  A Universe with finite age also has a finite causal scale. Larger scales can
not affect our local measurements or modeling, but far away locations could
have different cosmological parameters. The size of our causal Universe depends
on the details of inflation and is usually assumed to be larger than our
observable Universe today. To account for causality, we propose a new boundary
condition, that can be fulfill by fixing the cosmological constant (a free
geometric parameter of gravity). This forces a cancellation of vacuum energy
with the cosmological constant. As a consequence, the measured cosmic
acceleration can not be explained by a simple cosmological constant or constant
vacuum energy. We need some additional odd properties such as the existence of
evolving dark energy (DE) with energy-density fine tuned to be twice that of
dark matter today. We show here that we can instead explain cosmic acceleration
without DE (or modified gravity) assuming that the causal scale is smaller than
the observable Universe today. Such scale corresponds to half the sky at z=1
and 60 degrees at z=1100, which is consistent with the anomalous lack of
correlations observed in the CMB. Late time cosmic acceleration could then be
interpreted as the smoking gun of primordial Inflation.
