Logarithm of the scale factor as a generalised coordinate in a lagrangian for dark matter and dark energy

TL;DR

This paper develops a Lagrangian framework using the logarithm of the scale factor as a coordinate to model dark matter and dark energy, deriving cosmological dynamics consistent with observations.

Contribution

It introduces a novel Lagrangian with a generalized coordinate for dark sector modeling, providing solutions that match key cosmological parameters.

Findings

Dark energy density remains constant, acting as a cosmological constant.

Dark matter component scales as a^{-3}, consistent with standard cosmology.

Equation of state parameter w approaches -1 for dark energy and 0 for matter.

Abstract

A lagrangian for the $k-$ essence field is set up with canonical kinetic terms and incorporating the scaling relation of [1]. There are two degrees of freedom, {\it viz.},$q(t)= ln\enskip a(t)$ ($a(t)$ is the scale factor) and the scalar field $\phi$, and an interaction term involving $\phi$ and $q(t)$.The Euler-Lagrange equations are solved for $q$ and $\phi$. Using these solutions quantities of cosmological interest are determined. The energy density $\rho$ has a constant component which we identify as dark energy and a component behaving as $a^{-3}$ which we call dark matter. The pressure $p$ is {\it negative} for time $t\to \infty$ and the sound velocity $c_{s}^{2}={\partial p\over\partial\rho} << 1$. When dark energy dominates, the deceleration parameter $Q\to -1$ while in the matter dominated era $Q\sim {1\over 2}$. The equation of state parameter $w={p\over \rho}$ is shown to be consistent with $w={p\over\rho}\sim -1$ for dark energy domination and during the matter dominated era we have $w\sim 0$. Bounds for the parameters of the theory are estimated from observational data. Keywords: k-essence models, dark matter, dark energy PACS No: 98.80.-k