Dark Energy:
An invisible, hypothetical form of energy with repulsive gravitythat permeates all of space and that may explain recent observations that the universe appears to be expanding at an accelerating rate. In some models of cosmology, dark energy accounts for 74% of the total mass-energy of the universe. Its exact nature remains a mystery, although Einstein’s hypothesized “cosmological constant” is now considered a promising candidate.
Dark Matter:
Matter that gives out no light and does not interact with the electromagnetic force, but whose presence can be inferred from gravitational effects on visible matter. It is estimated that there may be between 6 and 7 times as much dark matter as normal, bright matter in the universe, although its exact nature remains a mystery.
Decoherence:
The process by which bodies and quantum systems lose some of their more unusual quantum properties (e.g. superposition, or the ability to appear in different places simultaneously) as they interact with their environments. When a particle decoheres, itsprobability wave collapses, any quantum superpositionsdisappear and it settles into its observed state under classical physics.
Density:
The mass of an object divided by its volume, a measure of how much it is compacted or crowded together (e.g. air is low in density, iron is high). Boyle’s Law dictates that a substance increases in density as its pressure is increased or as its temperature is decreased.
Dimensions:
Independent directions in space-time. We are familiar with the three dimensions of space (length, width and height, or east-west, north-south and up-down) and one of time (past-future), but superstring theory, for example, requires the universe to have ten dimensions.
DNA:
Deoxyribonucleic acid (DNA) molecules consist of two long intertwined polymers of nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds, structured as the familiar double helix. DNA is responsible for the long-term storage of genetic information, and specifies the sequence of the amino acids within proteins. It is organized into structures called chromosomes, and contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The first accurate model of the structure of DNA was formulated by James Watson and Francis Crick in 1953. The genetic information from DNA is transmitted into the nucleus of cells by molecules of RNA, which controls certain chemical processes in the cell. Both DNA and RNA are considered essential building blocks of life.
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