Compare · The Origin of Our Universe

Modern Slow-Roll Inflation vs Old Inflation

Inflationary Big Bang· within family

	Modern Slow-Roll Inflation Strongly supported	Old Inflation Historical
Proposed	1983 / 2018	1980 / 1981
Key figures	Andrei Linde, Alexei Starobinsky, many others	Alan Guth
In one sentence	Modern slow-roll inflation treats the inflationary epoch as an effective field theory of a scalar field on a flat potential, with many candidate potentials consistent with Planck's measured scalar spectral index and tensor-to-scalar ratio constraints.	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	Scalar spectral index n_s approximately 0.965, slightly less than 1, consistent across many viable potentials Gaussian, adiabatic perturbations with minimal non-Gaussianity (small f_NL) The tensor-to-scalar ratio r (how strong primordial gravitational-wave ripples are relative to density ripples) is set by the shape of the inflaton potential. Single-field models predict a fixed link between r and the gravitational-wave tilt, n_t = -r/8, that future experiments could check Specific acoustic peak structure and polarization patterns in the CMB	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	Initial conditions for inflation itself may require a fine-tuned homogeneous patch and specific scalar field values. Embedding viable inflaton potentials in a UV-complete [[quantum gravity]] (e.g., [[string theory]]) remains nontrivial; swampland conjectures challenge whether sufficiently flat potentials are generic. Most slow-roll potentials lead to [[eternal inflation]] in some regime, raising measure and predictability concerns.	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 quantum gravity fit problem: nobody has shown how inflation's flat potential fits inside a full quantum gravity theory, and swampland conjectures argue the kind of flat potential it needs may not be allowed at all.	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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Modern Slow-Roll Inflation

1983 / 2018 · Strongly supported

Old Inflation

1980 / 1981 · Historical

Proposed

1983 / 2018

1980 / 1981

Key figures

Andrei Linde, Alexei Starobinsky, many others

Alan Guth

In one sentence

Modern slow-roll inflation treats the inflationary epoch as an effective field theory of a scalar field on a flat potential, with many candidate potentials consistent with Planck's measured scalar spectral index and tensor-to-scalar ratio constraints.

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

Scalar spectral index n_s approximately 0.965, slightly less than 1, consistent across many viable potentials
Gaussian, adiabatic perturbations with minimal non-Gaussianity (small f_NL)
The tensor-to-scalar ratio r (how strong primordial gravitational-wave ripples are relative to density ripples) is set by the shape of the inflaton potential. Single-field models predict a fixed link between r and the gravitational-wave tilt, n_t = -r/8, that future experiments could check
Specific acoustic peak structure and polarization patterns in the CMB

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

Initial conditions for inflation itself may require a fine-tuned homogeneous patch and specific scalar field values.
Embedding viable inflaton potentials in a UV-complete [[quantum gravity]] (e.g., [[string theory]]) remains nontrivial; swampland conjectures challenge whether sufficiently flat potentials are generic.
Most slow-roll potentials lead to [[eternal inflation]] in some regime, raising measure and predictability concerns.

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 quantum gravity fit problem: nobody has shown how inflation's flat potential fits inside a full quantum gravity theory, and swampland conjectures argue the kind of flat potential it needs may not be allowed at all.

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

No votes yet