Cosmic inflation

Physical cosmology

Age of the universe Big Bang Comoving distance Cosmic microwave background Dark energy Dark matter FLRW metric Friedmann equations Galaxy formation Hubble's law Inflation Large-scale structure Lambda-CDM model Nucleosynthesis Observable universe Redshift Shape of the universe Timeline of the Big Bang Timeline of cosmology Ultimate fate of the universe Universe

Related topics

Astrophysics General relativity Particle physics Quantum gravity

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Cosmic inflation is the idea, first proposed by Alan Guth in 1981, that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density. This expansion is similar to a de Sitter universe with positive cosmological constant. As a direct consequence of this expansion, all of the observable universe originated in a small causally-connected region. Quantum fluctuations in this microscopic region, magnified to cosmic size, then became the seeds for the growth of structure in the universe (see galaxy formation and evolution). The particle responsible for inflation is generally called the inflaton.

The name of the theory was a semi-humorous reference to the economic inflation in the United States in the late 1970s.

Motivation

Inflation resolves several problems in the Big Bang cosmology that were pointed out in the 1970s. Among these are the observed flatness of the universe (the flatness problem), its extraordinary homogeneity on large (non-causally-connected) scales (the horizon problem), and its lack of any observed topological defects (the monopole problem), predicted by many Grand Unified Theories. Predictions of the standard model of inflation include geometrical flatness of the universe and near scale invariance of the primordial density fluctuations of the universe. These have been confirmed to great accuracy by precision measurements of the cosmic microwave background (such as those made by the WMAP satellite) and surveys of the distribution of galaxies observed by galaxy surveys (such as the Sloan Digital Sky Survey).

There are also consequences for high-energy particle physics near or at the GUT scale, as the simplest models of inflation have energies around the GUT scale, at 10¹⁵ GeV. During the 1980s, there were many attempts to relate the field that generates the vacuum energy to specific fields that were predicted by Grand Unified Theories or to use observations of the universe to constrain those theories. These efforts were largely fruitless and the exact nature of the particle or field that generates the vacuum energy density for inflation (the "inflaton") remains a mystery: inflation is understood principally by its detailed predictions of the initial conditions for the hot big bang, and the particle physics is largely ad hoc modelling.

Mechanism

The original model of inflation, proposed by Alan Guth, had the universe in a false vacuum. The universe was in an exactly de Sitter phase. In this model, regions of non-inflating universe are created through the nucleation of bubbles of true vacuum, while the rest of the universe continues inflating. When two such bubbles collide, the vast energy of the bubble walls is converted into the particles seen at the beginning of the big bang. This process is called reheating. Alan Guth has described the inflationary universe as the ultimate "free lunch": new universes, similar to our own, are continuously produced in a vast inflating background. Gravitational interactions, in this case, circumvent the arrow of time problem (i.e. the second law of thermodynamics) and conservation of energy.

However, the original model of Guth fails because, in order to guarantee a sufficient amount of inflation to solve the standard problems, the bubble nucleation rate must be too low for bubble walls to collide and for the reheating process to actually work. This is called the "graceful exit problem" and Guth's original model is now called "old inflation." Andrei Linde and, independently, Andreas Albrecht and Paul Steinhardt proposed a "new inflation" or "slow-roll inflation" in which the inflaton is modelled by a scalar field slowly rolling down a flat potential. In this model, the expansion of the universe is only approximately de Sitter, and the Hubble parameter is actually decreasing: the expansion is slowing. While the spectrum of fluctuations generated in the false vacuum de Sitter universe of old inflation is exactly scale-invariant, new inflation produces only a nearly scale invariant spectrum. This means that information about the potential during inflation can be extracted, in principle, from the cosmic microwave background by measuring the spectral index.

New inflation is generally eternal: that is, the process continues eternally. Although the scalar field is classically rolling down the potential, quantum fluctuations occasionally bring it back up the potential. These regions expand much faster than regions in which the inflaton has a lower potential energy. Thus, while inflation ends in some regions, the regions in which it continues are growing exponentially, and thus continue to dominate. This equilibrium, which was first described by Andrei Linde, in which inflation ends in some regions while quantum mechanical fluctuations keep it going in the majority of the universe, is called "eternal inflation". Inflation, however, cannot be eternal in the past, and so does not solve the problem of initial conditions for the universe.

One theoretical challenge for inflation arises from the need to fine tune the potentials for the fields which may give rise to inflation: while the inflaton must have a large vacuum energy it must have a low mass (and a large Compton wavelength). In addition, inflation causes rapid cooling of the universe and so it must be followed by a period of reheating before the hot big bang can begin. It is not known how reheating occurs, although several models have been proposed. Several different models have been proposed, including brane inflation and hybrid inflation, but on the whole inflation is believed to be difficult to derive naturally from string theory. One popular idea that has been suggested in the context of string theory and quantum gravity is that the universe actually contains many more dimensions of space than the three we experience, but that the universe only inflated along the three normal dimensions of space. This theory, called string gas cosmology, was proposed by Robert Brandenberger and Cumrun Vafa. It suggested that we have three large dimensions because of certain topological properties of colliding strings. However, considerable doubt has been cast on the practicability of these ideas.

The ekpyrotic, cyclic models and variable speed of light cosmology are considered competitors to inflation.

Observations

Observationally, it is hoped that improved measurements of the cosmic microwave background will tell us more about inflation. In particular, high precision measurements of the polarization of the background radiation will tell us if the energy scale of inflation predicted by the simplest models is correct, and measurements of the spectrum of primordial fluctuations will tell us if our naive models of inflation can produce the correct primordial fluctuations. A perfectly scale invariant spectrum is generally considered incompatible with the simplest models of inflation as is a running spectral index (a spectrum with curvature). These sorts of measurements are expected to be performed by the Planck satellite, CLOVER array and other ground-based cosmic microwave background experiments.

As of 2005, it is unclear what relationship if any the period of cosmic inflation has to do with observations of dark energy in the universe. Dark energy, particularly quintessence is broadly similar to inflation, but occurs at a much lower energy, 10^-12GeV, at least 27 orders of magnitude less than the scale of inflation.