  The origin of negative pressure fluid (the dark energy) is investigated in
the quantum model of the homogeneous, isotropic and closed universe filled with
a uniform scalar field and a perfect fluid which defines a reference frame. The
equations of the model are reduced to the form which allows a direct comparison
between them and the equations of the Einsteinian classical theory of gravity.
It is shown that quantized scalar field has a form of a condensate which
behaves as an antigravitating medium. The theory predicts an accelerating
expansion of the universe even if the vacuum energy density vanishes. An
antigravitating effect of a condensate has a purely quantum nature. It is shown
that the universe with the parameters close to the Planck ones can go through
the period of exponential expansion. The conditions under which in
semi-classical approximation the universe looks effectively like spatially flat
with negative deceleration parameter are determined. The reduction to the
standard model of classical cosmology is discussed.
