The mysterious vacuum
Dark energy — the unknown substance that is causing the expansion of the universe to
accelerate — could be due to quantum fluctuations of the vacuum, which can be viewed as the
"springiness" of empty space (see main text). Figuring out whether the quantum field is static
or whether it varies over time would provide us with important clues about the origin of dark
energy. Einstein's cosmological constant, with which the current data are consistent, is like a
field of identical springs that do not change in time (upper insert), while "quintessence" is like
a field that varies in space and time (lower insert).
Source: Lawrence Berkeley National Laboratory