Testing the cosmological principle with the Pantheon+ sample and the region-fitting method

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
e-mail: fayinwang@nju.edu.cn
² Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ Institute of Astronomy and Information, Dali University, Dali 671003, PR China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China

Received: 8 June 2023
Accepted: 17 October 2023

Abstract

The cosmological principle is fundamental to the standard cosmological model. It assumes that the Universe is homogeneous and isotropic on very large scales. As the basic assumption, it must stand the test of various observations. In this work, we investigated the properties of the Pantheon+ sample, including redshift distribution and position distribution, and we give its constraint on the flat ΛCDM model: Ω_m = 0.36 ± 0.02 and H₀ = 72.83 ± 0.23 km s⁻¹ Mpc⁻¹. Then, using the region fitting (RF) method, we mapped the all-sky distribution of cosmological parameters (Ω_m and H₀) and find that the distribution significantly deviates from isotropy. A local matter underdensity region exists toward (308.4°_−48.7^+47.6, −18.2°_−28.8^+21.1) as well as a preferred direction of the cosmic anisotropy (313.4°_−18.2^+19.6, −16.8°_−10.7^+11.1) in galactic coordinates. Similar directions may imply that local matter density might be responsible for the anisotropy of the accelerated expansion of the Universe. Results of statistical isotropy analyses including Isotropy and Isotropy with real-data positions (RP) show high confidence levels. For the local matter underdensity, the statistical significances are 2.78σ (isotropy) and 2.34σ (isotropy RP). For the cosmic anisotropy, the statistical significances are 3.96σ (isotropy) and 3.15σ (isotropy RP). The comparison of these two kinds of statistical isotropy analyses suggests that inhomogeneous spatial distribution of real sample can increase the deviation from isotropy. The similar results and findings are also found from reanalyses of the low-redshift sample (lp+) and the lower screening angle (θ_max = 60°), but with a slight decrease in statistical significance. Overall, our results provide clear indications for a possible cosmic anisotropy. This possibility must be taken seriously. Further testing is needed to better understand this signal.