Pantheon+ supernovae corrected for progenitor age indicate the universe is decelerating

Technical Summary: Pantheon+ Supernovae Corrected for Progenitor Age Indicate the Universe is Decelerating

Problem Statement
The standard cosmological model ($\Lambda$CDM) posits that the Universe is undergoing isotropic accelerated expansion, driven by dark energy or a Cosmological Constant. This inference relies heavily on Type Ia supernovae (SNe Ia) acting as standard candles, under the assumption that their intrinsic properties do not evolve with redshift. However, recent analyses suggest that the inferred acceleration may be an artifact of the observer's motion within a local bulk flow, manifesting as a dipole anisotropy in the deceleration parameter ($q_0$) rather than a global monopole. Furthermore, emerging evidence indicates a correlation between SNe Ia luminosity and the age of their progenitor stellar populations, suggesting a redshift-dependent bias in standard candle calibration that has not been fully accounted for in previous cosmographic analyses.

Methodology
The authors re-analyze the Pantheon+ catalogue of 1,701 SNe Ia using a statistically principled framework (Sah et al. 2025) that models the deceleration parameter as a combination of a monopole ($q_m$) and a dipole ($\mathbf{q}_d$) aligned with the local bulk flow, decaying over a characteristic scale $S$.

The core methodological innovation is the application of a redshift-dependent correction to the apparent magnitudes of the supernovae to account for progenitor age evolution. Following Son et al. (2025), the authors modify the distance modulus formula:
$$\mu_{SN} = m_B - M + \alpha x_1 - \beta c - \Delta m(z)$$
where the correction term is defined as $\Delta m(z) = \Delta \text{age}(z) \times 0.030 \text{ mag Gyr}^{-1}$. The term $\Delta \text{age}(z)$ represents the change in the mean progenitor age relative to $z=0$, derived from the Supernova progenitor-age distribution (SPAD) by convolving the delay-time distribution with the cosmic star formation history.

The analysis employs a Maximum Likelihood Estimator (Nielsen et al. 2016) on the corrected apparent magnitudes ($m^*_B$). The authors impose a redshift cut of $z_{hel} \leq 0.8$ to ensure the convergence of the cosmographic Taylor expansion of the luminosity distance. They test the model across four reference frames: Heliocentric, Local Group (LG), Cosmic Microwave Background (CMB), and the Hubble Diagram (CMB frame with peculiar velocity corrections). The dipole direction is initially fixed to the CMB dipole but is noted to remain consistent when allowed to vary.

Key Contributions

1. Application of Progenitor Age Correction: The paper applies the first cosmographic analysis of the Pantheon+ sample that explicitly corrects for the luminosity evolution driven by progenitor age, utilizing the slope of $0.030 \text{ mag Gyr}^{-1}$ established by recent studies (Chung et al. 2025; Son et al. 2025).
2. Decoupling Dipole and Monopole Effects: The study demonstrates that correcting for progenitor age leaves the local dipole anisotropy of $q_0$ (aligned with the bulk flow) essentially unchanged, while significantly altering the global monopole component.
3. Robustness Checks: The authors verify that their results are insensitive to the assumed cosmological model used to derive progenitor ages (testing $\Omega_\Lambda=0.73$ vs. $\Omega_\Lambda=0$) and remain consistent when varying the redshift cut (e.g., $z < 0.5$).

Results

• Shift in Monopole ($q_m$): Without age corrections, the monopole component of the deceleration parameter is negative (indicating acceleration) in most frames, consistent with $\Lambda$CDM expectations. Upon applying the progenitor age correction, the monopole shifts to positive values across all tested frames. For instance, in the Hubble Diagram (HD) frame, $q_m$ shifts from $-0.14$ to $+0.21$.
• Stability of Dipole ($\mathbf{q}_d$): The amplitude and direction of the dipole component remain statistically unchanged after the correction. The dipole remains aligned with the local bulk flow and decays at a scale of $z \sim 0.01$ ($\approx 30 h^{-1}$ Mpc), consistent with a local kinematic effect rather than a global cosmological phenomenon.
• Deceleration Indication: The corrected data shows no indication of accelerated expansion. The results are consistent with a currently decelerating Universe.
• Frame Independence: The conclusion that the Universe is decelerating holds true in the Heliocentric, Local Group, CMB, and Hubble Diagram frames, though the specific numerical values of the monopole vary slightly between frames.

Significance and Claims
The paper claims that the evidence for isotropic cosmic acceleration, derived from SNe Ia, may be illusory when two factors are considered: the observer's embedding in a local bulk flow and the redshift-dependent evolution of SNe Ia progenitor ages.

The authors assert that once the progenitor age bias is corrected, the monopole component of the deceleration parameter becomes positive, indicating deceleration. Consequently, they conclude there is no evidence for an isotropic accelerated expansion of the Universe that can be ascribed to a Cosmological Constant ($\Lambda$) or general dark energy. The observed "acceleration" in previous analyses is attributed to a combination of uncorrected progenitor age evolution and the dipolar modulation caused by local bulk flows. The authors note that while the existence of the progenitor age bias is still debated (citing Wiseman et al. 2026), the application of this correction fundamentally alters the cosmological interpretation of the Pantheon+ data.