Percent-Level Test of Isotropic Expansion Using Type Ia Supernovae

Area of Science:

• Cosmology
• Astrophysics
• Statistical Analysis

Background:

• The expansion of the Universe is a fundamental concept in cosmology.
• Testing the isotropy of cosmic expansion is crucial for understanding fundamental physics.
• Previous studies have explored variations in the Hubble parameter using various cosmological probes.

Purpose of the Study:

• To propose and implement a novel, robust, and nonparametric statistical test for the isotropy of the Universe's expansion.
• To apply this test to a large sample of Type Ia supernovae to constrain spatial variations in the Hubble parameter.

Main Methods:

• Angular clustering analysis of supernova magnitude residuals from the Pantheon sample.
• Comparison of observed clustering to expected noise under the isotropic assumption.
• Assessment and mitigation of potential systematic effects in the data and analysis.

Main Results:

• A novel, robust, and nonparametric statistical test for cosmic expansion isotropy was developed and applied.
• Analysis of ~1000 Type Ia supernovae from the Pantheon sample was performed.
• An upper limit on the root-mean-square (rms) spatial variation of the Hubble parameter was established.

Conclusions:

• The study demonstrates that spatial variations in the Hubble parameter are constrained to be less than 1% at 99.7% confidence for a Gaussian smoothing of 60°.
• The findings support the assumption of large-scale isotropy in the Universe's expansion at late times.
• Systematic effects were found to be negligible or adequately accounted for in the procedure.