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.
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