Updates on dipolar anisotropy in local measurements of the Hubble constant from Cosmicflows-4

TL;DR

Using the Cosmicflows-4 catalog, the study tests for dipolar and higher-order anisotropies in the local Hubble constant by mapping $H_0$ in radial shells and fitting spherical harmonics up to the octupole. It contrasts uncorrected ($CF4_{obs}$) and peculiar-velocity-corrected ($CF4_{pec}$) data, finding a distance-dependent dipole with subdominant multipoles and a weakened signal after velocity corrections. The uncorrected dipole does not align with the CMB dipole and is linked to local structures, while the corrected case suggests a smaller bulk flow, indicating potential differential expansion but no clear new physics. The work also assesses SN calibrator distributions and finds no strong correlation with the dipole, implying the Hubble tension is not driven by local anisotropy. Overall, the results reveal subtle local anisotropies and emphasize careful bias treatment in local $H_0$ measurements.

Abstract

Recent observations show a persistent tension in the Hubble constant $H_0$, suggesting an incomplete understanding of cosmic expansion and local dynamics. Using the Cosmicflows-4 catalogue, we mapped the angular and radial variations of $H_0$ in radial shells with a distance modulus $μ\in [29,36]$ (approximately corresponding to $[20,100]$ $h^{-1}$ Mpc) and equal-area sky patches, applied adaptive weighing, and fitted spherical harmonics up to the octupole. Our results reveal a clear, distance-decreasing dipole that remains coherent across shells, with subdominant higher-order multipoles, and the octupole fit capturing the main anisotropic features except in sparsely sampled or SDSS-dominated shells. The direction and amplitude of the dipole depend on whether the observed radial recessional velocities are corrected for peculiar velocities or not. If the correction is not applied, the dipole aligns with the major gravitational structures in the local universe. If it is applied, a global dipole still seems to be present, but the signal is much weaker and with much lower statistical significance. This decrease in the amplitude of the dipole supports the idea of a differential expansion rate in our universe, but does not clarify whether the origin is astrophysical or cosmological. Finally, we verify that, while this anisotropy could influence local measurements of the Hubble constant, its effect on the large-scale Hubble tension appears to be limited, as the distribution of galaxies hosting SNeIa, both used as calibrators to constrain $H_0$ and in the Hubble-flow, does not show a strong correlation with the dipole signal.

Figures (11)

• Figure 1: Hubble-Lemaître law from the Cosmicflows-4 data. Horizontal (vertical) dashed lines in the top (bottom) panel indicate the interval of distance on which our nominal analysis is based.
• Figure 2: Positions of the objects in the CF4$_{obs}$ catalog in galactic coordinates.
• Figure 3: Hubble constant average, $\langle \log H_0 \rangle$, in radial shells of width $\Delta \mu = 0.2$ and at average distance $\mu$. Full CF4 sample, gray; $v^{obs}_{CMB}> 4000$ km s$^{-1}$, red; $\{|v^{X}_{LS}|,|v^{Y}_{LS}|,|v^{Z}_{LS}|\}< 16000$ km s$^{-1}$, blue; $\{|v^{X}_{LS}|,|v^{Y}_{LS}|,|v^{Z}_{LS}|\}< 8000$ km s$^{-1}$, cyan; $\{|v^{X}_{LS}|,|v^{Y}_{LS}|,|v^{Z}_{LS}|\}< 5000$ km s$^{-1}$, magenta; $\sigma_{\mu}/\mu < 0.006$, green.
• Figure 4: Angular distribution in galactic coordinate system of the radial shells described in Sec. \ref{['sec:H0_map']}.
• Figure 5: Radial+Angular radial shells in Galactic coordinate system in the CMB frame. Coordinates: galactic longitude $l_g$, from $0^{\circ}$ to $360^{\circ}$; galactic latitude $b_g$, from $-90^{\circ}$ to $90^{\circ}$. The anisotropy signal in the Hubble constant from the data is shown as a colour gradient ranging from red, which indicates higher values, to violet, which indicates lower values. The black dashed lines represent the best fit to the signal, from the harmonic expansion up to the octupole.