  The cosmological constant, which was introduced by Einstein a century ago to
allow for a static universe, experienced a revival two decades ago under the
label dark energy as a parameter to model the observed accelerated expansion of
the universe. Its physical nature has however remained enigmatic. Here we use
the Einstein equations without cosmological constant to show that the origin of
the accelerated expansion is not in the equations but has to do with the
boundary conditions related to the causal horizon, which exists because the age
of the universe is finite. Via transformations to conformal coordinates and
Euclidian spacetime we find a resonance condition that uniquely determines the
dimensionless parameter $\Omega_\Lambda$ that governs the observed cosmic
acceleration: $\Omega_\Lambda =\textstyle{2\over 3}\,(\pi\, t_H/t_c)^2$, where
$t_H$ is the Hubble time and $t_c$ is the conformal age of the universe. This
explanation leads to a somewhat modified cosmology, in which the expansion rate
of the early universe is 2.1 times faster than in the standard model. We show
that Big Bang nucleosynthesis calculations with the faster expansion rate
requires the mean baryon density to be raised to the level of the total matter
density to agree with the observed deuterium abundance. This appears to
eliminate the need to invoke the existence of some yet to be discovered exotic
particles to explain dark matter, since all of it may be baryonic while still
remaining consistent with the observed abundances of the light elements.
