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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 Fluctuation s 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 ''. The name of the theory was a semi-humorous reference to the economic Inflation in the United States in the late 1970s. seen on the left. Image from WMAP press release, 2006 .]] MOTIVATION 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 Survey s (such as the Sloan Digital Sky Survey ). There are also consequences for high-energy for the hot early universe, and the particle physics is largely ''ad hoc'' modelling. MECHANISM The original model of inflation,A. H. Guth, "The Inflationary Universe: A Possible Solution to the Horizon and Flatness Problems", ''Phys. Rev. D'' 23, 347 (1981). proposed by or Energy Conservation and the Second Law Of Thermodynamics or the Arrow Of Time problem. However, the original model of Guth fails because, in order to guarantee a sufficient amount of inflation to solve the standard problems, the bubble nucleation rate must be too low for bubble walls to collide and for the reheating process to actually work, because the space between bubbles - which is still in the inflating phase - expands so fast that the separation between bubbles grows faster than the bubbles themselves. Each bubble expands in a finite rate - no faster than the speed of light, so we are supposed to see the bubble walls when we look at the universe, but we don't. This is called the "graceful exit problem" and Guth's original model is now called "old inflation." . In "slow-roll inflation", inflation terminates when the inflaton potential reaches the end of its nearly-flat part, where its slope starts to increase and the roll speeds up. This is when ''reheating'' occurs in this scenario, as particles are created via ineractions with the inflaton, on the expense of the potential's energy density. New inflation is generally eternal: that is, the process continues eternally. Although the scalar field is classically rolling down the potential, quantum fluctuations occasionally bring it back up the potential. These regions expand much faster than regions in which the inflaton has a lower potential energy. Thus, while inflation ends in some regions, the regions in which it continues are growing exponentially, and thus continue to dominate. This steady state, which was first described by Andrei Linde ,1 2 in which inflation ends in some regions while quantum mechanical fluctuations keep it going in the majority of the universe, is called "eternal inflation". Inflation, however, cannot be eternal in the ''past'', and so does not solve the problem of initial conditions for the universe.3 4 5 A set of models called ''hybrid inflation'' solves this problem by introducing (at least) one more scalar field (a second ''inflaton''), so that one of the inflatons is responsible for most of the energy density (thus determining the rate of expansion), while the other is responsible for the slow roll (thus determining the period of inflation and its termination). Thus fluctuations in the former inflaton won't affect inflation termination, while fluctuations in the latter won't affect the rate of expansion. Therefor hybrid inflation is not eternal. When the second (slow-rolling) inflaton reaches at the bottom of its potential, it changes the location of the minimum of the first inflaton's potential, which leads to a fast roll of the this inflaton down its potential, leading to termination of inflation {Link without Title} . Inflation causes rapid cooling of the universe and so it must be followed by a period of reheating to allow for a hot early universe. Reheating is thought to involve a phenomenon (still under study) called ''parametric resonance'', involving a resonant decay of the inflaton, as it oscillates during the termination of inflation, into other particles. Recent observation of the it must have a low Mass (and a large Compton Wavelength ). However high energy physics is thought to include many scalar fields (this is, for example, the situation in String Theory ) and a large number of possible solutions is also expected, especially in String Theory (see Landscape ). 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. This theory, called string gas cosmology, was proposed by Robert Brandenberger and Cumrun Vafa . It suggested that we have three large dimensions because of certain topological properties of colliding strings. However, considerable doubt has been cast on the practicability of these ideas. The Ekpyrotic , Cyclic Model s and Variable Speed Of Light cosmology are considered competitors to inflation. OBSERVATIONS 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 Planck Satellite , Clover Project and other ground-based cosmic microwave background experiments. However, the first experimental confirmation of some predictions of cosmic inflation theories has been provided by data from the WMAP mission in March 2006. The WMAP polarization data seem to favor the simplest versions of inflation. As of 2006, 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. REFERENCES FURTHER READING
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