# Crossing the phantom divide line in universal extra dimensions

## TL;DR

This paper explores how universal extra dimensions can enable the crossing of the dark energy equation of state parameter through the cosmological constant boundary, providing insights into cosmic acceleration beyond standard 4D cosmology.

## Contribution

It demonstrates that in universal extra dimensions, the dark energy EoS can cross the -1 boundary, unlike in conventional 4D cosmology, by solving higher-dimensional cosmological equations empirically.

## Key findings

- Dark energy EoS crosses -1 in UED models.
- Pressure transitions from positive to negative during cosmic acceleration.
- The no-go theorem remains valid in 4D but is bypassed in UED.

## Abstract

We investigate the cosmic acceleration and the evolution of dark energy across the cosmological constant boundary in universal extra dimensions UED. We adopt an empirical approach to solve the higher-dimensional cosmological equations so that the deceleration parameter $q$ is consistent with observations. The expressions for the jerk and deceleration parameters are independent of the number of dimensions $n$. The behavior of pressure in $4$D shows a positive-to-negative transition corresponding to the deceleration-to-acceleration cosmic transition. This pressure behavior helps in providing an explanation to the cosmic deceleration-acceleration transition although the reason behind the transition itself remains unknown. In the conventional $4$D cosmology, there is a no-go theorem prevents the EoS parameter of a single perfect fluid in FRW geometry to cross the $\omega=-1$ boundary. The current model includes a single homogenous but anisotropic perfect fluid in a homogenous FRW metric with two different scale factors in the ordinary $4$D and the UED. In contrast to the conventional $4$D cosmology, we have found that the dark energy evolution in UED shows $\omega=-1$ crossing. however, the no-go theorem is still respected in $4$D where the EoS parameter doesn't cross the $\omega=-1$ boundary.