Carnegie–Chicago Hubble Program (CCHP) Overview

• Carnegie–Chicago Hubble Program is a coordinated effort that integrates multiple stellar distance indicators, including Cepheids, TRGB, and JAGB, to recalibrate the extragalactic distance scale.
• It employs a multi-wavelength, space-based approach using HST and JWST data to minimize systematics such as metallicity effects, crowding, and extinction.
• The program advances SN Ia standardization and refines H0 measurements, contributing critical insights into the Hubble tension and cosmological models.

The Carnegie–Chicago Hubble Program (CCHP) is a coordinated effort to recalibrate the extragalactic distance scale and achieve a sub-percent precision measurement of the Hubble constant ($H_0$) using multiple, independent stellar distance indicators in nearby galaxies. CCHP integrates Population I (Cepheid variables), Population II (RR Lyrae variables and the Tip of the Red Giant Branch, TRGB), and, more recently, the J-region Asymptotic Giant Branch (JAGB) as standard candles, systematically leveraging data from both the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST). The program is explicitly designed to address the principal systematics—metallicity, crowding/blending, and extinction—limiting previous determinations of $H_0$, and to provide robust cross-checks across calibration rungs in the extragalactic distance ladder (Madore et al., 1 Jun 2025, Hoyt et al., 14 Mar 2025, Freedman et al., 12 Aug 2024, Freedman et al., 2019).

1. Scientific Aims and Rationale

CCHP seeks a highly precise ($\pm$1%–2%) local measurement of $H_0$ by independently calibrating three distance indicators—TRGB, JAGB, and Cepheids—within the same set of SN Ia host galaxies, all tied to a geometric anchor (the megamaser host NGC 4258 with $\mu_0 = 29.397 \pm 0.033$ mag). By conducting all calibrations within the same photometric system using contemporary space-based imaging, CCHP minimizes cross-instrument zero-point uncertainties. The inclusion of multiple independent rungs addresses systematic discrepancies observed in previous single-method measurements and allows a differential assessment of the so-called “Hubble tension” between local and cosmological ($\Lambda$CDM) expansion rates (Freedman et al., 12 Aug 2024).

2. Multi-Rung Distance Ladder: Methodologies and Calibration

Cepheid Variables and Metallicity Effects

CCHP pioneered a multi-wavelength approach to Cepheid period–luminosity (PL) calibration, targeting the propagation of metallicity (heavy element abundance) through band-dependent offsets in observed brightness. PHOENIX static stellar-atmosphere models are used to map the bandpass-dependent metallicity responses of Cepheid SEDs across [Fe/H] from $-2$ to $+0.5$ dex, covering $0.1$–$5\,\mu$m. These models demonstrate:

• Very large metallicity terms ($\sim$0.67 mag/dex) in the ultraviolet (U band), progressively decreasing toward the optical ($\sim$–0.02 mag/dex in $V$, $I$), and becoming negligible in the near-IR ($\sim$0.00 mag/dex in $J$).
• The 4.5 $\mu$m band shows a metallicity/cooling-dependent CO absorption feature, offset up to –0.07 mag/dex (Madore et al., 1 Jun 2025, Scowcroft et al., 2016).

Empirical tests confirm metallicity sensitivities in Cepheid PL relations are $\lesssim$0.02–0.03 mag (typically $<$0.5% in distance), undetectable given current single-star photometric accuracy. The “Wesenheit” hybrid PL relation, $W(J,3.6) \equiv J - 0.242\,(J - [3.6])$, is specially constructed to be insensitive to both reddening and metallicity over a 2.5 dex range in [Fe/H], enabling adoption as the CCHP’s primary Cepheid distance indicator (Madore et al., 1 Jun 2025).

Tip of the Red Giant Branch (TRGB)

The TRGB marks the luminosity at which low-mass, metal-poor red giants ignite core helium, producing a standard-candle in both the $I$ and $J$ bands. The TRGB method is nearly insensitive to metallicity for $[M/H] \lesssim -0.7$ (Freedman et al., 2019). CCHP employs both HST (F606W/F814W) and JWST (F115W, F356W) data to detect the TRGB discontinuity in the stellar halo regions of host galaxies, minimizing crowding and internal extinction uncertainties. Calibration employs a color-slope correction in the near-IR:

$M_{F115W}^{\text{TRGB}}(c) = M_0 + \beta [c - c_0]$

with $M_0 = -5.208 \pm 0.036$ mag at $c_0 = 1.411$ mag, and $\beta = -0.863 \pm 0.206$ mag mag$^{-1}$, tied to the maser distance of NGC 4258 (Hoyt et al., 14 Mar 2025).

Agreement between JWST and HST TRGB distances is $<$0.01 mag on average ($\sim$0.1%), with $\sim$0.08 mag rms (4%) per galaxy, confirming both are equally precise. Systematic errors in the TRGB zero point are now $\lesssim$0.04 mag ($\sim$2%).

J-region Asymptotic Giant Branch (JAGB)

The JAGB method uses carbon-rich TP-AGB stars as one-epoch, color-selected near-IR distance indicators. Empirically, $$M_{J, \mathrm{JAGB}} = -5.99 \pm 0.05_\text{stat} \pm 0.04_\text{sys}$$ mag anchored at NGC 4258. Optimal field selection minimizes crowding by defining annuli in each host where the observed modal $J$-band magnitude stabilizes (Lee et al., 6 Aug 2024). Distances from JAGB are consistent with TRGB at the 1% level and agree with Cepheid distances at the just over 1% level.

