Ultimate fate of the universe
Repulsive force

Starting in 1998, observations of supernovas in distant galaxies have been interpreted as consistent with a universe whose expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by invoking dark energy, which in its simplest form is just a positive cosmological constant. In general, dark energy is a catch-all term for any hypothesized field with negative pressure, usually with a density that changes as the universe expands. Some cosmologists are studying whether dark energy which varies in time (due to a portion of it being caused by a scalar field in the early universe) can solve the crisis in cosmology. Upcoming galaxy surveys from the Euclid, Nancy Grace Roman and James Webb space telescopes (and data from next-generation ground-based telescopes) are expected to further develop our understanding of dark energy (specifically whether it is best understood as a constant energy intrinsic to space, as a time varying quantum field or as something else entirely).
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