Ch.02 The Origin of Our UniverseInflationary Big Bang

The fix for Guth. A slowly rolling field ended inflation coherently across the universe.

New Inflation

1982Andrei Linde, Andreas Albrecht, Paul SteinhardtStrongly supported3 primary sources, 3 established Reviewed May 15, 2026

Inflation driven by a scalar field slowly rolling down a flat potential, replacing bubble nucleation with a graceful coherent exit.

Historical

Visualisation · Inflationary Big Bang

inflation: A brief burst of accelerating expansion in the very early universe, smoothing out and stretching whatever existed before.
scalar field: A field with one value at each point in space, like a temperature map. The inflaton is one example.
inflaton field: The hypothetical scalar field whose slow decay drove cosmic inflation. The fuel, when it burns, space stretches.
slow roll: When the inflaton field falls in value gradually rather than suddenly, sustaining inflation long enough to match observations.
quantum fluctuation: A spontaneous, unpredictable variation in a quantum field. Tiny in normal conditions, decisive at the smallest scales.
e-fold: One factor-of-e expansion in scale during inflation. Roughly 60 e-folds are needed to explain the universe's flatness.

§1 · The claim, in one sentence

Linde and independently Albrecht and Steinhardt replaced Old Inflation's bubble nucleation with a scalar field slowly rolling down a flat potential, producing a coherent end to inflation across whole Hubble regions and the first viable predictions for cosmological perturbations.

§2 · Why it might be true

New Inflation kept Guth's core idea that vacuum energy drives exponential expansion but changed the dynamics fundamentally. Instead of a first-order phase transition with quantum tunneling, the inflaton field starts near the top of a very flat potential and rolls slowly downward. During this slow roll, the potential energy behaves like a quasi-constant vacuum energy driving quasi-exponential expansion. When the field reaches the steeper part of the potential, inflation ends, the field oscillates, and the universe reheats into a hot Big Bang phase.

This slow-roll mechanism solves the graceful exit problem by ending inflation coherently across a Hubble region rather than via rare bubble percolation. Quantum fluctuations of the rolling scalar field generate nearly scale-invariant Gaussian density perturbations, whose theory was developed immediately after by Mukhanov-Chibisov, Hawking, Starobinsky, Guth-Pi, and Bardeen-Steinhardt-Turner.

The family stance

Our universe began with a brief epoch of exponential expansion driven by a scalar field, followed by reheating into the hot Big Bang phase. The same inflaton field that drove expansion also generated the seed perturbations that became galaxies.

§2.5 · Evidence

Generic predictions of slow-roll inflation match Planck observations of CMB scalar spectrum and adiabaticity
The framework's perturbation theory remains foundational for modern inflation

§3 · What you'd need to test it

Nearly scale-invariant spectrum of primordial density perturbations with scalar spectral index slightly less than 1
Approximately Gaussian, adiabatic primordial fluctuations
Small but nonzero gravitational wave background depending on the potential shape

§4 · Where it breaks

Requires the inflaton to start very near the top of a flat potential, which is a finely tuned initial state.
Realizing sufficiently flat small-field potentials compatible with particle physics is difficult; quantum corrections tend to spoil the required flatness.
Specific potentials of this class are now disfavored or ruled out by Planck data, even though the slow-roll mechanism itself remains the working framework.

Go deeper

Mukhanov and Chibisov (1981) first computed the spectrum of perturbations from slow-roll inflation. Hawking (1982), Starobinsky (1982), Guth and Pi (1982), and Bardeen, Steinhardt, and Turner (1983) followed quickly with the full perturbation theory.

The scale-invariance of the perturbation spectrum follows from the slow-roll mechanism itself. During inflation, quantum fluctuations of the inflaton field with wavelengths shorter than the Hubble radius behave as normal vacuum modes. As inflation stretches space exponentially, each mode's wavelength grows past the Hubble radius and freezes, becoming a classical density perturbation with amplitude set by the Hubble rate at horizon crossing. Because H changes only slowly during slow roll, modes that cross the horizon at different times receive nearly identical amplitudes, producing a nearly scale-invariant spectrum. The small departure from exact scale invariance, the spectral tilt n_s close to but less than 1, encodes how the inflaton's potential V(φ) and its derivatives evolve during the observable e-folds. Planck's measurement of n_s = 0.965 thus constrains the shape of the potential during the period our observable universe's modes crossed the horizon, roughly 50 to 60 e-folds before inflation ended.

▸§5 · Who built it, and when(3 sources, 3 established)

Variants in this family

Compare variants

0votes

Currently #1 in this family · #3 in Ch.2

Up next

Spotted an error? Have a source to add?

Prefer email?

You can also send a prefilled email with the variant URL already filled in.