A new analysis of Type Ia supernovae from the Pantheon+ catalogue challenges the established understanding of cosmic expansion, revealing evidence that the universe may be decelerating. Researchers at the Tata Institute of Fundamental Research, led by Animesh Sah and Mohamed Rameez, found that the corrections shift the monopole component of q0 to positive values, indicating deceleration and suggesting a departure from the previously assumed accelerating expansion. Their work builds on previous observations of a strong dipole anisotropy in q0, aligned with the bulk flow, which diminishes by a redshift of approximately 0.1, indicating local motion plays a significant role in observed cosmic expansion. This shift to positive values for the monopole component of q0, while leaving the local dipole component essentially unchanged, suggests supernova evolution must be accounted for when measuring cosmological parameters.
Pantheon+ Supernovae and Dipole Anisotropy of q0
Researchers Animesh Sah, Mohamed Rameez, and Subir Sarkar, from institutions including the Tata Institute of Fundamental Research and the University of Oxford, have demonstrated that incorporating corrections for the age of progenitor stars significantly alters interpretations of cosmic expansion. This challenges the established understanding based on observations initially interpreted as indicative of accelerating expansion. The team’s methodology centers on a detailed examination of the deceleration parameter, denoted as q0, which describes the rate of change in the universe’s expansion velocity. Previous analyses, including those of the SDSS-II/SNLS3 Joint Lightcurve Analysis, hinted at a dipolar anisotropy in q0, aligning with the direction of the local bulk flow of galaxies. However, these findings were criticized by Rubin and Heitlauf, who argued for a different approach involving boosting to the CMB frame and applying standard peculiar velocity corrections.
Sah and colleagues maintain that the non-isotropic distribution of matter, as observed in quasars and radio galaxies, renders that criticism invalid, citing studies by Secrest et al., Wagenveld et al., and Böhme et al. as supporting evidence. Their recent work with the Pantheon+ catalogue further solidified this claim, showing a pronounced dipole that diminishes by a redshift of approximately 0.1. Applying corrections for progenitor age shifts the monopole component of q0 to positive values, while leaving the local dipole component essentially unchanged. The team utilized a correction factor, Δm(z), proposed by Son et al. based on the work of Chung et al., which accounts for the relationship between supernova luminosity and the age of the star from which it originated. This correction is based on the premise that younger progenitor stars tend to produce fainter supernovae at a given luminosity parameter.
While acknowledging ongoing debate, Wiseman et al. recently questioned the need for these corrections, the team’s analysis demonstrates that accounting for progenitor age fundamentally alters the inferred cosmological parameters. This finding implies that the perceived cosmic acceleration may be a local phenomenon, a relativistic effect stemming from our universe’s anomalous bulk flow rather than a universal constant.
Progenitor Age-Luminosity Correlation in Type Ia Supernovae
The prevailing cosmological model, built upon observations of Type Ia supernovae, currently posits an accelerating expansion of the universe driven by dark energy. However, recent analyses are prompting a re-evaluation of this understanding, focusing on subtle effects within the supernova data itself. Specifically, researchers are investigating the potential impact of progenitor age, the age of the star systems giving rise to these crucial standard candles, on measurements of the universe’s expansion history. The assumption that supernova properties remain constant with redshift has long been a cornerstone of cosmological calculations, but evidence suggests this may not be entirely accurate.
This anisotropy suggests that our location in the universe, and our peculiar motion within it, plays a substantial role in how we perceive cosmic expansion. “Our recent redshift tomographic analysis showed that locally q0 has a strong dipole anisotropy aligned approximately with the bulk flow, and only a small monopole component remains at distances exceeding a few hundred Mpc,” the team reports. This local motion, they argue, could be masking the true nature of cosmic acceleration. The team’s methodology incorporates corrections for progenitor age-dependent luminosity evolution, building on work by Chung et al., who identified a correlation between supernova luminosity and host galaxy age, and applying a redshift-dependent bias correction to the Phillips-Tripp formula, a standard tool for calculating supernova distances.
This correction, denoted as Δm(z), accounts for the tendency of supernovae with younger progenitors to appear fainter at a given luminosity. The value of the slope, 0.03 mag Gyr-1, was taken as an average of measurements made by Chung et al. from samples of Gupta and others and Rose et al. This indicates a decelerating universe, a result that directly challenges the established understanding of cosmic acceleration. However, the local dipole component diminishes by a redshift of approximately 0.1, reinforcing the idea that local motion continues to exert a significant influence on observed expansion rates. The debate surrounding progenitor age corrections is ongoing; a recent study by Wiseman et al. criticizes Chung et al. and Son et al. for not performing host-mass standardization and argues that correcting for these removes any correlation between the Hubble residuals and the progenitor age. Despite these criticisms, the Sah, Rameez, and Sarkar analysis demonstrates the sensitivity of cosmological parameter estimates to subtle effects within the supernova data, highlighting the importance of accounting for supernova evolution when measuring the universe’s expansion history.
