# Beyond self-acceleration: force- and fluid-acceleration

## TL;DR

This paper critiques the traditional concept of self-acceleration in cosmology, introduces more general notions of force-acceleration and field-acceleration, and demonstrates their application to modified gravity models like f(R) and coupled dark energy.

## Contribution

It proposes new, broader definitions of acceleration in cosmology that encompass non-universal couplings and models excluded by the traditional self-acceleration concept.

## Key findings

- Introduced force-acceleration and field-acceleration as generalizations of self-acceleration.
- Applied these concepts to f(R) models and coupled dark energy.
- Showed these models remain consistent with current data.

## Abstract

The notion of self-acceleration has been introduced as a convenient way to theoretically distinguish cosmological models in which acceleration is due to modified gravity from those in which it is due to the properties of matter or fields. In this paper we review the concept of self-acceleration as given, for example, by [1], and highlight two problems. First, that it applies only to universal couplings, and second, that it is too narrow, i.e. it excludes models in which the acceleration can be shown to be induced by a genuine modification of gravity, for instance coupled dark energy with a universal coupling, the Hu-Sawicki f(R) model or, in the context of inflation, the Starobinski model. We then propose two new, more general, concepts in its place: force-acceleration and field-acceleration, which are also applicable in presence of non universal cosmologies. We illustrate their concrete application with two examples, among the modified gravity classes which are still in agreement with current data, i.e. f(R) models and coupled dark energy.