  In this work, a flat Friedmann-Robertson-Walker (FRW) universe with dust and
a cosmological constant is quantized. By means of a canonical transformation,
the classical Hamiltonian is reduced to that of either a harmonic oscillator or
anti-oscillator, depending on whether $\Lambda<0$ or $\Lambda>0$, respectively.
In this way exact solutions to the Wheeler-DeWitt equation can easily be
obtained. It turns out that a positive cosmological constant alone may account
for an early inflationary regime and a later accelerated expansion phase, with
a period of decelerated expansion in between. This suggests that quantum
gravitational effects can influence most of the history or even the entire
history of the Universe.
