An effective description of Laniakea: impact on cosmology and the local determination of the Hubble constant

Abstract

We propose an effective model to describe the bias induced on cosmological observables by Laniakea, the gravitational supercluster hosting the Milky Way, which was defined using peculiar velocity data from Cosmicflows-4 (CF4). The structure is well described by an ellipsoidal shape exhibiting triaxial expansion, reasonably approximated by a constant expansion rate along the principal axes. Our best fits suggest that the ellipsoid, after subtracting the background expansion, contracts along the two smaller axes and expands along the longest one, predicting an average expansion of ∼ -1.1 km/s/Mpc. The different expansion rates within the region, relative to the mean cosmological expansion, induce line-of-sight-dependent corrections in the computation of luminosity distances. We apply these corrections to two low-redshift datasets: the Pantheon+ catalog of type Ia Supernovae (SN Ia), and 63 measurements of Surface Brightness Fluctuations (SBF) of early-type massive galaxies from the MASSIVE survey. We find corrections on the distances of order ∼ 2-3%, resulting in a shift in the inferred best-fit values of the Hubble constant H 0 of order ΔH 0 SN Ia ≈ 0.5 km/s/Mpc and ΔH 0 SBF ≈ 1.1 km/s/Mpc, seemingly worsening the Hubble tension.

Bulk Flow Motion Detection in the Local Universe with Pantheon+ Type Ia Supernovae

“…Recent works (Kenworthy et al 2019;Giani et al 2024;Mazurenko et al 2024) have investigated the possibility that overdensities (i.e., large galaxy clusters) and underdensities (i.e., large voids) affect the H 0 measurements in the Local Universe, because such structures produce large inflows and outflows, respectively. In fact, this biasing effect exists, being important to quantify the impact of such phenomena in the measurements of H 0 .…”

“…In fact, this biasing effect exists, being important to quantify the impact of such phenomena in the measurements of H 0 . According to Giani et al (2024), the large supercluster Laniakea, which hosts the Milky Way, produces a negative average expansion of ∼ −1.1 km s −1 Mpc −1 , inducing a variation in the Hubble constant ΔH 0 ≈ 0.5 km s −1 Mpc −1 . Therefore, the inflows produced by Laniakea can increase the Hubble tension when applying this correction to the SNe Ia data set.…”

A Critical Discussion on the H0 Tension

“…The presence of local inhomogeneities that could affect the late-time measurements of H 0 , which may be either due to possible observational issues like incomplete sky sampling, astrophysical problems like incorrect modelling of the local structures, or a more fundamental nature, like the departure of the FLRW assumption at very small scales [262][263][264][265][266].…”