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.

中文翻译：

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跨越通用额外维度中的幻影分界线

我们研究了宇宙加速和暗能量在通用额外维度 UED 中跨越宇宙常数边界的演化。我们采用经验方法来求解高维宇宙学方程，使减速参数 $q$ 与观测结果一致。jerk 和 deceleration 参数的表达式与维数 $n$ 无关。$4$D 中的压力行为显示了对应于减速到加速的宇宙过渡的正到负过渡。这种压力行为有助于解释宇宙减速-加速转变，尽管转变背后的原因仍然未知。在传统的 4 美元 D 宇宙学中，有一个不通过定理阻止 FRW 几何中单个完美流体的 EoS 参数越过 $\omega=-1$ 边界。当前模型包括在同质 FRW 度量中的单一同质但各向异性完美流体，在普通 $4$D 和 UED 中具有两个不同的比例因子。与传统的 $4$D 宇宙学相反，我们发现 UED 中的暗能量演化显示 $\omega=-1$ 交叉。然而，在 $4$D 中仍然遵守 no-go 定理，其中 EoS 参数不跨越 $\omega=-1$ 边界。我们发现UED中暗能量演化显示$\omega=-1$交叉。然而，在 $4$D 中仍然遵守 no-go 定理，其中 EoS 参数不跨越 $\omega=-1$ 边界。我们发现UED中暗能量演化显示$\omega=-1$交叉。然而，在 $4$D 中仍然遵守 no-go 定理，其中 EoS 参数不跨越 $\omega=-1$ 边界。