Cosmic inflation in the context of "Physical cosmology"

The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models based on the Big Bang concept explain a broad range of phenomena, including the abundance of light elements, the cosmic microwave background (CMB) radiation, and large-scale structure. The uniformity of the universe, known as the horizon and flatness problems, is explained through cosmic inflation: a phase of accelerated expansion during the earliest stages. Detailed measurements of the expansion rate of the universe place the initial singularity at an estimated 13.787±0.02 billion years ago, which is considered the age of the universe. A wide range of empirical evidence strongly favors the Big Bang event, which is now widely accepted.

Extrapolating this cosmic expansion backward in time using the known laws of physics, the models describe an extraordinarily hot and dense primordial universe. Physics lacks a widely accepted theory that can model the earliest conditions of the Big Bang. As the universe expanded, it cooled sufficiently to allow the formation of subatomic particles, and later atoms. These primordial elements—mostly hydrogen, with some helium and lithium—then coalesced under the force of gravity aided by dark matter, forming early stars and galaxies. Measurements of the redshifts of supernovae indicate that the expansion of the universe is accelerating, an observation attributed to a concept called dark energy.

The horizon problem, also known as the homogeneity problem, is a cosmological fine-tuning problem within the Big Bang model of the universe. Observations of widely separated regions of space appear homogeneous, yet normal physical processes that create homogeneity require are causal connection and these regions are too far apart. Given the Einsteinian speed limit on communication, they have not had sufficient time to interact with each other since the Big Bang. This creates a difficulty in explaining the observed homogeneity without a mechanism that sets the same initial conditions everywhere. It was first pointed out by Wolfgang Rindler in 1956.

The most commonly accepted hypothesis to explain the horizon problem is cosmic inflation. Alternative solutions propose a cyclic universe or a variable speed of light.

The flatness problem (also known as the oldness problem) is a cosmological fine-tuning problem within the Big Bang model of the universe. Measurements find the current universe close to perfectly flat and expansion of the universe increases flatness. Consequently the early universe must have been exceptionally close to flat. In standard cosmology based on the Friedmann equations the density of matter and energy in the universe affects the curvature of space-time, with a very specific critical value being required for a flat universe. The current density of the universe is observed to be very close to this critical value. Since any departure of the total density from the critical value would increase rapidly over cosmic time, the early universe must have had a density even closer to the critical density, departing from it by one part in 10 or less. This leads cosmologists to question how the initial density came to be so closely fine-tuned to this 'special' value.

The problem was first mentioned by Robert Dicke in 1969. The most commonly accepted solution among cosmologists is cosmic inflation, the idea that the universe went through a brief period of extremely rapid expansion in the first fraction of a second after the Big Bang; along with the monopole problem and the horizon problem, the flatness problem is one of the three primary motivations for inflationary theory.

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 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 PhD from the Lebedev Physical Institute in Moscow. He worked at CERN (European Organization for Nuclear Research) since 1989 and moved to the United States in 1990, where he became professor of physics at Stanford University. Among the various awards he has 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 Prize in Cosmology for the development of inflationary cosmology. In 2012 he, along with Alan Guth, was an inaugural awardee of the Breakthrough Prize in Fundamental Physics. In 2014 he received the Kavli Prize in Astrophysics "for pioneering the theory of cosmic inflation", together with Alan Guth and Alexei Starobinsky. In 2018 he received the Gamow Prize.

An emergent Universe scenario is a cosmological model that features the Universe being in a low-entropy "dormant" state before the Big Bang or the beginning of the cosmic inflation. Several such scenarios have been proposed in the literature.

In cosmology, eternal inflation is a prediction of some forms of the model of cosmic inflation, a theory developed to explain issues in the original Big Bang theory. According to eternal inflation, the decay of the meta-stable false vacuum in one region led to the inflationary phase of the universe's expansion. In surrounding regions, the false vacuum continues to expand and decay can occur in surrounding regions, resulting in other universes inaccessibly distant from ours. This process lasts forever. Eternal inflation, therefore, produces a hypothetically infinite multiverse, in which only an insignificant fractal volume ends inflation.

Paul Steinhardt, one of the original researchers of the inflationary model, introduced the first example of eternal inflation in 1983, and Alexander Vilenkin showed that it is generic.