What can galaxy shapes tell us about physics beyond the standard model?

TL;DR

Galaxy shapes, including shear and flexion, can reveal higher-spin physics like vector and tensor modes, offering a new way to probe early Universe phenomena and cosmic strings with upcoming surveys.

Contribution

This study provides the first comprehensive analysis of how galaxy shape statistics can detect higher-spin modes, including detailed correlators and observational forecasts.

Findings

Shear $EB$ and $BB$ spectra are sensitive to parity properties of sources.

Galaxy shear is a powerful probe of cosmic strings via vector mode sourcing.

Flexion offers limited additional constraining power, mainly on small scales.

Abstract

The shapes of galaxies trace scalar physics in the late-Universe through the large-scale gravitational potential. Are they also sensitive to higher-spin physics? We present a general study into the observational consequences of vector and tensor modes in the early and late Universe, through the statistics of cosmic shear and its higher-order generalization, flexion. Higher-spin contributions arise from both gravitational lensing and intrinsic alignments, and we give the leading-order correlators for each (some of which have been previously derived), in addition to their flat-sky limits. In particular, we find non-trivial sourcing of shear $EB$ and $BB$ spectra, depending on the parity properties of the source. We consider two sources of vector and tensor modes: scale-invariant primordial fluctuations and cosmic strings, forecasting the detectability of each for upcoming surveys. Shear is found to be a powerful probe of cosmic strings, primarily through the continual sourcing of vector modes; flexion adds little to the constraining power except on very small scales ($\ell\gtrsim 1000$), though it could be an intriguing probe of as-yet-unknown rank-three tensors or halo-scale physics. Such probes could be used to constrain new physics proposed to explain recent pulsar timing array observations.