Redshift-Dependent Corrections to SNe Ia Distance Moduli
Their work builds on previous analyses suggesting that the observed acceleration of the universe may not be as straightforward as it appears, and that local motions play a significant role in shaping our perception of cosmic expansion. The team’s latest investigation centers on the impact of accounting for the fact that older supernovae populations exhibit different luminosity characteristics than younger ones, a factor often overlooked in standard distance calculations. Previous studies, including those analyzing the SDSS-II/SNLS3 Joint Lightcurve Analysis, have hinted at a directional component to the universe’s expansion. The current analysis extends this work by incorporating progenitor age corrections into the Pantheon+ data, revealing a shift in the monopole component of the deceleration parameter to positive values. The methodology employed by Sah and his team involves a detailed cosmographic expansion of the luminosity distance, incorporating the Hubble rate, deceleration parameter, and jerk to third order in redshift.
The value of the slope, 0.03 mag Gyr-1, was taken as an average of measurements made by Chung et al. from the samples of Gupta and others and Rose et al. While debates continue, with Wiseman et al. criticizing the methodology and arguing for host-mass standardization, the results presented by Sah, Rameez, and Sarkar offer a compelling alternative perspective on the nature of cosmic expansion, suggesting that our interpretation of the universe may be significantly biased by our unique location within it. The team acknowledges the ongoing refinement of these corrections, noting that the value of 0.03 mag Gyr-1 is subject to further investigation.
CMB Frame Isotropy and Bulk Flow Criticism
The quest to understand the universe’s expansion rate continues to yield surprising results, with recent analyses suggesting our cosmic perspective may be fundamentally skewed. Researchers are now focusing on the impact of subtle effects, like the age of progenitor stars within those supernovae, and the implications for interpreting the cosmic microwave background. A key point of contention revolves around the deceleration parameter, denoted as q0. Applying redshift-dependent corrections for progenitor age to the Pantheon+ catalogue, we find that this shifts the monopole component of q0 to positive values, indicating deceleration, while leaving the local dipole component essentially unchanged. This was demonstrated previously for the Pantheon+ compilation, with q0 exhibiting a dipole anisotropy that diminishes by a redshift of approximately 0.1. The assumption that the universe is isotropic in the CMB frame has been a long-standing point of debate.
This invalidates the core assumption underpinning the criticism, reinforcing the possibility of a local bulk flow influencing observed expansion rates. The team acknowledges the ongoing debate surrounding progenitor age corrections. While Wiseman et al. criticize Chung et al. and Son et al. for not performing host-mass standardization, and question the accuracy of estimated progenitor age differences, the current analysis demonstrates the significant impact of even a modest correction. The value of the slope, 0.03 mag Gyr-1, was taken as an average of measurements made by Chung et al. from the samples of Gupta and others and Rose et al. Their analysis models the deceleration parameter as a combination of a monopole and dipole component. The results indicate that accounting for progenitor age is crucial for accurately determining cosmological parameters and understanding the true nature of cosmic expansion.
Impact of Progenitor Age on Deceleration Parameter q0
Conventional cosmological models posit a universe undergoing accelerated expansion, driven by dark energy. However, recent analyses are prompting a reassessment of this established understanding, suggesting that observed acceleration may not be intrinsic but rather a consequence of our location within a substantial cosmic flow. Their work challenges the notion of constant acceleration, indicating that the universe may, in fact, be slowing down after accounting for specific supernova characteristics. This suggests that our motion through the cosmos influences how we perceive expansion. The researchers utilized a correction factor proposed by Son et al. based on the work of Chung et al. to account for the influence of progenitor age on supernova luminosity. This model incorporates a term, Δm(z), representing the redshift-dependent bias arising from the age of the exploding star’s progenitor.
The value of the slope, 0.03 mag Gyr-1, was taken as an average of measurements made by Chung et al. from the samples of Gupta and others and Rose et al. While debates continue, Wiseman et al. criticize the methodology, arguing for the importance of host-mass standardization, the findings underscore the importance of carefully accounting for supernova evolution when determining cosmological parameters. The implications are profound, potentially reshaping our understanding of dark energy and the fundamental nature of cosmic expansion.
Source: https://arxiv.org/abs/2606.09650
