Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the very early universe. This enormous expansion supercooled the universe and ended when the energy content of the field driving inflation condensed into hot, dense particles, a process called reheating. Following the inflationary period, the universe continued to expand, but at a slower rate.
Inflation theory was developed in the late 1970s and early 1980s, with notable contributions by several theoretical physicists, including Alexei Starobinsky at Landau Institute for Theoretical Physics, Alan Guth at Cornell University, and Andrei Linde at Lebedev Physical Institute. Starobinsky, Guth, and Linde won the 2014 Kavli Prize "for pioneering the theory of cosmic inflation". It was developed further in the early 1980s. It explains the origin of the large-scale structure of the cosmos. Quantum fluctuations in the microscopic inflationary region, magnified to cosmic size, become the seeds for the growth of structure in the Universe. Many physicists also believe that inflation explains why the universe appears to be the same in all directions, why the cosmic microwave background radiation is distributed evenly, why the universe is flat, and why no magnetic monopoles have been observed.
The detailed particle physics mechanism responsible for inflation is unknown. A number of inflation model predictions have been confirmed by observation; for example temperature anisotropies observed by the COBE satellite in 1992 exhibit nearly scale-invariant spectra as predicted by the inflationary paradigm and WMAP results also show strong evidence for inflation. However, some scientists dissent from this position. The hypothetical field thought to be responsible for inflation is called the inflaton.
In 2002, three of the original architects of the theory were recognized for their major contributions; physicists Alan Guth of M.I.T., Andrei Linde of Stanford, and Paul Steinhardt of Princeton shared the Dirac Prize "for development of the concept of inflation in cosmology". In 2012, Guth and Linde were awarded the Breakthrough Prize in Fundamental Physics for their invention and development of inflationary cosmology.
Concept
Cosmic inflation is the hypothesis that the very early universe expanded exponentially fast. Distances between points doubled every 10-37 seconds; the expansion lasted at least 10-35 seconds, but its full duration is not certain. All of the mass-energy in all of the galaxies currently visible started in a sphere with a radius around 4 x 10-29 m then grew to a sphere with a radius around 0.9 m by the end of inflation. At the end of inflation the driving field converts to particles, leading to a quark-soup phase of the universe, a phase that retains small density variations due to quantum fluctuations in the original small smooth patch of the universe.Motivations
Inflation resolves several problems in Big Bang cosmology that were discovered in the 1970s. The Big Bang model successfully explained the cosmic microwave background and synthesis of primordial elements. However these successes relied on assuming initial conditions that were difficult to justify. For example, the model has no mechanism to create density fluctuation which could explain the formation of galaxies. When particle physicists took up the problem of the very early universe they immediately found additional problems. Inflation was first proposed by particle physicist Alan Guth in 1979 while investigating the problem of why no magnetic monopoles are seen today; he found that a positive-energy false vacuum would, according to general relativity, generate an exponential expansion of space. The expansion also resolves other long-standing problems including the flatness problem and the horizon problem as discussed below.Structure formation
The Big Bang theory postulates an initial very hot uniform plasma that expands according to the equations of general relativity and ultimately produces all of the stars and galaxies. The production of stars assumes that gravity causes mass to clump, but this requires density contrast: a completely uniform mass density has no force to drive clumping. Statistical variations in density would provide the force, but expansion of the universe works faster, pulling the mass apart before it can concentrate into a star. Without an additional source of variation, Big Bang models could not produce stars.Magnetic-monopole problem
Stable magnetic monopoles are a problem for Grand Unified Theories, which propose that at high temperatures, the electromagnetic force, strong, and weak nuclear forces are not actually fundamental forces but arise due to spontaneous symmetry breaking from a single gauge theory. These theories predict a number of heavy, stable particles that have not been observed in nature. The most notorious is the magnetic monopole, a kind of stable, heavy "charge" of magnetic field.Monopoles are predicted to be copiously produced following Grand Unified Theories at high temperature, and they should have persisted to the present day, to such an extent that they would become the primary constituent of the Universe. Not only is that not the case, but all searches for them have failed, placing stringent limits on the density of relic magnetic monopoles in the Universe.
A period of inflation that occurs below the temperature where magnetic monopoles can be produced would offer a possible resolution of this problem: Monopoles would be separated from each other as the Universe around them expands, lowering their observed density by many orders of magnitude.
While solving the monopole problem motivated the original hypothesis, not every cosmologist was impressed. Martin Rees has written,
However, the flatness and especially the horizon problem are also solved by inflation theory.
Flatness problem
The flatness problem is a cosmological fine-tuning problem within the Big Bang model of the universe. Observations of the cosmic microwave background have demonstrated that the Universe is flat to within a few percent. The 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 1062 or less. This leads cosmologists to question how the initial density came to be so closely fine-tuned to this 'special' value.
Horizon problem
The horizon problem is the problem of determining why the universe appears statistically homogeneous and isotropic in accordance with the cosmological principle. For example, molecules in a canister of gas are distributed homogeneously and isotropically because they are in thermal equilibrium: gas throughout the canister has had enough time to interact to dissipate inhomogeneities and anisotropies. In the Big Bang model without inflation, gravitational expansion separates regions too quickly: the early universe does not have enough time to equilibrate. In a Big Bang with only the matter and radiation known in the Standard Model, two widely separated regions of the observable universe cannot have equilibrated because they move apart from each other faster than the speed of light and thus have never come into causal contact.Theory
Each of the motivations for inflation are issues related to the initial conditions for the expansion of the universe.The inflation solution starts with a tiny universe in thermal equilibrium then expands it much faster than the speed of light, so fast that the equilibrated parts are widely separated by the time gravitational expansion takes over. The results is a homogeneous and isotropic universe as the initial conditions for the expansion predicted by general relativity.
The theory of inflation thus explains why the temperatures and curvatures of different regions are so nearly equal. It also predicts that the total curvature of a space-slice at constant global time is zero. This prediction implies that the total ordinary matter, dark matter and residual vacuum energy in the Universe have to add up to the critical density, and the evidence supports this. More strikingly, inflation allows physicists to calculate the minute differences in temperature of different regions from quantum fluctuations during the inflationary era, and many of these quantitative predictions have been confirmed.
Space expands
Looking backwards to smaller scales and higher energy in the very early universe eventually leads to a point where existing models may not be valid. There is no particular reason to expect normal physics to apply. The inflation hypothesis is that in this very early time space expands exponentially by many orders of magnitude.In a space that expands exponentially with time, any pair of free-floating objects that are initially at rest will move apart from each other at an accelerating rate, at least as long as they are not bound together by any force. From the point of view of one such object, the spacetime is something like an inside-out Schwarzschild black hole—each object is surrounded by a spherical event horizon. Once the other object has fallen through this horizon it can never return, and even light signals it sends will never reach the first object.
In the approximation that the expansion is exactly exponential, the horizon is static and remains a fixed physical distance away. This patch of an inflating universe can be described by the following metric:
This exponentially expanding spacetime is called a de Sitter space, and to sustain it there must be a cosmological constant, a vacuum energy density that is constant in space and time and proportional to Λ in the above metric. For the case of exactly exponential expansion, the vacuum energy has a negative pressure p equal in magnitude to its energy density ρ; the equation of state is p=−ρ.
Inflation is typically not an exactly exponential expansion, but rather quasi- or near-exponential. In such a universe the horizon will slowly grow with time as the vacuum energy density gradually decreases.