Speed of Gravity and Gravitational Horizon
  Pioneer-10 unexpected anomalous deceleration hurtling through the solar system might have revealed a secret of nature that gravitational information propagating at very high but finite speed rushing to gravitational horizon opens a window to Universe neighborhood.   Dark energy may be owing to interactive gravitational horizons of cousin universes. The necessary condition for the conjecture to be true is that the speed of gravity must be faster than the speed of light.The sufficient condition is that the dark energy content would be increasing with the age of our Universe, while more and more gravitational horizons of cousin universes might reach the gravitational horizon of ours in the neighborhood that may be boundless in space-time flow without beginning or end.   Keys: Inseparability Principle, Scale Conversion, Speed of Gravity, Gravitational Horizon, Neighborhood of universes, Dark Energy. 
Chao Yuan Yang