# Andrei Linde : biography

**Andrei Dmitriyevich Linde** ( born March 2, 1948) is a Russian-American theoretical physicist, the Harald Trap Friis Professor of Physics at Stanford University. Linde is one of the main authors of the inflationary universe theory, as well as the theory of eternal inflation and inflationary multiverse. He received his Bachelor of Science degree from Moscow State University. In 1975, Linde was awarded a Ph.D. from the Lebedev Physical Institute in Moscow. He worked at CERN (European Organization for Nuclear Research) since 1989 and moved to USA in 1990 where he became Professor of Physics at Stanford University. Among the various awards he's received for his work on inflation, in 2002 he was awarded the Dirac Medal, along with Alan Guth of MIT and Paul Steinhardt of Princeton University. In 2004 he received, along with Alan Guth, the Gruber Cosmology Prize for the development of inflationary cosmology. He is a member of the National Academy of Sciences of the USA and of the American Academy of Arts and Sciences.

## Chaotic inflation

In 1983, Linde abandoned some of the key principles of old and new inflation and proposed a more general inflationary theory, chaotic inflation. Chaotic inflation occurs in a much broader class of theories, without any need for the assumption of initial thermal equilibrium. The basic principles of this scenario became incorporated in most of the presently existing realistic versions of inflationary theory. Chaotic inflation changed the way we think about the beginning of inflation. Later on, Linde also proposed a possible modification of the way in which inflation may end, by developing the hybrid inflation scenario. In that model, inflation ends due to the "waterfall" instability.

## Cosmological phase transitions and old inflation

In 1972-1976, David Kirzhnits and Andrei Linde developed a theory of cosmological phase transitions. According to this theory, there was not much difference between weak, strong and electromagnetic interactions in the very early universe. These interactions became different from each other only gradually, after the cosmological phase transitions while the universe expanded and cooled down. In 1974 Linde found that the energy density of scalar fields breaking symmetry between different interactions can play the role of the vacuum energy density (cosmological constant) in the Einstein equations. In 1976-1978 he demonstrated that the release of this energy during the cosmological phase transitions may be sufficient to heat up the universe.

These observations became the main ingredients of the first version of the inflationary universe theory, which was proposed by Alan Guth in 1980. This theory, which is now called "old inflation," was based on the assumption that the universe was initially hot. It then experienced the cosmological phase transitions and was temporarily stuck in a supercooled metastable vacuum state (false vacuum). Then the universe expanded exponentially (inflated) until the false vacuum decayed, and the universe became hot again. This idea attracted lots of attention because it could provide a unique solution to many difficult problems of the standard Big Bang theory. In particular, it could explain why the universe is so large and so uniform. However, as Guth immediately realized, this scenario did not quite work as intended: the decay of the false vacuum would make the universe extremely inhomogeneous.

## Inflation and string theory

A significant advance in this area was obtained when the theory of inflationary multiverse was implemented in the context of string theory. In 2000, Bousso and Polchinski proposed to use the regime of eternal inflation and transitions between many different vacua in string theory for solving the cosmological constant problem. At that time, no stable or metastable vacua of string theory were actually known. A possible mechanism of string theory vacuum stabilization was proposed in 2003 by Kachru, Kallosh, Linde, and Trivedi, who also found that all of these vacua describing expanding universe are metastable, i.e. they must eventually decay. Then Douglas and his collaborators estimated that the total number of different stringy vacua can be as large as 10500, or even more, and Susskind developed the string theory landscape scenario based on investigation of cosmological phase transitions between different string theory vacua.