Inflation is the idea—first proposed by Alan Guth (1981)—that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. This expansion can be modelled by a non-zero cosmological constant. As a direct consequence of this expansion, all of the observable universe is posited to have originated in a small, initially causally-connected region. Quantum fluctuations in this microscopic region, magnified to cosmic size, then became the seeds for the growth of structure in the universe (see Galaxy Formation and Evolution).

Inflation resolves several problems in the Big Bang cosmology that were pointed out in the 1970s. Among these are the observed flatness of the universe (the flatness problem), its extraordinary homogeneity on large (non-causally-connected) scales (the horizon problem), and its lack of any observed topological defects (e.g., the monopole problem), predicted by many Grand Unified Theories.

Predictions of the standard model of inflation include geometrical flatness of the universe and near scale invariance of the primordial density fluctuations of the universe. These have been confirmed to great accuracy by WMAP's precision measurements of the cosmic microwave background and surveys of the distribution of galaxies observed by galaxy surveys. There are also consequences for high-energy particle physics near or at the GUT scale. During the 1980s, there were many attempts to relate the field that generates the vacuum energy to specific fields that were predicted by Grand Unified Theories or to use observations of the universe to constrain those theories. These efforts proved fruitless and the exact nature of the particle or field that generates the vacuum energy density for inflation (the "inflaton") remains a mystery. One major theoretical issue involving inflation is that it is unclear how one gets inflation to stop once it has started. In addition, inflation implies rapid cooling of the universe and therefore there must be a period of reheating after the end of the period of inflation.

One popular idea that has been suggested in the context of string theory and quantum gravity is that the universe actually contains many more dimensions of space than the three we experience in daily life, but that the universe only experienced inflation along the three normal dimensions of space, leaving the other dimensions to be microscopic. At present, there is no clear way to get inflation from string theory. A competing model is the ekpyrotic cosmology.

As of 2004, it is unclear what relationship if any the period of cosmic inflation has to do with observations of a present-day accelerating universe which has a much lower energy than cosmic inflation.

The name of the theory was a semi-humorous reference to the economic inflation in the United States in the late 1970s.

See also

• Cosmology
• Quantum mechanics

External links

• Was Cosmic Inflation The 'Bang' Of The Big Bang? (http://nedwww.ipac.caltech.edu/level5/Guth/Guth_contents.html), by Alan Guth, 1997
• An Introduction to Cosmological Inflation (http://arxiv.org/abs/astro-ph/9901124) by Andrew Liddle, 1999
• update 2004 (http://www2.iap.fr/Conferences/Colloque/col2004/Docs/20040628_liddle.pdf) by Andrew Liddle
• hep-ph/0309238 Laura Covi: Status of observational cosmology and inflation (http://arxiv.org/abs/hep-ph/0309238)
• hep-th/0311040 David H. Lyth: Which is the best inflation model? (http://arxiv.org/abs/hep-th/0311040)

References

• Alan H. Guth, "The Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems", Physical Review D23, 347 (1981).
• Andrei Linde (2002). Inflationary Theory versus Ekpyrotic/Cyclic Scenario. Presented at Stephen Hawking's 60th birthday conference. (online) (http://de.arxiv.org/abs/hep-th/0205259)

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