The fact is that our universe is fairly homogenious & isotropic at larger scale larger than 150 h*-1MPC where 1MP~ 3x10*24 Cm and h~0.7
The fact is also that our universe is expanding. The Mean distance between galaxies is 1MPC and our visible universe is about 300h*-1MPC
Our universe has a critical energy density and that critical energy density have different components ( Like baryons density, Dark Matter density, Radiation density). Cosmic microwave Back ground Radiation Study(CMBR) also prove that that we live in a universe with Critical density
Total ammount of Baryons in the universe produced in the Big Bang nuecleosynthesis OmegaB=(0.024+-0.0012)h*-2 where h=0.72+-0.7 and all baryons exist in the universe today irrespective of whether they are luminious or not. But most of the universe is Non baryonic
universe is populated by non luminiuos component of Dark matter made of WIMP and this component is OmegaDM~=0.15-0.3
so there must be one or more components to the energy density of universe contributing aBout 70% of critical density and this component is unclustered and has negetive pressure and this component omegaDE~=o.7-0.85
The universe has also radiation contributing to OmegaRh*2=2.56x10*5 to day which are mostly photon in CMBR and these photons are at red shift larger than Zeq~- OmegaDM/OmegaR~-4x10*4DMh*2
and So our universe must has omegaDE~-0.7,~-0.26 beyond the CMBR ie observable universe
The dark energy with suffieciently negetive pressure( can be treated as fluid with stress tensors) will accereate the expansion of universe once it can dominate the matter
If Dark energy is taken as cosmological constant Omegatot=1 and when omegam=1 with no dark energy, universe is then Decelerating at all epoch aND When Omega m=0 and Omega DE=1 then universe is accelrating at all epoch. And in between this lies that universe is were decelerating in the past and to accelrating when dark energy is dominating as above shown

See the Following Links on Thread by Pranab" What is beyond the Boundaries of Our Visible Universe"
http://www.extremeastronomy.com/foru...-universe.html
See thread"Big Bang" By Pranab
Big Bang

Mr. Rupak Bhattacharya
Mr. Ritwik Bhattacharya
prof. Pranab Kr Bhattacharya
www.unipathos.com