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Evidence for anisotropy of cosmic acceleration (1808.04597v3)

Published 14 Aug 2018 in astro-ph.CO

Abstract: Observations reveal a bulk flow' in the local Universe which is faster and extends to much larger scales than is expected around a typical observer in the standard $\Lambda$CDM cosmology. This is expected to result in a scale-dependent dipolar modulation of the acceleration of the expansion rate inferred from observations of objects within the bulk flow. From a maximum-likelihood analysis of the Joint Lightcurve Analysis (JLA) catalogue of Type Ia supernovae we find that the deceleration parameter, in addition to a small monopole, indeed has a much bigger dipole component aligned with the CMB dipole which falls exponentially with redshift $z$: $q_0 = q_\mathrm{m} + \vec{q}_\mathrm{d}.\hat{n}\exp(-z/S)$. The best fit to data yields $q_\mathrm{d} = -8.03$ and $S = 0.0262~(\Rightarrow d \sim 100~\mathrm{Mpc})$, rejecting isotropy ($q_\mathrm{d} = 0$) with $3.9\sigma$ statistical significance, while $q_\mathrm{m} = -0.157$ and consistent with no acceleration ($q_\mathrm{m} = 0$) at $1.4\sigma$. Thus the cosmic acceleration deduced from supernovae may be an artefact of our being non-Copernican observers, rather than evidence for a dominant component ofdark energy' in the Universe.

Citations (173)

Summary

  • The paper demonstrates that cosmic acceleration has a pronounced dipole anisotropy, challenging the isotropic assumption of the standard ΛCDM model.
  • It employs maximum likelihood estimation on JLA Type Ia supernova data to separate monopole and dipole components, showing a dominant dipole effect up to z~0.1.
  • The results imply that local bulk flows may distort our observations of cosmic expansion, calling for a reassessment of dark energy and the Copernican Principle.

Evidence for Anisotropy of Cosmic Acceleration

The paper "Evidence for anisotropy of cosmic acceleration" by Colin, Mohayaee, Rameez, and Sarkar presents an analysis that questions the isotropic expansion of the universe, a key assumption of the standard ΛCDM cosmological model. This paper critically examines the deceleration parameter inferred from the Joint Lightcurve Analysis (JLA) catalog of Type Ia supernovae (SNe Ia), exploring the idea that our observations of cosmic acceleration might be largely influenced by our non-Copernican location within a bulk flow of the universe.

Key Observations and Analysis

The researchers focus on the 'bulk flow'—a coherent large-scale motion of galaxies that appears to extend much farther and faster than expected in the standard ΛCDM framework. Their primary analysis involves a maximum likelihood estimation using the JLA catalog of SNe Ia, which is a widely used tool in cosmology to trace and measure cosmic expansion due to the 'standard candle' nature of these exploding stars.

The team proposes that the apparent cosmic acceleration could be an artifact, influenced by our particular position within a bulk flow. They fit the deceleration parameter not only with a monopole (reflecting overall isotropic expansion) but also introduce a dipole component. The deceleration parameter is modeled as:

q0=qm+qdn^exp(z/S)q_0 = q_\mathrm{m} + \vec{q}_\mathrm{d} \cdot \hat{n} \exp(-z/S)

where qmq_\mathrm{m} is the monopole component, representing isotropic contributions, and qd\vec{q}_\mathrm{d} is the dipole component aligned with the Cosmic Microwave Background (CMB) dipole, indicating anisotropy dependent on direction. The parameter SS characterizes the scale over which this dipole decays with redshift.

Significant Findings

  • The dipole component qdq_\mathrm{d} is estimated at 8.03-8.03, while the monopole qmq_\mathrm{m} is significantly smaller at 0.157-0.157.
  • The decay parameter SS implies that the dipole effect is dominant out to redshift z0.1z \sim 0.1, correlating with scales of around 100 Mpc.
  • The hypothesis of isotropy (qd=0q_\mathrm{d} = 0) is rejected with a statistical significance of 3.9σ, suggesting a substantial directional dependence in the inferred cosmic acceleration.
  • This result contrasts with the standard interpretation of cosmic acceleration, suggesting the deceleration parameter has a redshift and directional dependence potentially caused by the local cosmic flow rather than a cosmological constant or dark energy.

Implications and Future Research

This work has profound implications for our understanding of cosmic acceleration and questions the solidification of dark energy as a necessary component of the universe's composition. If the observed acceleration is an artifact of our unique observer position in a bulk flow, this challenges the Copernican Principle and invites a reevaluation of the ΛCDM model.

Furthermore, the findings encourage a re-examination of previous analyses that did not account for possible anisotropies in the cosmic expansion. The prospect of anisotropic expansion may necessitate revised interpretations of supernova data and possibly the deployment of new methodologies in cosmological inference.

Future studies will need to scrutinize these anisotropic effects more thoroughly, potentially incorporating more expansive and uniform data sets, such as those introduced by surveys following JLA and Pantheon. An explicit understanding of cosmic flows and their imprints on observable parameters remains critical, potentially calling for more complex models that integrate local inhomogeneities and anisotropies into the broader cosmological narrative.

In conclusion, the paper presents a compelling argument for directional dependence in the observations of cosmic acceleration, pushing the boundaries of how cosmologists interpret supernova data and the fundamental cosmological principle of isotropy.

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