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 $\sim -1.1 ~\rm{km}/\rm{s}/\rm{Mpc}$. 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 $\sim 2-3\%$, resulting in a shift in the inferred best-fit values of the Hubble constant $H_0$ of order $\Delta H_0^{\rm{SN~Ia}}\approx 0.5 ~\rm{km}/\rm{s}/\rm{Mpc}$ and $\Delta H_0^{\rm{SBF}}\approx 1.1 ~\rm{km}/\rm{s}/\rm{Mpc}$, seemingly worsening the Hubble tension.

• The paper introduces an anisotropic ellipsoidal model for Laniakea that quantifies its influence on local H₀ using SN Ia and SBF data.
• The methodology reveals a systematic H₀ increase of about 0.5 km/s/Mpc for SN Ia and 1.1 km/s/Mpc for SBF, intensifying the Hubble tension.
• The study highlights Laniakea's role as a weak backreaction source, urging the integration of local gravitational effects into broader cosmological models.

An Effective Description of Laniakea: Impact on Cosmology and the Local Determination of the Hubble Constant

The paper presents a model addressing the influence of Laniakea, our local gravitational supercluster, on cosmological measurements, particularly focusing on the nearby determination of the Hubble constant, $H_0$. The study employs an effective description of Laniakea using an anisotropic, triaxial ellipsoidal model to delineate its impact on cosmological observables derived from two datasets: type Ia Supernovae (SN~Ia) from the Pantheon+ catalog and Surface Brightness Fluctuations (SBF) measurements.

Model Construction

Laniakea, which embodies our local cosmic environment, is shown to exert inhomogeneous effects on standard cosmological assumptions of isotropy and homogeneity, primarily adopted under the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. This paper constructs an ellipsoidal model to characterize Laniakea's influence. The shape's principal axes exhibit different expansion rates: contraction along the smaller axes and expansion along the longest. This anisotropic expansion generates peculiar velocity effects leading to deviations in line-of-sight-dependent corrections in luminosity distances.

The model utilizes peculiar velocity data from the Cosmicflows-4 (CF4) survey to fit an effective ellipsoidal shape to Laniakea's structure. The best-fit parameters suggest significant triaxial anisotropy. This model challenges the traditional flexibility provided by spherically symmetric models like Lemaître-Tolman-Bondi (LTB) but aligns with the defined spatial and velocity structures according to the CF4 data.

Results and Implications

1. Impact on $H_0$: The application of the anisotropic model for Laniakea affects the inferred values of the Hubble constant from nearby measurements. The study finds systematic shifts in $H_0$, with calculated corrections leading to an increased $H_0$ estimate by approximately 0.5 km/s/Mpc for SN~Ia and 1.1 km/s/Mpc for SBF sources. This shift appears to exacerbate the existing tension between local measurements of $H_0$ and those obtained from the cosmic microwave background (CMB).
2. Cosmological Backreaction: The analysis reveals that Laniakea behaves as a significant gravitational oversource within the local Universe, contributing to anisotropies observable at low redshifts. The study conceptualizes this as a 'weak backreaction'—an area of inquiry exploring how local universe structures might alter or bias cosmological parameter determinations.
3. Theoretical and Observational Consistency: The study complements recent observations that suggest an excess of anisotropic expansion in local cosmography and highlights implications for current cosmological paradigms. The methodology underscores the necessity of integrating local environmental inferences into broader cosmological models when estimating fundamental parameters like the Hubble constant.

Future Directions

The work suggests several pathways for further research. With upcoming enhanced peculiar velocity measurements and deeper surveys, the anisotropic model could be refined and validated more definitively, supporting a re-evaluation of local universe impacts. These developments could illuminate correlations or discrepancies between local and global cosmological models, potentially informing adjustments to theoretical frameworks or interpretations of observational data.

This paper's insights pave the way for integrating small-scale effects into broader structure formation models and cosmological inference, underscoring the criticality of understanding local environments' contributions to our understanding of the Universe's large-scale properties. Future explorations might encompass a detailed investigation of neighboring structures' effects or deploy higher-fidelity models to encapsulate more complex gravitational dynamics within the local supercluster network.