  We study a holographic theory of general spacetimes that does not rely on the
existence of asymptotic regions. This theory is to be formulated in a
holographic space. When a semiclassical description is applicable, the
holographic space is assumed to be a holographic screen: a codimension-1
surface that is capable of encoding states of the gravitational spacetime. Our
analysis is guided by conjectured relationships between gravitational spacetime
and quantum entanglement in the holographic description. To understand basic
features of this picture, we catalog predictions for the holographic
entanglement structure of cosmological spacetimes. We find that qualitative
features of holographic entanglement entropies for such spacetimes differ from
those in AdS/CFT but that the former reduce to the latter in the appropriate
limit. The Hilbert space of the theory is analyzed, and two plausible
structures are found: a direct sum and "spacetime equals entanglement"
structure. The former preserves a naive relationship between linear operators
and observable quantities, while the latter respects a more direct connection
between holographic entanglement and spacetime. We also discuss the issue of
selecting a state in quantum gravity, in particular how the state of the
multiverse may be selected in the landscape.
