The Hubble parameter in the early universe with viscous QCD matter and finite cosmological constant

摘要

The evolution of a flat, isotropic and homogeneous universe is studied. Thebackground geometry in the early phases of the universe is conjectured to befilled with causal bulk viscous cosmological fluid and dark energy. The energydensity relations obtained from the assumption of covariant conservation ofenergy-momentum tensor of the background matter in the early universe are usedto derive the basic equation for the Hubble parameter $H$. The viscousproperties described by ultra-relativistic equations of state and bulkviscosity taken from recent heavy-ion collisions and lattice QCD calculationshave been utilized to give an approximate solution of the field equations. Thecosmological constant is conjectured to be related to the energy density of thevacuum. In this treatment, there is a clear evidence for singularity atvanishing cosmic time $t$ indicating the dominant contribution from the darkenergy. The time evolution of $H$ seems to last for much longer time than theideal case, where both cosmological constant and viscosity coefficient areentirely vanishing.