Testing cosmic anisotropy with Padé approximations and the latest Pantheon+ sample

Abstract

Cosmography can be used to constrain the kinematics of the Universe in a model-independent way. In this work, we attempt to combine the Padé approximations with the latest Pantheon+ sample to test the cosmological principle. Based on the Padé approximations, we first applied cosmographic constraints to different-order polynomials including third-order (Padé(2, 1)), fourth-order (Padé(2, 2)), and fifth-order (Padé(3, 2)) ones. The statistical analyses show that the Padé(2, 1)polynomial has the best performance. Its best fits areH0 = 72.53 ± 0.28 km s−1Mpc−1,q0= −0.35−0.07+0.08, andj0= 0.43−0.56+0.38. By further fixingj0 = 1.00, it can be found that the Padé(2, 1)polynomial can describe the Pantheon+ sample better than the regular Padé(2, 1)polynomial and the usual cosmological models (including the ΛCDM,wCDM, CPL, andRh= ct models). Based on the Padé(2, 1)(j0 = 1) polynomial and the hemisphere comparison method, we tested the cosmological principle and found the preferred directions of cosmic anisotropy, such as (l, b) = (304.6°−37.4+51.4, −18.7°−20.3+14.7) and (311.1°−8.4+17.4, −17.53°−7.7+7.8) forq0andH0, respectively. These two directions are consistent with each other at a 1σconfidence level, but the corresponding results of statistical isotropy analyses including isotropy and isotropy with real positions are quite different. The statistical significance ofH0is stronger than that ofq0; that is, 4.75σand 4.39σfor isotropy and isotropy with real positions, respectively. Reanalysis with fixedq0 = −0.55 (corresponds to Ωm= 0.30) gives similar results. Overall, our model-independent results provide clear indications of a possible cosmic anisotropy, which must be taken seriously. Further testing is needed to better understand this signal.