  In cosmology based on general relativity, the universe is modeled as a fluid.
The transition from the Einstein field equation to its large-scale
(cosmological) version is thus analogous to the transition, for a system
consisting of a large number of molecules, from the molecular/kinetic
description to thermodynamics and hydrodynamics. The cosmic fluid is an
effective continuum defined on the cosmological scales (only); for such a
continuum, the appearance of new emergent properties should be expected.
(Emergence of space-time and gravity is not discussed here.) When these new
properties are calculated, the following predictions result: (a) the universe
is spatially flat; (b) its expansion is accelerating; (c) dark energy makes up
75% of the total energy density of the universe; (d) the pressure of dark
energy is equal and opposite to its density. All of these are in good agreement
with the observational data. Also in favor of the present model are the absence
of adjustable parameters, and consistency with the second law of
thermodynamics. The distance-redshift relation predicted by the model is in
good agreement with the Hubble diagram of Type Ia supernovae.
