Old Inflation
The universe is trapped in a high-energy false vacuum that expands exponentially until first-order tunneling decays it into the true vacuum.
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
Guth's original 1981 model proposed that exponential expansion is driven by a false vacuum state which decays via quantum tunneling into bubbles of true vacuum, solving the horizon, flatness, and monopole problems of the standard Big Bang.
The claim
Guth's original inflationary model assumed the early universe was trapped in a metastable false vacuum associated with a Grand Unified Theory phase transition. The false vacuum has nearly constant energy density, which drives exponential expansion through Einstein's equations. This rapid expansion stretches a small causally connected patch to encompass the entire observable universe, explaining the observed CMB uniformity across the sky.
The mechanism was supposed to end via quantum tunneling: bubbles of true vacuum nucleate at random points and expand at nearly the speed of light, eventually filling all of space and thermalizing into the hot Big Bang phase. The model elegantly solved three major puzzles at once but contained a fatal flaw that led to its abandonment within two years.
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
- Spatial flatness with Omega close to 1
- Homogeneous and isotropic large-scale universe
- Absence of GUT-scale magnetic monopoles at observable densities
Evidence
- Established the framework that all subsequent inflationary models build on
- Generic inflationary predictions of flatness and homogeneity match later CMB observations
Counterpoints
- Bubble nucleation cannot percolate to fill all space if inflation lasts long enough to solve the horizon problem, leaving isolated bubbles in an eternally inflating sea.
- Where bubbles do collide, they produce large inhomogeneities incompatible with the observed smoothness of the CMB.
- The density perturbation spectrum produced by bubble nucleation does not match the nearly scale-invariant, Gaussian, adiabatic spectrum observed.
Variants in this family
▸Go deeperTechnical detail with proper terminology
References
- EstablishedGuth (1981) Inflationary universe: A possible solution to the horizon and flatness problems, Phys. Rev. D 23, 347
- EstablishedGuth & Weinberg (1981) Cosmological consequences of a first-order phase transition in the SU(5) grand unified model, Phys. Rev. D 23, 876
Last reviewed May 15, 2026
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