A possible space-time diagram for the Universe
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FIGURE 1. A possible space-time diagram for the Universe on ultra-large scales.
A natural state for a Universe with a positive vacuum energy is empty de Sitter space; this is the background universe in the figure, with time running vertically. In the presence of an appropriate scalar field, quantum fluctuations in such a background can lead to the nucleation of baby universes. Each baby universe is created in a proto-inflationary state, which then expands and reheats into a universe like that we observe. These universes are portrayed as evolving 'sideways', to emphasize that the local direction of time may not be related to that of the background space-time.