@DScol
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I'm a dad; I’m a professor doing cosmology at Duke U. and I have some opinions about basketball.
Joined May 2009
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Two new papers this last week about important challenge for the JAGB distance indicator: stars selected by usual “J-region” color cuts are not necessarily the carbon-rich AGB stars we thought they were. And two very different approaches are finding the same thing.
How come? It looks like pulsating O-rich AGB stars in metal rich spirals like M101 or the MW appear to produce substantially more silicate dust than their LMC counterparts, reddening them into the “carbon-star” color region. So the same broadband color cut can select different physical populations in different environments.
While this doesn't mean carbon-rich AGB stars can't be useful distance indicators, it now seems not safe to assume they are C-rich AGB stars in spirals like in the LMC. Before treating them as a standard candle to calibrate SN Ia, need to establish that we're actually measuring the same physical population across the distance ladder. From these papers, variability, water bands and metallicity measurements should help.
Awesome getting to spend day with the infinitely curious @McConaughey as well as so many from @NASARoman team and @DukePhysics. What an incredible day and great video Matthew!
Got to talk on NPR and WaPost about NASA Roman launch this Sunday!! I can't believe this is finally here! npr.org/2026/08/28/nx-s1-593… and washingtonpost.com/science/2…
Dan Scolnic retweeted
1/ With NASA's Roman Space Telescope launch 🚀 in T-20 days and our official team jerseys in hand, this is the perfect timing to congratulate my PhD student, @rujutapurohit, on her first-first author paper 🎉
Paper: arxiv.org/abs/2607.08886
Roman Hype: nitter.cf/nasa/status/2082526878…
LIVE: In one month, @NASARoman will lift off to help us study some of the greatest mysteries in the universe! Tune in as NASA experts give an overview of the mission and its status. nitter.cf/i/broadcasts/1qxvvvQRL…
The original MCP analysis, using the 2M++ peculiar-velocity model, gave H0 = 71.8 ± 2.7. Switching to M25 lowers the central value to ~69.7 +/- 2.6, not such a dramatic change. But allowing σv to vanish drives H0 lower and crucially the uncertainty down to ±0.9 km/s/Mpc, roughly 3× more precise than the original megamaser determination.
The paper then concludes that this suggests the Hubble tension may arise from an unidentified systematic in the distance ladder rather than new physics. Conclusion is roughly: six spatially correlated masers + a velocity model whose uncertainty has been dramatically compressed → a ~2σ difference with H0DN → evidence that the cross-checked distance ladder has an unidentified systematic?
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M25 doesn’t just give one peculiar velocity per galaxy. It uses constrained realizations to provide uncertainty and covariance in the reconstructed velocity field, which should be propagated into maser analysis. Instead, the paper uses only the central values and replaces all the rest of the information with a single independent velocity dispersion, σv, inferred from just these 6 masers.
Leaving out those posteriors and external information leaves little leverage on σv, and its driven toward ~0 km/s, which just can't be. The weights of the masers change dramatically, so that even nearby NGC 4258 itself, just outside the Local Group at ~7.6 Mpc away where peculiar velocity is usually considered a large fraction of the Hubble flow, is claimed to measure H0 very precisely.
New H0 paper arxiv.org/abs/2608.06247 which applies sophisticated Manticore (M25) peculiar-velocity reconstruction to the 6 megamaser galaxies from the MCP (P20). Important idea, but leaves out what makes M25 powerful.
Dan Scolnic retweeted
Prof Maria Vincenzi was awarded the New Horizons in Physics Prize last week! Watch our interview where she discusses her research! @brkthroughprize
Had a group goodbye dinner for @space_veggie before they go off and become fancy NASA Goddard scientist. Lauren is leading our supernova photometry pipeline for @NASARoman and will continue this work at Goddard.
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.
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.
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.
Was such an honor to host @ValerieFoushee and hear about all her work on the Space & Aeronautics Subcommittee!
It was great to join @DukeU's SPACE Initiative to discuss the importance of bolstering STEM education and innovation.
As the Ranking Member of the Space & Aeronautics Subcommittee, I am grateful to see our local universities taking steps to support these educational initiatives in the face of Trump's funding cuts.
