PantheonPlus: Cosmology with Type Ia Supernovae

• PantheonPlus is a comprehensive, cross-calibrated Type Ia supernova sample spanning redshifts from 0.001 to 2.26 for precise cosmological measurements.
• It integrates diverse surveys and employs advanced methods such as SuperCal-Fragilistic recalibration and SALT2 retraining to reduce systematic uncertainties.
• The dataset underpins robust constraints on cosmological parameters, including Ωm, w0, and H0, while distinguishing between relative-distance and calibrated distance ladder analyses.

Searching arXiv for PantheonPlus and closely related primary sources. Pantheon+—commonly written PantheonPlus—is the state-of-the-art compilation and analysis of Type Ia supernovae for cosmology, extending the original Pantheon sample into a larger, cross-calibrated, and systematically propagated Hubble-diagram dataset. The original analysis contains 1701 light curves of 1550 distinct SNe Ia spanning $z=0.001$ to $2.26$, and combines a retrained SALT2 light-curve model, an unbinned covariance treatment, and direct integration with Cepheid-calibrated SH0ES hosts when an absolute distance ladder is required (Brout et al., 2022). In subsequent usage, a distinction is usually made between PantheonPlus as a relative-distance supernova compilation and PantheonPlusSH0ES as the Cepheid-calibrated joint likelihood that directly constrains $H_0$ (Gsponer et al., 2023).

1. Definition, scope, and data content

PantheonPlus is a heterogeneous but homogenized SN Ia compilation built from low-redshift samples, SDSS-II, SNLS, HST high-redshift samples, PS1, DES 3-year, Foundation, and related inputs, with the Pantheon+ analysis describing 1550 distinct SNe Ia represented by 1701 cosmologically usable light curves over $0.001Brout et al., 2022). A key novelty of the release is the explicit inclusion of SNe in Cepheid-calibrated host galaxies, enabling a single likelihood that links local distance calibrators, nearby SNe, and Hubble-flow SNe within one covariance framework (Brout et al., 2022).

The compilation is used in two closely related but operationally distinct ways. In the first, PantheonPlus functions as a relative-distance Hubble diagram: the absolute magnitude is marginalized, and the dataset constrains the shape of $H(z)$ and parameters such as $\Omega_m$. In the second, often denoted PantheonPlusSH0ES, Cepheid host distances calibrate the SN absolute magnitude and convert the Hubble diagram into a direct late-time constraint on $H_0$ (Gsponer et al., 2023). Later cosmological analyses often apply a $z>0.01$ cut to suppress peculiar-velocity contamination, leaving 1590 light curves in one implementation and 1580 likelihood data points in another (Feng et al., 27 Oct 2025, Gsponer et al., 2023).

This distinction is not semantic. PantheonPlus alone is primarily a precision relative-distance probe, whereas PantheonPlusSH0ES is a full distance-ladder likelihood. Much of the apparent diversity in later cosmological inferences traces to which of these two objects is being used.

2. Cross-calibration, light-curve modeling, and systematic control

A central technical pillar of PantheonPlus is the SuperCal-Fragilistic recalibration program, which solves simultaneously for offsets in 25 photometric systems and 105 filters and yields a full calibration covariance matrix rather than survey-by-survey offset estimates (Brout et al., 2021). This recalibration incorporates the updated HST CALSPEC flux scale, whose change relative to earlier implementations is on the order of $1.5\%$ over $\Delta\lambda \sim 4000\,\text{\AA}$ (Brout et al., 2021).

The recalibration is coupled to a retraining of the SALT2 light-curve model, producing the SALT2-B22 surface used in the Pantheon+ cosmology analysis (Brout et al., 2022). The paper on the calibration pipeline shows that the change in calibration together with SALT2 retraining causes a net distance-modulus drift of $2.26$0 mag over $2.26$1, establishing that recalibration and light-curve-model retraining cannot be treated as separable corrections (Brout et al., 2021).

PantheonPlus also adopts the BS21 dust-plus-intrinsic-scatter model with host-dependent color distributions and propagates calibration, selection, redshift, Milky Way extinction, peculiar-velocity, and training uncertainties into an unbinned covariance matrix (Brout et al., 2022). The resulting calibration-only contribution to dark-energy constraints is quantified as $2.26$2, roughly half the sample statistical uncertainty, while the SN calibration contribution to the SH0ES $2.26$3 error budget is below $2.26$4 (Brout et al., 2021). This systematic program underlies the factor-of-two improvement in cosmological constraining power on $2.26$5 relative to the original Pantheon analysis (Brout et al., 2022).

3. Statistical construction of the Hubble diagram

PantheonPlus is built on SALT2 light-curve fitting, with the flux-amplitude parameter converted to a pseudo-$2.26$6-band magnitude through

$2.26$7

Distances are then inferred with a modified Tripp relation,

$2.26$8

where $2.26$9 and $H_0$0 describe stretch and color standardization, $H_0$1 is the fiducial SN absolute magnitude, $H_0$2 is the simulation-based bias correction, and $H_0$3 is the host-mass correction (Brout et al., 2022). In the baseline BS21 configuration, Pantheon+ reports $H_0$4 and $H_0$5 (Brout et al., 2022).

Cosmological inference uses the usual luminosity-distance relation

$H_0$6

and the model distance modulus

$H_0$7

The SN-only likelihood is a Gaussian in the residual vector,

$H_0$8

with an unbinned $H_0$9 covariance matrix that preserves survey-to-survey correlations and reduces the information loss associated with redshift binning (Brout et al., 2022). Pantheon+ explicitly argues that binning inflates the systematic error on $0.001Brout et al., 2022).

