Novel geometrical test of cosmological expansion from photometric data

TL;DR

This paper proposes a geometrical null test based on the BNT transform to constrain the Universe's expansion rate using weak lensing data, enhancing dark energy parameter constraints.

Contribution

It introduces a novel null test leveraging the BNT transform that isolates background expansion information from weak lensing data, independent of matter distribution.

Findings

The null test can significantly improve dark energy constraints.

Shape noise currently limits the method's effectiveness.

Future survey designs could greatly enhance the method's potential.

Abstract

In tomographic cosmic-shear observations, the BNT (Bernardeau, Nishimichi, Taruya) transform, Bernardeau et al. (2014), allows to build weak lensing transformed maps for which the contribution from low redshift lenses is nulled. As this transformation depends specifically on the expansion rate of the Universe but is independent of the matter distribution properties, it can be leveraged to extract information from large-scale structure probes at arbitrary non-linear scales, providing constraints on cosmological background evolution. We demonstrate this by proposing a specific null test for stage IV weak lensing projects. Using a Fisher matrix analysis and parameter sampling, we show that this approach can substantially enhance constraints on the dark energy equation of state. Notably, we find that shape noise currently limits this method's effectiveness making significant improvement possible in future designs. A detailed analysis of our null test in the context of the Euclid mission is presented in a companion paper Touzeau et al. (2025).