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# Andrei Linde

Professor of Physics

## Contact Information

- office:
Department of Physics

Varian blg. 386

Stanford University

Stanford, CA 94305

- phone: (650) 723 2687 and 650 494 6106
- FAX: (650) 725 6544
- email: alinde@stanford.edu

## Research Interests

I am one of the authors of the inflationary cosmology and of the theory
of the cosmological phase transitions. These two topics remain the main
subject of my work. Current research also involves the theory of dark
energy, investigation of the global structure and the fate of the
universe, cosmological constraints on the properties of elementary
particles, and quantum cosmology.

## Books

#### 1. A.D. Linde, INFLATION AND QUANTUM COSMOLOGY (Academic Press,
Boston 1990)

## Career History

- B.S., Moscow State University
- Ph.D., 1975, Lebedev Physical Institute, Moscow
- Professor, Lebedev Physical Institute, Moscow, 1985-89
- Staff Member of CERN, Switzerland, 1989-90
- Professor of Physics, Stanford University, 1990-present
- Lomonosov Award of the Academy of Sciences of the USSR, 1978
- Oskar Klein medal in physics, 2001
- Dirac medal for the development of inflationary cosmology, 2002
- Peter Gruber Prize for for the development of inflationary
cosmology, 2004
- Humboldt Research Award, Germany, 2004
- Robinson Prize for Cosmology, Newcastle University, UK, 2005
- Medal of the Institute of Astrophysics, Paris, France, 2006
- Member of the National Academy of Sciences, 2008
- Harald Trap Friis Professor in Physics at Stanford, 2009
- Corresponding Member of the Hamburg Academy of Sciences, Germany,
2010
- Member of the American Academy of Arts and Sciences, 2011
- Fundamental Physics Prize, 2012
- Kavli Prize, 2014

Inflationary theory describes the very early stages of the evolution of
the Universe, and its structure at extremely large distances from us.
For many years, cosmologists believed that the Universe from the very
beginning looked like an expanding ball of fire. This explosive
beginning of the Universe was called the big bang. In the end of the
70's a different scenario of the evolution of the Universe was
proposed. According to this scenario, the early universe came through
the stage of inflation, exponentially rapid expansion in a kind of
unstable vacuum-like state (a state with large energy density, but
without elementary particles). Vacuum-like state in inflationary theory
usually is associated with a scalar field, which is often called ``the
inflaton field.'' The stage of inflation can be very short, but the
universe within this time becomes exponentially large.
Initially, inflation was considered as an intermediate stage of the
evolution of the hot universe, which was necessary to solve many
cosmological problems. At the end of inflation the scalar field
decayed, the universe became hot, and its subsequent evolution could be
described by the standard big bang theory. Thus, inflation was a part
of the big bang theory. Gradually, however, the big bang theory became
a part of inflationary cosmology. Recent versions of inflationary
theory assert that instead of being a single, expanding ball of fire
described by the big bang theory, the universe looks like a huge
growing fractal. It consists of many inflating balls that produce new
balls, which in turn produce more new balls, ad infinitum. Therefore
the evolution of the universe has no end and may have no beginning.
After inflation the universe becomes divided into different
exponentially large domains inside which properties of elementary
particles and even dimension of space-time may be different. Thus the
universe looks like a multiverse consisting of many universes with
different laws of low-energy physics operating in each of them. Thus,
the
new cosmological theory leads to a considerable modification of the
standard point of view on the structure and evolution of the universe
and on our own place in the world.
A description of the new cosmological theory can be found, in
particular, in my article The Self-Reproducing
Inflationary Universe published in Scientific American, Vol. 271,
No. 5, pages 48-55, November 1994. A nice introduction to inflation was
written by the journalist and science writer John Gribbin Cosmology for
Beginners . The new cosmological paradigm may have non-trivial philosophical implications. In particular, it
provides a scientific
justification of the cosmological anthropic principle, and allows
one to discuss a
possibility to create the universe in a laboratory.

