Compare · The Origin of Our Universe

New Inflation vs Old Inflation

Inflationary Big Bang· within family

	New Inflation Historical	Old Inflation Historical
Proposed	1982	1980 / 1981
Key figures	Andrei Linde, Andreas Albrecht, Paul Steinhardt	Alan Guth
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.	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.
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	Spatial flatness with Omega close to 1 Homogeneous and isotropic large-scale universe Absence of GUT-scale magnetic monopoles at observable densities
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.	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.
Key unresolved problem	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.	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.
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New Inflation

1982 · Historical

Old Inflation

1980 / 1981 · Historical

Proposed

1982

1980 / 1981

Key figures

Andrei Linde, Andreas Albrecht, Paul Steinhardt

Alan Guth

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.

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.

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

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

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.

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.

Key unresolved problem

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.

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.

Reader vote

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