Constraints on galileons from the positions of supermassive black holes

TL;DR

This paper uses galaxy and black hole offset measurements to constrain Galileon scalar field theories, finding that the fifth force strength is limited to less than 16% of gravity at 1 sigma confidence.

Contribution

It introduces a novel method combining constrained N-body simulations and galaxy offset data to test Galileon theories, providing new bounds on their coupling strength.

Findings

Galileon coupling constrained to be less than 0.16 at 1 sigma

Developed a Monte Carlo forward model for galaxy-BH offsets

Used a large sample of 1916 galaxies for analysis

Abstract

Galileons are scalar field theories which obey the Galileon symmetry $\varphi \to \varphi + b + c_\mu x^\mu$ and are capable of self-acceleration if they have an inverted sign for the kinetic term. These theories violate the Strong Equivalence Principle, such that black holes (BHs) do not couple to the Galileon field, whereas non-relativistic objects experience a fifth force with strength $\Delta G / G_{\rm N}$ relative to gravity. For galaxies falling down a gradient in the Galileon field, this results in an offset between the centre of the galaxy and its host supermassive BH. We reconstruct the local gravitational and Galileon fields through a suite of constrained N-body simulations (which we dub CSiBORG) and develop a Monte Carlo-based forward model for these offsets on a galaxy-by-galaxy basis. Using the measured offset between the optical centre and active galactic nucleus of 1916 galaxies from the literature, propagating uncertainties in the input quantities and marginalising over an empirical noise model describing astrophysical and observational noise, we constrain the Galileon coupling to be $\Delta G / G_{\rm N} < 0.16$ at $1\sigma$ confidence for Galileons with crossover scale $r_{\rm C} \gtrsim H_0^{-1}$.