## A Brief History of the Multiverse

The idea of an inflationary multiverse (the universe consisting of many
universes with different properties) was first proposed in 1982 in my
Cambridge University preprint Nonsingular
Regenerating Inflationary Universe . A more detailed discussion of
this possibility was contained in my paper The New
Inflationary Universe Scenario published in the book "The Very
Early Universe," ed. G.W. Gibbons, S.W. Hawking and S.Siklos, Cambridge
University Press, 1983, pp. 205-249. Implications of this picture for
the "SUSY landscape" (the universe with different properties
corresponding to different vacua of
supersymmetric theories) was discussed in my paper Inflation Can Break Symmetry In SUSY, Phys.
Lett. B131, 330 (1983).
A major step in the development of the theory of the multiverse was
related to the
discovery of eternal inflation; for a discussion of its anthropic
implications see the last page of my paper
Eternally Existing Self-Reproducing Chaotic Inflationary Universe,
Phys. Lett. B175, 395 (1986).
The methods of calculation of the probability to live in the parts of
the universe with different properties were developed in my paper with
Dimitri Linde and Arthur Mezhlumian From the Big Bang Theory to
the Theory of a Stationary Universe, in my paper with Juan
Garcia-Bellido and Dimitri Linde Fluctuations of the
Gravitational Constant in the Inflationary Brans-Dicke Cosmology,
and in the paper by Alex Vilenkin Predictions from Quantum
Cosmology, who called these methods "the mediocrity principle."
One of the most important implications of the anthropic principle in
the context of inflationary multiverse is
the possility to solve the cosmological constant problem. The first
anthropic solution of the cosmological constant problem was proposed at
the last page of my review article The
Inflationary Universe , Rept. Prog. Phys. 47, 925 (1984). My
second proposal was made in my paper Inflation
and Quantum Cosmology. It was written in June 1986, and published
in the book "300 years of gravitation," (Eds.: S.W. Hawking and W.
Israel, Cambridge Univ. Press, 1987), 604-630. The main goal of these
two papers was to propose a physical mechanism which would allow the
existence of different exponentially large parts of the universe with
different values of the cosmological constant. Until the invention of
the inflationary theory, this was an unsolvable problem. In addition to
this problem addressed in my papers mentioned above, one must also show
that life can hardly exist in the parts of the universe where the
cosmological constant is much greater than the present energy density
in our part of the universe. Validity of this order-of-magnitude
condition was pretty obvious even 20 years ago, and it was taken for
granted in my works mentioned above. However, in order to have a
reliable anthropic solution for the cosmological constant problem one
should know a more precise anthropic bound on the cosmological
constant. The progress in this direction began in 1987 with the famous
paper by Steven Weinberg Anthropic
Bound on the Cosmological Cosntant .
His work and the subsequent developments confirmed the assumption that
the probability of existence of life of our type becomes strongly
suppressed if the cosmological constant is much greater than the
present energy density in the universe. The experimental discovery of
the cosmological constant satisfying the anthropic bound was greeted as
an experimental evidence in favour of the multiverse scenario.
One of the most important recent steps in the development of the
multiverse theory was a discovery of the KKLT mechanism of moduli
stabilization in string theory, which allows to explain accelerated
expansion of the universe and inflation in the context of string
theory. The KKLT mechanism can lead to an incredibly large number
of different vacua, perhaps 10^{100}
or even 10^{1000}, corresponding to different local minima of
energy in a vast string
theory landscape.
This means that our multiverse may consist of exponentially many
exponentially large domains (universes), each of which may live in
accordance to one of the exponentially large variety of laws of the
low-energy physics.

