A cosmological model describing the early inflation, the intermediate
  decelerating expansion, and the late accelerating expansion by a quadratic
  equation of state
  We develop a cosmological model based on a quadratic equation of state
p/c^2=-(\alpha+1){\rho^2}/{\rho_P}+\alpha\rho-(\alpha+1)\rho_{\Lambda} (where
\rho_P is the Planck density and \rho_{\Lambda} the cosmological density)
"unifying" vacuum energy and dark energy in the spirit of a generalized
Chaplygin gas model. For $\rho\rightarrow \rho_P$, it reduces to p=-\rho c^2
leading to a phase of early accelerated expansion (early inflation) with a
constant density equal to the Planck density \rho_P (vacuum energy). For
$\rho_{\Lambda}\ll\rho\ll \rho_P$, we recover the standard linear equation of
state p=\alpha \rho c^2 describing radiation (\alpha=1/3) or pressureless
matter (\alpha=0) and leading to an intermediate phase of decelerating
expansion. For $\rho\rightarrow \rho_{\Lambda}$, we get p=-\rho c^2 leading to
a phase of late accelerated expansion (late inflation) with a constant density
equal to the cosmological density \rho_{\Lambda} (dark energy). We show a nice
symmetry between the early universe (vacuum energy + \alpha-fluid) and the late
universe (\alpha-fluid + dark energy). In our model, they are described by two
polytropic equations of state with index n=+1 and n=-1 respectively.
Furthermore, the Planck density \rho_P in the early universe plays a role
similar to the cosmological density \rho_{\Lambda} in the late universe. They
represent fundamental upper and lower density bounds differing by 122 orders of
magnitude. This quadratic equation of state leads to a fully analytical model
describing the evolution of the universe from the early inflation (Planck era)
to the late accelerated expansion (de Sitter era). These two phases are bridged
by a decelerating algebraic expansion (\alpha-era). This model does not present
any singularity at t=0 and exists eternally in the past. It admits a scalar
field interpretation based on a quintessence field or a tachyon field.
