C:5. What is the nature of dark energy?
The accelerating expansion rate of the Universe requires gravity to be repulsive on large scales. This can be effected by Einstein’s cosmological constant, or by Dark Energy - a non-zero (but small) energy density of the vacuum, or a new field which evolves with time. More radically, the acceleration may be a manifestation of other modifications to Einstein gravity. Techniques focus on tests of the geometry of the Universe and the growth rate of fluctuations, both of which depend on the Dark Energy properties, and include weak gravitational lensing, large-scale galaxy redshift surveys and supernova searches.