  The causal limit usually considered in cosmology is the particle horizon,
delimiting the possibilities of causal connection in the expanding universe.
However it is not a realistic indicator of the effective local limits of
important interactions in spacetime. We consider here the matter horizon for
the Solar System, that is,the comoving region which has contributed matter to
our local physical environment. This lies inside the effective domain of
dependence, which (assuming the universe is dominated by dark matter along with
baryonic matter and vacuum-energy-like dark energy) consists of those regions
that have had a significant active physical influence on this environment
through effects such as matter accretion and acoustic waves. It is not
determined by the velocity of light c, but by the flow of matter perturbations
along their world lines and associated gravitational effects. We emphasize how
small a region the perturbations which became our Galaxy occupied, relative to
the observable universe -- even relative to the smallest-scale perturbations
detectable in the cosmic microwave background radiation. Finally, looking to
the future of our cosmic domain, we suggest simple dynamical criteria for
determining the present domain of influence and the future matter horizon. The
former is the radial distance at which our local region is just now separating
from the cosmic expansion. The latter represents the limits of growth of the
matter horizon in the far future.

Cosmology and Nongalactic Astrophysics