# Violation of the equivalence principle from light scalar fields: from Dark Matter candidates to scalarized black holes

## TL;DR

This paper reviews tensor-scalar theories of gravity, exploring how light scalar fields can violate the equivalence principle, and presents observational constraints from atomic clocks, free fall tests, and black hole measurements.

## Contribution

It introduces a microscopic model of scalar/matter coupling and applies it to ultralight scalar dark matter and scalarized black holes, deriving new observational constraints.

## Key findings

- Constraints on scalar field couplings from atomic clock comparisons
- Limits on equivalence principle violations from free fall experiments
- Bounds on scalarized black holes from S0-2 star redshift measurements

## Abstract

Tensor-scalar theory is a wide class of alternative theory of gravitation that can be motivated by higher dimensional theories, by models of dark matter or dark ernergy. In the general case, the scalar field will couple non-universally to matter producing a violation of the equivalence principle. In this communication, we review a microscopic model of scalar/matter coupling and its observable consequences in terms of universality of free fall, of frequencies comparison and of redshifts tests. We then focus on two models: (i) a model of ultralight scalar dark matter and (ii) a model of scalarized black hole in our Galactic Center. For both these models, we present constraints using recent measurements: atomic clocks comparisons, universality of free fall measurements, measurement of the relativistic redshift with the short period star S0-2 orbiting the supermassive black hole in our Galactic Center.