The universe is expanding, as first described by Edwin Hubble in 1929. He found that the light from distant sources was shifted toward the red end of the spectrum. This indicated that the light waves had been "stretched" during their journey to Earth, and the reason for this is that the entire universe was "stretched" by a corresponding amount as it expanded. Now, 80 years on, this red-shift has been confirmed many times over. What astronomers wanted to determine was the rate at which the expansion was slowing down due to the gravitational attraction of all the matter in the Universe. Using a a Type 1A supernova as a standard candle, a number of astronomers spent 10 years researching this question. What they found in 1998 was astounding. Instead of the expansion rate slowing, it was speeding up over time. This was based on a Type 1A supernova explosions from which light was emitted between 1 and 10 billion years ago, and the initial results came from two independent research groups.
These observations have been corroborated a number of times since 1998, and there is strong evidence for this from the WMAP and SDSS projects as discussed in the previous part of this section; Cosmic Microwave Background (see menu, left). The model that is the best fit for observation is the Lambda/Cold Dark Matter (or LCDM) model. This gives contributions from Dark Energy, Dark Matter and Baryonic matter that are in close agreement with values derived in other ways. Lambda (L) here is the Cosmological Constant, and represents the dark Energy component which causes the expansion of the Universe. Cold Dark Matter indicates that the Dark Matter in this model moves at low, non-relativistic, velocity, that it does not emit electro-magnetic energy (photons) and it does not interact with baryonic matter except via gravity. This model also makes predictions about the size of the Universe that fit well with Inflation theory.
There are many models that try to explain accelerating expansion due to dark energy: a cosmological constant, quintessence, or some form of phantom energy. The latest seven year WMAP data tends to indicate a cosmological constant. As the Universe expands, the average density of dark matter reduces. The average density of dark energy, which is homogenously distributed and has negative pressure, that is it is repulsive, tends to stay constant. While the average energy equivalent of the matter in the Universe exceeded the average density of dark energy, the expansion of the Universe slowed down. At some point, currently believed to have been about seven billion years ago, the average energy equivalent of the matter in the Universe fell below that of dark energy, so the repulsive effect caused the expansion to start.
Astronomy & Cosmology
Large Scale Structure of the Universe