Exploring the evidence for a large local void with supernovae Ia data

Abstract

Abstract In the present work we utilise the most recent publicly available SN Ia compilations and implement a well formulated cosmological model based on LTB metric in presence of cosmological constant Λ (ΛLTB) to test for signatures of large local inhomogeneities at z ≤ 0.15. Local underdensities in this redshift range have been previously found based on luminosity density data and galaxy number counts. Our main constraints on the possible local void using the Pantheon SN Ia dataset are: redshift size of $z_{\rm size}=0.068^{+0.021}_{-0.030}$; density contrast of $\delta \Omega _0/\Omega _0 = -10.5_{-7.4}^{+9.3}\%$ between 16th and 84th percentiles. Investigating the possibility to alleviate the ∼ $9\%$ disagreement between measurements of present expansion rate H0 coming from calibrated local SN Ia and high-z CMB data, we find large local void to be a very unlikely explanation alone, consistently with previous studies. However, the level of matter inhomogeneity at a scale of ∼100Mpc that is allowed by SN Ia data, although not expected from ΛCDM cosmic variance calculations, could be the origin of additonal systematic error in distance ladder measurements based on SN Ia. Fitting low-redshift Pantheon data with a cut 0.023 < z < 0.15 to the ΛLTB model and to the Taylor expanded luminosity distance formula we estimate that this systematic error amounts to $1.1\%$ towards the lower H0 value. A test for local anisotropy in Pantheon SN Ia data yields null evidence. Analysis of luminosity density data provides a constraint on contrast of large isotropic void $\delta \Omega _0/\Omega _0 = -51.9\%\pm 6.3\%$, which is in ∼ 4σ tension with SN Ia results. More data is necessary to better constrain the local matter density profile and understand the disagreement between SN Ia and luminosity density samples.