physical cosmology Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of f ...

, the graceful exit problem refers to an inherent flaw in the initial proposal of the inflationary universe theory proposed by

Alan Guth Alan Harvey Guth (; born February 27, 1947) is an American theoretical physicist and cosmologist. Guth has researched elementary particle theory (and how particle theory is applicable to the early universe). He is Victor Weisskopf Professor of ...

in 1981. In Guth’s model, the period of accelerated expansion (a.k.a.

inflation In economics, inflation is an increase in the general price level of goods and services in an economy. When the general price level rises, each unit of currency buys fewer goods and services; consequently, inflation corresponds to a reductio ...

) makes the universe homogeneous and flat but can never end. The problem with that model, later referred to as “old inflation,” is that an energy barrier prevents the universe from escaping inflation, blocking the universe from reheating and forming the stars and galaxies observed today. In 1982, a solution to the graceful exit problem was found independently by

Andrei Linde Andrei Dmitriyevich Linde (russian: Андре́й Дми́триевич Ли́нде; born March 2, 1948) is a Russian-American theoretical physicist and the Harald Trap Friis Professor of Physics at Stanford University. Linde is one of the ...

and the team of Andreas Albrecht and

Paul J. Steinhardt Paul Joseph Steinhardt (born December 25, 1952) is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University, where he ...

. The new model, subsequently referred to as “new inflation,” replaced the energy barrier with an “energy plateau” so that inflation takes place for a period but stops once the plateau is traversed. The Albrecht-Steinhardt paper also revealed an important gravitational effect known as “Hubble friction,” which helps sustain inflation for a sufficiently long period of time (referred to as the “slow-roll” effect). In 1983, James Bardeen, Steinhardt and Michael S. Turner showed that Hubble friction is essential for generating a spectrum of density fluctuations at the end of inflation that could explain the origin of galaxies and the temperature variations in the cosmic microwave background. Most subsequent inflationary models solve the graceful exit problem using similar principles.

References

Inflation (cosmology) {{physical-cosmology-stub