Potential signature of a quadrupolar hubble expansion in Pantheon+supernovae

The assumption of isotropy - that the Universe looks the same in all directions on large scales - is fundamental to the standard cosmological model. It is therefore critical to empirically test in which regimes this assumption holds. Anisotropies in the cosmic expansion are expected due to non-linear structures in the late Universe. However, the extent to which these anisotropies might impact our low-redshift observations remains to be fully tested. We use general relativistic simulations to determine that the expected anisotropies in the Hubble and deceleration parameters are quadrupolar and dipolar, respectively. We constrain these multipoles simultaneously in the new Pantheon+supernova compilation. In the rest frame of the cosmic microwave background (CMB), including peculiar velocity (PV) corrections, we find an ∼2σ deviation from isotropy. We constrain the eigenvalues of the quadrupole in the Hubble parameter to be λ₁ = 0.021 ± 0.011 and λ₂ = 0.00 ± 0.012 and place a 1σ upper limit on its amplitude of 2.88 per cent. We find no significant dipole in the deceleration parameter, with amplitude. However, in the rest frame of the CMB without PV corrections, we find a >2σ positive amplitude with. Incorporating these anisotropies, the monopole of the Hubble parameter shifts by only 0.30 km s^-1 Mpc^-1 with respect to the isotropic constraints.