# On the necessity of phantom fields for solving the horizon problem in scalar cosmologies

## TL;DR

This paper investigates the conditions under which the particle horizon problem arises in scalar cosmologies, demonstrating that phantom fields can eliminate horizons, unlike quintessence fields under typical energy conditions.

## Contribution

It shows that phantom scalar fields are necessary to avoid particle horizons in certain cosmological models with perfect fluids and specific curvature conditions.

## Key findings

- Particle horizons are unavoidable with quintessence fields under standard energy conditions.
- Phantom fields can prevent the formation of particle horizons in specific solvable models.
- The presence of phantom fields correlates with horizon avoidance in scalar cosmologies.

## Abstract

We discuss the particle horizon problem in the framework of spatially homogeneous and isotropic scalar cosmologies. To this purpose we consider a Friedmann-Lemaitre-Robertson-Walker (FLRW) spacetime with possibly non-zero spatial sectional curvature (and arbitrary dimension), and assume that the content of the universe is a family of perfect fluids, plus a scalar field that can be a quintessence or a phantom (depending on the sign of the kinetic part in its action functional). We show that the occurrence of a particle horizon is unavoidable if the field is a quintessence, the spatial curvature is non-positive and the usual energy conditions are fulfilled by the perfect fluids. As a partial converse, we present three solvable models where a phantom is present in addition to a perfect fluid, and no particle horizon~appears.