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Anisotropy in the cosmic acceleration inferred from supernovae

Mohamed Rameez

TL;DR

This work reevaluates the evidence for cosmic acceleration by analyzing large Type Ia SN catalogs within a tilted-Universe framework, arguing that local bulk flows and relativistic effects can mimic acceleration. Using a maximum-likelihood approach, it finds a strong, directionally dependent signal in the deceleration parameter $q$, aligned near the CMB dipole and extending to $z\lesssim0.1$, while highlighting how standard SN Ia corrections and the Cosmic Rest Frame assumptions can artificially induce or erase such anisotropies. The authors critique SN standardisation procedures and peculiar-velocity corrections, suggesting that the CP-based ΛCDM model may be premature and that data processing choices strongly influence inferred acceleration. They advocate CRF-free, blinded analyses and foresee Rubin-LSST as a crucial test, potentially enabling a >5σ detection of a scale-dependent dipolar modulation in the local expansion rate. The work thus emphasizes the need to model local inhomogeneities and relativistic effects before attributing observations to dark energy, with significant implications for cosmology’s standard paradigm.

Abstract

Under the assumption that they are standard(isable) candles, the lightcurves of Type Ia supernovae have been analyzed in the framework of the standard Friedmann-Lemaître-Robertson-Walker cosmology to conclude that the expansion rate of the Universe is accelerating due to dark energy. While the original claims in the late 1990s were made using overlapping samples of less than 100 supernovae in total, catalogues of nearly 2000 supernovae are now available. In light of recent developments such as the cosmic dipole anomaly and the larger than expected bulk flow in the local Universe (which does not converge to the Cosmic Rest Frame), we analyze the newer datasets using a Maximum Likelihood Estimator and find that the acceleration of the expansion rate of the Universe is unequivocally anisotropic. The associated debate in the literature highlights the artifices of using supernovae as standardisable candles, while also providing deeper insights into a consistent relativistic view of peculiar motions as departures from the Hubble expansion of the Universe. The effects of our being `tilted observers' embedded in a deep bulk flow may have been mistaken for cosmic acceleration.

Anisotropy in the cosmic acceleration inferred from supernovae

TL;DR

This work reevaluates the evidence for cosmic acceleration by analyzing large Type Ia SN catalogs within a tilted-Universe framework, arguing that local bulk flows and relativistic effects can mimic acceleration. Using a maximum-likelihood approach, it finds a strong, directionally dependent signal in the deceleration parameter , aligned near the CMB dipole and extending to , while highlighting how standard SN Ia corrections and the Cosmic Rest Frame assumptions can artificially induce or erase such anisotropies. The authors critique SN standardisation procedures and peculiar-velocity corrections, suggesting that the CP-based ΛCDM model may be premature and that data processing choices strongly influence inferred acceleration. They advocate CRF-free, blinded analyses and foresee Rubin-LSST as a crucial test, potentially enabling a >5σ detection of a scale-dependent dipolar modulation in the local expansion rate. The work thus emphasizes the need to model local inhomogeneities and relativistic effects before attributing observations to dark energy, with significant implications for cosmology’s standard paradigm.

Abstract

Under the assumption that they are standard(isable) candles, the lightcurves of Type Ia supernovae have been analyzed in the framework of the standard Friedmann-Lemaître-Robertson-Walker cosmology to conclude that the expansion rate of the Universe is accelerating due to dark energy. While the original claims in the late 1990s were made using overlapping samples of less than 100 supernovae in total, catalogues of nearly 2000 supernovae are now available. In light of recent developments such as the cosmic dipole anomaly and the larger than expected bulk flow in the local Universe (which does not converge to the Cosmic Rest Frame), we analyze the newer datasets using a Maximum Likelihood Estimator and find that the acceleration of the expansion rate of the Universe is unequivocally anisotropic. The associated debate in the literature highlights the artifices of using supernovae as standardisable candles, while also providing deeper insights into a consistent relativistic view of peculiar motions as departures from the Hubble expansion of the Universe. The effects of our being `tilted observers' embedded in a deep bulk flow may have been mistaken for cosmic acceleration.

Paper Structure

This paper contains 11 sections, 2 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Relativistic effects of the bulk flow
  3. Evidence for anisotropy of cosmic acceleration
  4. The Dialectics of Nature
  5. Issue 1 : Sample and Redshift dependence of SNe Ia standardisation
  6. Issue 2 : Peculiar velocity 'corrections' and a shell crossing singularity
  7. The sculpting of an elephant
  8. Anisotropy in the Pantheon+ compilation
  9. Peculiar velocities, the fitting problem in Cosmology and the myth of the Cosmic Rest Frame
  10. Discussion and Conclusions
  11. The future: Rubin-LSST and DESC

Figures (3)

  • Figure 1: The shift in the magnitudes of Pantheon+ SNe Ia caused by the applied peculiar velocity corrections ($\Delta\mu = {5v}/{\mathrm{log}(10)cz}$) versus redshift. The signal for cosmic acceleration, viz. the 0.15 mag dimming of high $z$ SNe Ia w.r.t. the low $z$ ones, is shown for comparison as a black horizontal line.
  • Figure 2: Dipole in the Hubble expansion rate, extracted for 17 distinct redshift shells each containing 100 SNe Ia from the Pantheon+ compilation. The redshift range in which the SH0ES measurements are performed is shaded in gray, with its vertical spread indicating the claimed uncertainty on $H_0$. Different colours correspond to redshift corrections for different choices of observer frames and peculiar velocities. See Ref. Sah:2024csa from which this figure is taken, for more details.
  • Figure 3: Dipole in the deceleration parameter, extracted for 17 distinct redshift shells each containing 100 SNe Ia from the Pantheon+ compilation. See Ref. Sah:2024csa from which this figure is taken, for more details.