SN Ia Calibration and $H_0$ Determination

TRGB-, JAGB-, and Cepheid-based distances to SN Ia hosts are used to calibrate the absolute magnitude of SNe Ia, which, after standardization, are used to determine $H_0$ via

$H_0 = 10^{0.2 [m_B - M_B^0 + 5 - DM(z)]}$

where $M_B^0$ is the mean absolute $B$-band magnitude of local calibrators and $a_B$ is the Hubble-flow SN intercept (Hoyt et al., 14 Mar 2025, Freedman et al., 12 Aug 2024).

3. Precision, Systematics, and Error Budget

Total $H_0$ uncertainty for the program’s key “rungs” is now dominated by systematic errors: TRGB zero-point calibration (1.5–2%), Cepheid metallicity scaling ($\gamma$), anchor distance (NGC 4258 maser), and SNe Ia standardization (host-mass correction, dust law, intrinsic scatter). The CCHP reports:

• Combined (Cepheids+TRGB+JAGB): $$H_0 = 69.96 \pm 1.05_\text{stat} \pm 1.12_\text{sys}$$ km\,s$^{-1}$\,Mpc$^{-1}$ (Freedman et al., 12 Aug 2024).
• TRGB-only (24 calibrators): $H_0$ typically $69$–$70$ km\,s$^{-1}$\,Mpc$^{-1}$ with $\simeq$1.5% total uncertainty (Hoyt et al., 14 Mar 2025).
• JAGB-only: $$H_0 = 67.80 \pm 2.17_\text{stat} \pm 1.64_\text{sys}$$ km\,s$^{-1}$\,Mpc$^{-1}$ (Lee et al., 6 Aug 2024).
• Cepheids: $$H_0 = 72.05 \pm 1.86_\text{stat} \pm 3.10_\text{sys}$$ km\,s$^{-1}$\,Mpc$^{-1}$ (Freedman et al., 12 Aug 2024).

Systematic uncertainties now dominate over statistical uncertainties. Recommended future work includes expanding the number of SN Ia calibrators (particularly in crowded hosts), independently calibrating all rungs with Gaia parallaxes and additional geometric anchors, and harmonizing SN Ia standardization.

4. Empirical Validation and Comparison of Distance Scales

A central objective of CCHP is validating mutual consistency among all stellar distance indicators. For systems with both Cepheid and TRGB distances, residuals are $\lesssim$0.05 mag (1–2%) (Jang et al., 2017, Beaton et al., 2019, Hatt et al., 2018), and for JAGB vs. TRGB the residual is $+0.017 \pm 0.031$ mag, i.e., below 1% (Lee et al., 6 Aug 2024).

Null results in detecting metallicity dependence of the optical and near-IR Cepheid PL relations at the $\lesssim$0.02–0.03 mag level are explained by the low amplitude of SED-based offsets, which require photometric precision at the 0.01 mag level or better to detect (Madore et al., 1 Jun 2025). The cross-method agreement with SH0ES and Planck/BAO $H_0$ determinations is within $1.7\sigma$ (Planck) and $3\sigma$ (SH0ES) (Hoyt et al., 14 Mar 2025, Freedman et al., 12 Aug 2024, Freedman et al., 2019).

5. Technical Advances: JWST, Algorithms, and Crowding Mitigation

The implementation of JWST/NIRCam imaging provides unprecedented resolution ($0.04''$ FWHM at $1.15\,\mu$m), sharply reducing blending and crowding, particularly in high-surface-brightness disks (Freedman et al., 12 Aug 2024). Specific algorithmic innovations include:

• 2D Hess diagram–based TRGB edge detection with elliptical contour analysis in the infrared (Hoyt et al., 14 Mar 2025).
• Radial convergence selection for JAGB mode stabilization (Lee et al., 6 Aug 2024).
• Blinded analysis pipelines (randomized photometric zero points) to prevent unconscious scalar bias in $H_0$ (Freedman et al., 12 Aug 2024).

Systematic testing—varied smoothing scales, field selection (e.g., masking spiral arms in JAGB determination), and artificial-star injections—ensures robustness against photometric biases.

6. Impact, Cosmological Context, and Future Prospects

CCHP’s results currently situate the local $H_0$ calibration between Planck/BAO ($\sim$67–68 km\,s$^{-1}$\,Mpc$^{-1}$) and SH0ES ($\sim$73 km\,s$^{-1}$\,Mpc$^{-1}$), with the multi-rung CCHP average at $H_0 \simeq 70$ km\,s$^{-1}$\,Mpc$^{-1}$. There is no compelling statistical evidence from CCHP alone requiring new cosmological physics beyond $\Lambda$CDM (Freedman et al., 12 Aug 2024).

The next phase aims to break the 1% accuracy barrier via:

• Expanded JWST and Roman Space Telescope imaging of SN Ia hosts and geometric anchors.
• Gaia DR3/DR4 zero-point anchoring of both TRGB and RR Lyrae.
• Increased spectrophotometric sample sizes for SNe Ia (e.g., with LSST) and advanced standardization modeling to reduce intrinsic SN scatter (Freedman et al., 12 Aug 2024).

CCHP’s independent, multi-method approach is now a critical reference in resolving the Hubble tension, enabling direct systematics cross-checks throughout the local extragalactic distance ladder. The methodology and data products also serve as a foundation for studying faint stellar systems (e.g., ultra-faint dwarfs) and galactic substructure in resolved halos (Lee et al., 2017).