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Cosmic inflation is the idea, first proposed by Alan Guth in 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 is similar to a de Sitter universe with positive cosmological constant. As a direct consequence of this expansion, all of the observable universe originated in a small 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). The particle responsible for inflation is generally called the inflaton.
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 (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 precision measurements of the cosmic microwave background (such as those made by the WMAP satellite) and surveys of the distribution of galaxies observed by galaxy surveys (such as the Sloan Digital Sky Survey).
There are also consequences for high-energy particle physics near or at the GUT scale, as the simplest models of inflation have energies around the GUT scale, at 1015GeV. 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 were largely fruitless and the exact nature of the particle or field that generates the vacuum energy density for inflation (the "inflaton") remains a mystery: inflation is understood principally by its detailed predictions of the initial conditions for the hot big bang, and the particle physics is largely ad hoc modelling.
One theoretical challenge for inflation arises from the need to fine tune the potentials for the fields which may give rise to inflation: while the inflaton must have a large vacuum energy it must have a low mass (and a large Compton wavelength). In addition, inflation causes rapid cooling of the universe and so it must be followed by a period of reheating before the hot big bang can begin. It is not known how reheating occurs, although several models have been proposed. The original model of inflation proposed by Guth, which postulated that inflation is caused by a false vacuum did not have a graceful exit: it ended too soon and the reheating process destroyed some of the predictions of inflation. This was resolved by new inflation, proposed by Andreas Albrecht and Paul Steinhardt in 1982.
Observationally, it is hoped that improved measurements of the cosmic microwave background will tell us more about inflation. In particular, high precision measurements of the polarization of the background radiation will tell us if the energy scale of inflation predicted by the simplest models is correct, and measurements of the spectrum of primordial fluctuations will tell us if our naive models of inflation can produce the correct primordial fluctuations. A perfectly scale invariant spectrum is generally considered incompatible with the simplest models of inflation as is a running spectral index (a spectrum with curvature). These sorts of measurements are expected to be performed by the WMAP and Planck satellites and future ground-based cosmic microwave background experiments.
As of 2005, it is unclear what relationship if any the period of cosmic inflation has to do with observations of dark energy in the universe. Dark energy, particularly quintessence is broadly similar to inflation, but occurs at a much lower energy, 10-12GeV, at least 27 orders of magnitude less than the scale 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, but that the universe only inflated along the three normal dimensions of space. The other dimensions remain microscopic and are not detectable at laboratory energy scales. However, at present, there is no clear way to get inflation from string theory. A competing model to inflation is the ekpyrotic cosmology.
- Was Cosmic Inflation The 'Bang' Of The Big Bang?, by Alan Guth, 1997
- An Introduction to Cosmological Inflation by Andrew Liddle, 1999
- update 2004 by Andrew Liddle
- hep-ph/0309238 Laura Covi: Status of observational cosmology and inflation
- hep-th/0311040 David H. Lyth: Which is the best inflation model?
- 13 things that do not make sense
- Alan H. Guth, "The Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems", Physical Review D23, 347 (1981).
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