## Are there any Alternatives to Inflation?

There were many attempts to replace inflation by other theories. One
attempt that attracted a lot of attention in the media is called the
ekpyrotic/cyclic scenario. However, ekpyrotic/cyclic
scenario scenario is plagued by numerous problems. The original
version of the ekpyrotic theory, which was supposed to be a true
alternative to inflation, did not work. It was replaced by the cyclic
scenario, which also suffers from many problems, including the yet
unsolved problem of the cosmological singularity. Independently of
these issues, solving the homogeneity problem in the cyclic scenario
requires an infinite sequence of periods of accelerated expansion of
the universe in a vacuum-like state, i.e. an infinite number of
inflationary stages. In this sense, instead of being a true alternative
to inflation, the cyclic scenario is a rather unusual and problematic
version of inflationary theory. Thus, at present, inflation remains the
only robust mechanism that produces
density perturbations with a flat spectrum and simultaneously solves
all
major cosmological problems.

Observational data indicate that the universe accelerates. If this is
caused by the positive vacuum energy (cosmological constant),
acceleration of the universe will continue forever. However, recently
we have found that in a broad class of theories describing the present
stage of acceleration of the universe, this acceleration may end and
the universe may eventually collapse. Rather unexpectedly, we found
that this may happen not in an extremely distant future, as one could
expect, but in about 10-20 billion years. This may happen in a broad
class of realistic theories of elementary particles, including, in
particular the popular theories based on M-theory and supergravity.
This is not a doomsday prediction because other outcomes (such as
eternal acceleration of the universe) are also theoretically possible
and are equally compelling. The only way to find out which of these
possibilities is more realistic is to make cosmological observations.
These results may have important implications. One may argue: Why do I
care about the most abstract theories of elementary particles, such as
M-theory, string theory or supergravity? Why do I care about precise
measurements of cosmological parameters? Why do we need to spend
billions of dollars for the development of science?
Now we can add something new to the existing arguments: Without the
development of these theories and without cosmological observations we
will be unable to know the fate of the universe and the fate of the
mankind.
Here one can find
a popular discussion of our work (see also an article in
SF Chronicle).

### 1. Self-Reproduction of the Universe

Inflationary cosmology is different in many respects from the standard
big bang cosmology. Domains of the inflationary universe with
sufficiently large energy density permanently produce new inflationary
domains due to stochastic processes of generation of the long-wave
perturbations of the scalar field. Therefore the evolution of the
universe in the inflationary scenario has no end and may have no
beginning.
Here we present the results of computer simulations of generation of
quantum fluctuations in the inflationary universe. These processes
should occur in the very early universe, at the densities just below
the Planck density.
1) Series of figures in gold show generation of fluctuations of the
scalar field $\varphi$ during inflation. Classically, the value of this
field should decrease, but quantum perturbations lead to formation of
exponentially large domains containing the scalar field which is much
bigger than its initial value. In particular, calculation of the volume
of the parts of the universe corresponding to the peaks of the
``mountains'' shows that it is much bigger than the volume of the parts
where the scalar field rolled to the minimum of its energy density.
2) Series of figures in red, blue and green show evolution of another
scalar field, which has three different minima of its potential energy
density. In the regions when the inflaton field is large (it is
represented by the hight of the mountains), the second field strongly
fluctuates. In the domains where the inflaton field is small, the
second field relaxes near one of the three minima of its potential
energy density, shown by red, blue and green correspondingly. Each such
domain is exponentially large. If the second field is responsible for
symmetry breaking in the theory, then the laws of low-energy physics
inside domains of different colors are different. The universe globally
looks not like an expanding ball, but like a huge fractal consisting of
exponentially large domains permanently produced during inflation.
3) The third movie shows only the evolution of the second field,
determining the choice of the symmetry breaking (shown by dirrerent
colors), so the images are two-dimensional. This made it possible to
perform simulations on a much greater scale and with a much better
resolution. We called this series of images ``Kandinsky universe,''
after the famous Russian abstractionist.

## Images

## PowerPoint Talks

Stanford2008
Paris2008
Nikko2008
Kyoto2008
SUSY2008
Varna 2008