Cosmic inflation

A brief rapid expansion

Cosmic inflation is the hypothesis that the universe underwent a very early period of accelerated, nearly exponential expansion. During inflation, the scale factor grew roughly as

$a(t)propto e^{Ht}$

where $H$ was approximately constant.

Inflation was proposed to solve several puzzles in the standard Big Bang model and to explain the origin of primordial density fluctuations.

Horizon problem

The cosmic microwave background has nearly the same temperature in regions of the sky that, without inflation, would not have had time to exchange light signals before recombination. Why are these regions so similar?

Inflation solves this by proposing that regions now widely separated were once close enough to interact before being stretched far apart.

Flatness problem

Observations show that the universe is close to spatially flat. In standard expansion without inflation, exact flatness is unstable: tiny deviations from flatness grow in importance over time. This means the early universe would have needed extremely fine-tuned curvature.

Inflation stretches space so dramatically that any initial curvature becomes diluted, like a small patch on a sphere appearing flat when magnified.

Relic problem

Some high-energy theories predict heavy relic particles such as magnetic monopoles. If produced abundantly in the early universe, they should still be common, but they are not observed. Inflation can dilute such relics to negligible densities.

Quantum fluctuations

Inflation also provides a mechanism for structure formation. Quantum fluctuations in the inflaton field and spacetime metric were stretched to cosmic scales. After inflation, these became tiny density fluctuations.

These fluctuations later grew into galaxies and clusters.

The observed CMB anisotropy level is about

rac{Delta T}{T}sim10^{-5}

consistent with small primordial perturbations.

Reheating

Inflation must end. The energy driving inflation is converted into particles and radiation in a process called reheating. This begins the hot, radiation-filled universe described by standard Big Bang evolution.

The details depend on the inflation model.

Observational predictions

Inflation generally predicts a nearly flat universe and a nearly scale-invariant spectrum of primordial fluctuations. These predictions match observations well. Some models also predict primordial gravitational waves, which could leave a distinctive pattern in CMB polarization.

A common measure of fluctuation scale dependence is the spectral index $n_s$. Exact scale invariance would be

$n_s=1$

Observations find a value close to, but slightly less than, 1.

Open questions

Inflation is powerful but not fully settled. The identity of the inflaton field is unknown. There are many inflationary models. Questions about initial conditions, eternal inflation, and connections to particle physics remain active research topics.

The big idea

Inflation proposes a short era of accelerated early expansion. It explains why the universe is so uniform, flat, and free of unwanted relics, while also generating the tiny fluctuations that became cosmic structure. It is a leading idea, but its underlying physics remains under investigation.