New Constraints on Anisotropic Expansion from Supernovae Type Ia

Abstract

We re-examine the contentious question of constraints on anisotropic expansion from Type Ia supernovae (SNIa) in the light of a novel determination of peculiar velocities, which are crucial to test isotropy with SNe out to distances ∼ < 200/ℎ Mpc. We re-analyze the Joint Light-Curve Analysis (JLA) Supernovae (SNe) data, improving on previous treatments of peculiar velocity corrections and their uncertainties (both statistical and systematic) by adopting state-of-the-art flow models constrained independently via the 2M++ galaxy redshift compilation. We also introduce a novel procedure to account for colour-based selection effects, and adjust the redshift of low-𝑧 SNe self-consistently in the light of our improved peculiar velocity model.We adopt the Bayesian hierarchical model BAHAMAS to constrain a dipole in the distance modulus in the context of the ΛCDM model and the deceleration parameter in a phenomenological Cosmographic expansion. We do not find any evidence for anisotropic expansion, and place a tight upper bound on the amplitude of a dipole, |𝐷 𝜇 |< 5.93 × 10 −4 (95% credible interval) in a ΛCDM setting, and |𝐷 𝑞 0 |< 6.29 × 10 −2 in the Cosmographic expansion approach. Using Bayesian model comparison, we obtain posterior odds in excess of 900:1 (640:1) against a constant-in-redshift dipole for ΛCDM (the Cosmographic expansion). In the isotropic case, an accelerating universe is favoured with odds of ∼ 1100 : 1 with respect to a decelerating one.

The quadrupole in the local Hubble parameter: first constraints using Type Ia supernova data and forecasts for future surveys

“…for 𝐻 0 in Peterson et al 2021). In addition, Rahman et al (2021) used an improved flow model to correct for PVs and found no evidence for departures from isotropy once the PV corrections were applied, consistent with our findings. The theoretical framework developed by Heinesen (2020) in principle allows us to account for anisotropic expansion of space and to infer peculiar velocities around a background model for both the observer and the sources.…”

“…We find that the cosmological parameters, 𝑞 𝑚 , 𝑞 𝑑 are uncorrelated with the SN Ia standardisation parameters, using a Pearson 𝑟 test and finding |𝑟 | values < 0.1. A similar correlation between the nuisance parameters and anisotropic cosmologies for the JLA compilation is presented in Dam et al (2017); Rahman et al (2021).…”

“…Previous anisotropy searches in the literature have employed various forms of F , including constant, linear, and exponential laws in redshift (Colin et al 2019b). Recent Bayesian model comparison studies strongly disfavour a constant-in-redshift dipole in data over scales of ∼ 1Gpc (Rahman et al 2021). The redshift range of the survey is important for the interpretation of the (amplitude of) anisotropic coefficients in the cosmographic fit (Macpherson & Heinesen 2021).…”

Cosmology Intertwined: A Review of the Particle Physics, Astrophysics, and Cosmology Associated with the Cosmological Tensions and Anomalies

“…Differences in the treatment of SNIa redshifts have even spilled over into a debate on the evidence for dark energy[1674][1675][1676].…”

The angular scale of homogeneity with SDSS-IV DR16 Luminous Red Galaxies

“…These results have been confirmed by analyses relying on different methods and data [23][24][25][26][27][28], although some authors have claimed the absence of such homogeneity scale [29][30][31], arguing that these measurements could be biased by the survey window function [32]. Moreover, some tests of cosmic isotropy revealed potential hints at FLRW breakdown [33][34][35][36][37][38][39][40], albeit disputed by [41][42][43][44][45][46][47]. In light of these results, it is crucial to perform new tests of the CP in order to determine whether it constitutes a valid physical assumption.…”