New Inflation
Inflation driven by a scalar field slowly rolling down a flat potential, replacing bubble nucleation with a graceful coherent exit.
Placeholder for a 3D visualisation of Inflationary Big Bang. The interactive scene will land in Phase 3. The standard Big Bang model assumes a hot dense early universe but cannot explain why it is so uniform across causally disconnected regions, why spatial curvature is so close to zero, or why we don't see the GUT-scale monopoles particle physics predicts. Inflationary cosmology prepends a brief epoch of exponential expansion to the standard model, stretching a small causally connected patch to encompass the entire observable universe and diluting any pre-existing relics to unobservable density. The observed near-perfect flatness and the nearly scale-invariant primordial perturbation spectrum measured by Planck are taken as strong empirical support for the inflationary framework, even though the specific potential driving inflation remains unknown.
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.
The claim
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.
Predictions
- 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
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
Counterpoints
- 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.
Variants in this family
▸Go deeperTechnical detail with proper terminology
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.
References
- EstablishedLinde (1982) A new inflationary universe scenario, Phys. Lett. B 108, 389
- EstablishedAlbrecht & Steinhardt (1982) Cosmology for Grand Unified Theories with Radiatively Induced Symmetry Breaking, Phys. Rev. Lett. 48, 1220
- EstablishedMukhanov & Chibisov (1981) Quantum fluctuations and a nonsingular universe, JETP Lett. 33, 532
Last reviewed May 15, 2026
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