For PantheonPlusSH0ES, the residual vector is modified so that Cepheid-host SNe are compared to host-distance calibrators rather than to a cosmological model,

$0.001

and the covariance includes both the SN and Cepheid blocks (Brout et al., 2022). This is the formal step that turns PantheonPlus from a relative-distance compilation into a calibrated distance ladder.

4. Cosmological constraints in the original Pantheon+ analysis

The original Pantheon+ cosmology paper reports that SNe Ia alone give

$0.001

in flat $0.001Brout et al., 2022). In flat $0.001

$0.001

while Pantheon+ alone in flat $0.001

$0.001

both consistent with a cosmological constant (Brout et al., 2022).

With Cepheid calibration included, PantheonPlusSH0ES gives

$0.001

in flat $H(z)$0CDM and

$H(z)$1

in flat $H(z)$2CDM (Brout et al., 2022). The persistence of this calibrated $H(z)$3 across background parameterizations is one reason PantheonPlusSH0ES became central to late-time distance-ladder analyses.

When combined with CMB and BAO, Pantheon+ substantially sharpens dark-energy constraints. The original paper quotes

$H(z)$4

for flat $H(z)$5CDM and

$H(z)$6

for flat $H(z)$7CDM when Pantheon+ is combined with CMB and BAO (Brout et al., 2022). In the same analysis, the non-accelerating point $H(z)$8 is excluded at $H(z)$9 using SNe alone, and the paper concludes that systematic uncertainties in the SN distance ladder comprise less than one third of the total $\Omega_m$0 uncertainty and cannot explain the Hubble tension (Brout et al., 2022).

5. PantheonPlus as a baseline in post-2022 cosmology

After 2022, PantheonPlus became a standard late-time baseline in a wide range of beyond-$\Omega_m$1CDM analyses. In full-shape early-dark-energy studies, the baseline combination

$\Omega_m$2

already limits the EDE fraction to

$\Omega_m$3

with

$\Omega_m$4

whereas replacing PantheonPlus by the calibrated PantheonPlusSH0ES likelihood drives the preferred solution to

$\Omega_m$5

with

$\Omega_m$6

depending on nuisance priors (Gsponer et al., 2023). This illustrates how the difference between relative-distance PantheonPlus and calibrated PantheonPlusSH0ES propagates directly into EDE inference.

PantheonPlus is also routinely used as a conservative cross-check against DESY5. In a PEDE model with a free dark-matter equation of state, the combination CMB+DESI+PantheonPlus yields

$\Omega_m$7

whereas CMB+DESI+DESY5 gives

$\Omega_m$8

so the preference for $\Omega_m$9 weakens from about $H_0$0 to about $H_0$1 when PantheonPlus is used (Li et al., 11 Jun 2025). A broader review of evolving dark energy reaches a similar conclusion: combinations that simultaneously include PantheonPlus and SDSS BAO significantly weaken the preference for CPL evolution, with CMB+PantheonPlus giving only $H_0$2 and CMB+DESI+PantheonPlus giving $H_0$3, lower than comparable DESY5 or Union3 combinations (Giarè et al., 14 Feb 2025).

The same pattern appears in recent distance-based tests with FRBs, CC, BAO, and CMB. Using FRB+PantheonPlus+DESI+CC+CMB, one analysis reports

$H_0$4

but finds weaker Bayesian support for extended models than when DESY5 is substituted (Feng et al., 27 Oct 2025). This suggests that PantheonPlus often functions as a comparatively conservative SN anchor in late-time model selection, even when its parameter constraints remain precise.

6. Model-independent applications and continuing significance

PantheonPlus has also become a standard input for model-independent reconstructions. A Gaussian-process reconstruction using PantheonPlus+SH0ES+GRB+OHD+DESI finds that the reconstructed dimensionless Hubble parameter $H_0$5 is consistent with $H_0$6CDM at the $H_0$7 level for $H_0$8, while the mean reconstructed dark-energy equation of state crosses $H_0$9 at

$z>0.01$0

a suggestive but not decisive deviation from a cosmological constant (Zhang et al., 4 Nov 2025). In a model-independent test of the cosmic distance duality relation, PantheonPlus+DESI DR2+CC gives, for the linear parametrization $z>0.01$1,

$z>0.01$2

supporting the CDDR within $z>0.01$3 and yielding results consistent with DES Dovekie (Wu, 29 Mar 2026). In cosmography with strong-lensing time delays and DESI-DR2, the combination SGL+PantheonPlus+DESI-DR2 gives

$z>0.01$4

consistent with flatness and with $z>0.01$5CDM kinematics at current precision (Kumar et al., 2 Nov 2025). In a Buchert-backreaction setting fitted only to PantheonPlus+SH0ES, the inferred optical depth to reionization is

$z>0.01$6

with a modest reduction of the Hubble tension (Pandey et al., 15 Apr 2026).

These later applications show that PantheonPlus is more than a large SN compilation. It is simultaneously a precision relative-distance dataset, a calibrated distance ladder when coupled to SH0ES, and a benchmark late-time geometric probe for model-independent and model-extended cosmology. A plausible implication is that much of its continuing importance lies in this dual role: PantheonPlus supplies stringent low-redshift distance information, while the choice between its uncalibrated and SH0ES-calibrated forms determines whether the analysis is testing late-time geometry alone or the full absolute cosmic distance scale.