Inflationary Universe Theory

The Inflationary Universe Theory proposes a brief period of extremely rapid accelerating expansion in the very early universe, before the radiation dominated era called the hot big bang. This acceleration is believed to be driven by a quantum field (in effect, some exotic kind of matter) with a repulsive gravitational effect. This can be achieved if the pressure of the field is extremely large and negative (unlike ordinary matter, which has positive pressure).

A specific example is a scalar field associated with a potential energy. Such a field "rolls down" the energy surface defined by the potential, and if it is slow-rolling can act like an effective cosmological constant, driving an exponential expansion with constant acceleration. During this epoch, any matter or radiation density other than that of the scalar field is negligible; one is left with an almost constant energy density of the field, often called a false vacuum because it behaves like the highly energetic vacuum of quantum field theory. Every 10-37 seconds the size of an inflating patch doubles with its energy density remaining constant, so the total mass in the region increases by a huge factor. Inflation ends through decay of the repulsive material into a mixture of matter and radiation, this decay taking place by quantum processes similar to radioactive decay of ordinary matter. The resulting hot expanding gas provides the starting point for the hot big bang era in the early universe.

This scenario provides explanations for some puzzles in cosmology: why the universe is so large, why it is so uniform, and why it is so nearly flat (scientists can not detect the large-scale spatial curvature effects associated with general relativity). Most importantly, this scenario provides an explanation for the origin of large-scale structure in the universe: Clusters of galaxies arise from seed perturbations generated by quantum fluctuations in the very early universe, amplified vastly in size by the inflationary expansion of the universe and in amplitude by gravitational instability after the decoupling of matter and radiation. A major triumph of the theory is that the subtle variations in the cosmic background radiation it predicted have been observed from satellites and balloons.

One popular version of the theory (Chaotic Inflation ) proposes that ever more inflationary bubbles are generated and expand to vast size, so that on the largest scales the universe is an eternally reproducing foam-like structure of interleaved inflating and post-inflation regions. It should be noted, however, that this proposition is not observationally testable. Indeed, despite its successes, inflation is not yet a fully developed physical theory; in particular the field (or fields) causing inflation (the inflaton ) has neither been identified nor shown actually to exist. Moreover, various theoretical conundrums remain, for example the problem of exactly how inflation ends, how probable it is that inflation will succeed in starting in an extremely inhomogeneous and anisotropic situation, and how successful inflation can be in smoothing out the universe if arbitrary initial conditions are allowed. (A cosmology is anisotrophic if the physical situation appears very different when we observe from different directions in the sky.) Despite these theoretical problems, and the difficulties in testing the physics proposed, inflation is currently the dominant explanatory paradigm for the physics of the early universe. It has generated immense interest because it provides a major link between particle physics and cosmology, allowing cosmological observations to be used for testing theories in particle physics.

See also Big Bang Theory; Cosmology, Physical Aspects; Physics, Particle; Physics, Quantum

Bibliography

guth, alan. the inflationary universe: the quest for a new theory of cosmic origins. reading, mass.: addison wesley, 1997.

kolb, edward w., and turner, michael s. the early universe. new york: wiley, 1990.

liddle, andrew r., and lyth, david h. cosmological inflation and large-scale structure. cambridge, uk: cambridge university press, 2000.

linde, andrei d. particle physics and inflationary cosmology. chur, switzerland: harwood academic, 1990.

peacocke, j. a. cosmological physics. cambridge, uk: cambridge university press, 1999.

george f. r. ellis