The exact nature of dark energy has yet to be established. However, dark energy is believed to be extremely homogeneous (evenly spread throughout space) and of very low density, roughly 6.9 × 10-29 kilogram per cubic meter. This is much lower than the density of ordinary matter or energy with the Milky Way Galaxy and makes it probably undetectable in laboratory experiments.
Various models of dark energy have been proposed, including a cosmic field associated with inflation; a different, low-energy field called quintessence; and the cosmological constant, or vacuum energy of empty space. Unlike Einstein's famous fudge factor, the cosmological constant in its present incarnation doesn't delicately (and artificially) balance gravity in order to maintain a static universe; instead, it has negative pressure that causes expansion to accelerate.
Discovery of dark energyUntil the late 1990s, astronomers were convinced that the expansion of the Universe had to be slowing: it might slow and eventually reverse if the overall energy density were hig enough, or it might slow but continue forever. It was true that no slowing had been observed but the assumption seemed reasonable since gravity was the only significant force known to be acting over cosmic distances. Then, 1998, a startling discovery was announced. Observations of remote supernovae by the Hubble Space Telescope showed that, billions of years ago, the Universe was expanding more slowly than it is today. To everyone's surpise the rate at which the Universe is expanding has increased – a fact that could only be explained if there were a previously unknown phenomenon acting to pull the Universe apart.
Related categories COSMOLOGY
• SPACE AND TIME
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