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The multipole expansion of the local expansion rate
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abstract
We design a new observable, the expansion rate fluctuation $\eta$, to characterize deviations from the linear relation between redshift and distance in the local universe. We also show how to compress the resulting signal into spherical harmonic coefficients in order to better decipher the structure and symmetries of the anisotropies in the local expansion rate. We apply this analysis scheme to several public catalogs of redshift-independent distances, the Cosmicflows-3 and Pantheon data sets, covering the redshift range $0.01<z<0.05$. The leading anisotropic signal is stored in the dipole. Within the standard cosmological model, it is interpreted as a bulk motion ($307 \pm 23$ km/s) of the entire local volume in a direction aligned at better than $4$ degrees with the bulk component of the Local Group velocity with respect to the CMB. This term alone, however, provides an overly simplistic and inaccurate description of the angular anisotropies of the expansion rate. We find that the quadrupole contribution is non-negligible ($\sim 50\%$ of the anisotropic signal), in fact, statistically significant, and signaling a substantial shearing of gravity in the volume covered by the data. In addition, the 3D structure of the quadrupole is axisymmetric, with the expansion axis aligned along the axis of the dipole. Implications for the determination of the $H_0$ parameter are also discussed.
Forward citations
Cited by 6 Pith papers
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Cosmography for a General Spacetime Centred at Arbitrary Redshift
A general-relativistic cosmographic expansion centered at arbitrary redshift is derived and tested on void and overdensity LTB models, showing that its coefficients lose strict local meaning near density gradients.
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Multipolar structure of the local expansion rate from incomplete sky data
CF4 data yield a 3.3σ excess dipole in the local expansion-rate fluctuation field at (l,b)=(290°,-4°)±5°, sourced mainly by z∈[0.03,0.05], with quadrupole/octupole consistent with ΛCDM and no multipole-vector alignments.
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Covariant cosmography in the presence of local structures: comparing exact solutions and perturbation theory
Off-center observers in a spherical LTB overdensity get accurate cosmographic distances up to δc≈2.5 near the structure, while linear perturbation theory is better beyond ~3Rs; a gauge dictionary links the two.
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Anisotropy in Pantheon+ supernovae
A maximum-likelihood analysis of Pantheon+ supernovae finds a statistically significant dipole in the local Hubble expansion rate and in the deceleration parameter, which the authors interpret as evidence against a co...
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Redshift Dependence of $H_0$ Dipole in Pantheon+ Supernovae
A dipole in the locally measured Hubble constant appears at 2-3 sigma in the lowest-redshift Pantheon+ supernova bins, points near the Shapley supercluster and CMB dipole, and disappears for higher redshift thresholds.
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Probing the Cosmological Principle with weak lensing shear
A Euclid-like weak lensing survey could detect the E-B shear cross-correlation produced by late-time anisotropic expansion, yielding a new probe of the Cosmological Principle.
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