Dark energy: a hypothetical force that is regarded as the cause of the geometrical properties and accelerated expansion of the universe.

While it is like dark matter in being invisible, dark energy is in other respects even more mysterious. Since it does not have any effect on gravitational fields in the universe, it falls short of being a genuine form of energy. We encounter it only in the context of the formula for distance, where it determines as a "cosmological constant" the connection between redshift and actual distance.

Detailed comparisons between distance and redshift indicate that dark energy apparently accounts for 73% of the expansion of the universe. The remaining 27% is due to the gravitational impacts of radiation and normal as well as dark matter. Thus, dark energy provides just the right amount to explain the "flatness", that is, the Euclidian geometry of the universe.

The exact nature of dark energy is unknown. Perhaps, like the speed of light, it is merely a property of space for which there is no further explanation. Perhaps it is a basic form of vacuum energy whose existence is expressed by nothing but the continuous creation and elimination of virtual particles. Some scientists speculate that dark energy really is a fifth physical force called quintessence. It may eventually turn out that string theory has the final answer to the questions of what this mysterious energy consists in and by what mechanism it causes space to expand.

Links Relating to the Topic

■ Video: What is Dark Energy?
■ The Elegant Universe

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