Accelerating universe

The accelerating universe is the observation that the universe appears to be expanding at an increasing rate, which in formal terms means that the cosmic scale factor has a positive second derivative,^[1] implying that the velocity at which a given galaxy is receding from us should be continually increasing over time^[2] (here the recession velocity is the same one that appears in Hubble's law; defining 'velocity' in cosmology is somewhat subtle, see Comoving Distance - Uses of the proper distance for a discussion). In 1998, observations of type Ia supernovae suggested that the expansion of the universe has been accelerating^[3][4] since around redshift of z~0.5.^[5] The 2006 Shaw Prize in Astronomy and the 2011 Nobel Prize in Physics were both awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess for the 1998 discovery of the accelerating expansion of the Universe through observations of distant supernovae.^[6][7]

Corroboration

After the initial discovery in 1998, these observations were corroborated by several independent sources: the cosmic microwave background radiation and large scale structure,^[8] apparent size of baryon acoustic oscillations,^[9] age of the universe,^[10] as well as improved measurements of the supernova,^[11][12] and X-ray properties of galaxy clusters.

Density drops

An expanding universe means that density drops due to continual space being added between all matter. If acceleration continues, eventually all galaxies beyond our own local supercluster will redshift so far that it will become hard to detect them, and the distant universe will turn dark.

Explanatory models

Models attempting to explain accelerating expansion include some form of dark energy, dark fluid or phantom energy. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space.

Divergent expansion

Phantom energy in a scenario known as the Big Rip causes an exponentially increasing divergent expansion, which overcomes the gravitation of the local group and tears apart our Virgo supercluster; it then tears apart the Milky Way Galaxy, our solar system, and finally even atoms. Measurements of acceleration are thought crucial to determining the ultimate fate of the universe, however we should expect the implications of such a major discovery to develop slowly over many years in the same way the big bang model has continued to develop.

Dark energy dominates

As the Universe expands, the density of dark matter declines more quickly than the density of dark energy (see equation of state) and, eventually, the dark energy dominates. Specifically, when the volume of the universe doubles, the density of dark matter is halved but the density of dark energy is nearly unchanged (it is exactly constant if the dark energy is a cosmological constant). In the cosmological constant models, the dark energy already dominates the mass-energy of matter, including dark matter, and the expansion of the universe is approximately exponential with time. In this scenario the scale factor doubling time of the expansion, in the future, will be approximately 11.4 billion years.

See also

• High-z Supernova Search Team
• Supernova Cosmology Project
• Metric expansion of space
• List of multiple discoveries

References