Multipoles of the relativistic galaxy bispectrum

TL;DR

This paper analyzes the relativistic effects on the galaxy bispectrum's multipoles, highlighting how odd multipoles uniquely indicate relativistic signatures and are significant on large scales, aiding primordial non-Gaussianity detection.

Contribution

It provides an analytical method to decompose the relativistic galaxy bispectrum into multipoles, revealing the significance of odd multipoles as relativistic signatures.

Findings

Odd multipoles have no Newtonian contribution, serving as relativistic indicators.

Relativistic effects significantly impact dipole and octopole on large scales.

Even multipoles receive notable corrections on very large scales.

Abstract

Above the equality scale the galaxy bispectrum will be a key probe for measuring primordial non-Gaussianity which can help differentiate between different inflationary models and other theories of the early universe. On these scales a variety of relativistic effects come into play once the galaxy number-count fluctuation is projected onto our past lightcone. By decomposing the Fourier-space bispectrum into invariant multipoles about the observer's line of sight we examine in detail how the relativistic effects contribute to these. We show how to perform this decomposition analytically, which is significantly faster for subsequent computations. While all multipoles receive a contribution from the relativistic part, odd multipoles arising from the imaginary part of the bispectrum have no Newtonian contribution, making the odd multipoles a smoking gun for a relativistic signature in the bispectrum for single tracers. The dipole and the octopole are significant on equality scales and above where the Newtonian approximation breaks down. This breakdown is further signified by the fact that the even multipoles receive a significant correction on very large scales.