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

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

2007-10-29

arxiv.org/abs/0710.5366v2

Phys.Lett.B665:121-124,2008

Astronomy and Astrophysics

Astrophysics

General Relativity and Quantum Cosmology

16 pages, latex, paper shortened by 2 pages for journal publication

Scientific paper

10.1016/j.physletb.2008.06.025

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

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