Compare · The Origin of Our Universe

Old Inflation vs New Inflation

Inflationary Big Bang· within family

	Old Inflation Historical	New Inflation Historical
Proposed	1980 / 1981	1982
Key figures	Alan Guth	Andrei Linde, Andreas Albrecht, Paul Steinhardt
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.	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.
Predictions	Spatial flatness with Omega close to 1 Homogeneous and isotropic large-scale universe Absence of GUT-scale magnetic monopoles at observable densities	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
Where it breaks	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.	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.
Key unresolved problem	The graceful exit problem: bubbles of ordinary space form too slowly to ever join up and fill the universe, so they stay marooned as isolated pockets in a sea that keeps inflating around them.	The flatness fine-tuning problem: the model needs an exceptionally flat potential for the field to roll down, but quantum corrections from particle physics tend to wrinkle that potential and ruin the flatness it depends on.
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Old Inflation

1980 / 1981 · Historical

New Inflation

1982 · Historical

Proposed

1980 / 1981

1982

Key figures

Alan Guth

Andrei Linde, Andreas Albrecht, Paul Steinhardt

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.

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.

Predictions

Spatial flatness with Omega close to 1
Homogeneous and isotropic large-scale universe
Absence of GUT-scale magnetic monopoles at observable densities

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

Where it breaks

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.

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.

Key unresolved problem

The graceful exit problem: bubbles of ordinary space form too slowly to ever join up and fill the universe, so they stay marooned as isolated pockets in a sea that keeps inflating around them.

The flatness fine-tuning problem: the model needs an exceptionally flat potential for the field to roll down, but quantum corrections from particle physics tend to wrinkle that potential and ruin the flatness it depends on.

Reader vote

No votes yet