25, but miss that they went the necessary step to model selection - see here:

Thought on new paper arxiv.org/abs/2601.22215 that claims “physical” distance priors shift SH0ES H0 downward by assuming all distance indicators are uniformly distributed in volume. But realistic Bayesian priors must include selection — and it’s clear from the data that selection dominates the effective prior. Neglecting that, not new data, drives the reduction in H0. 🧵

Two important results today in supernova cosmology: a consistent set of host-galaxy stellar masses (Union3.1 led by @taylorjhoyt) and a new Bayesian model that solves for two SN Ia subclasses (UNITY1.8 by me). Both tackle one of our biggest challenges: astrophysical systematics.

We are now collaborating with the @b612foundation, who has studied similar questions, to understand this problem even better and figure out how well we can predict the orbit when we discover these objects. I think some numbers in our paper seem scary, but they are a big improvement of where things have been in the past!

We spend a lot of time trying to understand why we miss ones we miss, and also look at the Argus Array as a complementary high-cadence, shallower depth unit. Basically, VRO will be great, but cannot carry the burden of planetary defense all by itself.

Really proud of my grad student Qifeng Cheng for her first paper "Assessing the Vera Rubin Observatory's Ability to Discover Asteroid Impactors Before They Collide with Earth". arxiv.org/abs/2601.16255 . A thread about @VRubinObs, discovery and warning times...

All in all, multiple aspects of their model and predictions dont match our large SN samples. The universe is accelerating just fine. (end)

Finally, we show that the assumptions that their model is based on (how many SNe are in high mass hosts) doesn’t look anything like the data.

New supernova cosmology ‘rebuttal’ paper led by @astro_pwise addressing viral claim that the universe isn’t accelerating. I discussed this before, but now # of people in community have written more formal response: “Still Accelerating: Type Ia supernova cosmology is robust to host galaxy age evolution” arxiv.org/abs/2601.13785