Dark Energy the reason for expanding universe at an accelerating rate.

In 1998 studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. Yet, according to Einstein's theory of General Relativity, gravity should lead to a slowing of the expansion. To explain cosmic acceleration, cosmologists are faced with two possibilities: Either 75% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

            The evidence came from studying distant “Type Ia” supernovae. This type of supernova results from having a white dwarf star a binary system. Matter transfers from the normal star to the white dwarf until the white dwarf attains a critical mass and undergoes a thermonuclear explosion. Because all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity and can be used by astronomers as "standard candles." Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r² law.

            By knowing the distance to these supernovae, we know how long ago they occurred. In addition, the light from the supernova has been red-shifted by the expansion of the Universe. By measuring this redshift from the spectrum of the supernova, astronomers can determine how much the Universe has expanded since the explosion. By studying many supernovae at different distances, astronomers can piece together a history of the expansion of the Universe.

            In the 1990's two teams of astronomers, the Supernova Cosmology Project and the High-Z Supernova Search were looking for distant Type Ia supernovae in order to measure the expansion rate of the Universe with time. They expected that the expansion would be slowing, which would be indicated by the supernovae being brighter than their redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the Universe was accelerating!

In addition, measurements of the cosmic microwave background indicate that the Universe has a flat geometry on large scales. Because there is not enough matter in the Universe either ordinary or dark matter to produce this flatness, the difference must be attributed to a "dark energy". This same dark energy causes the acceleration of the expansion of the Universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times.

            Astronomers know dark matter is there by its gravitational effect on the matter that we see, and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name "dark energy" refers to the fact that some kind of "stuff" must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a "field" just like an electric field or a magnetic field, both of which are produced by electromagnetic energy.

            Some astronomers identify dark energy with Einstein's Cosmological Constant. Einstein introduced this constant into his general relativity when he saw that his theory was predicting an expanding universe, which was contrary to the evidence for a static universe that he and other physicists had in the early 20th century. This constant balanced the expansion and made the Universe static. With Edwin Hubble's discovery of the expansion of the Universe, Einstein dismissed his constant. It later became identified with what quantum theory calls the energy of the vacuum.