# Spinning particle orbits around a black hole in an expanding background

## TL;DR

This paper analyzes how spinning particles orbit black holes within an expanding universe, revealing that cosmic expansion and spin-orbit interactions deform orbits in complex ways.

## Contribution

It provides an analytical and numerical study of spinning particle orbits around black holes considering cosmic expansion effects, highlighting new deformation mechanisms.

## Key findings

- Cosmic expansion influences orbit deformation through attractive or repulsive interactions.
- Spin-orbit coupling causes additional orbit deformation depending on orientation.
- Orbits deviate from circular shape due to combined effects of expansion and spin.

## Abstract

We investigate analytically and numerically the orbits of spinning particles around black holes in the post Newtonian limit and in the presence of cosmic expansion. We show that orbits that are circular in the absence of spin, get deformed when the orbiting particle has spin. We show that the origin of this deformation is twofold: a. the background expansion rate which induces an attractive (repulsive) interaction due to the cosmic background fluid when the expansion is decelerating (accelerating) and b. a spin-orbit interaction which can be attractive or repulsive depending on the relative orientation between spin and orbital angular momentum and on the expansion rate.