Compare · The Origin of Our Universe

New Inflation vs Modern Slow-Roll Inflation

Inflationary Big Bang· within family

	New Inflation Historical	Modern Slow-Roll Inflation Strongly supported
Proposed	1982	1983 / 2018
Key figures	Andrei Linde, Andreas Albrecht, Paul Steinhardt	Andrei Linde, Alexei Starobinsky, many others
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.	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.
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	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
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.	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.
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 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.
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New Inflation

1982 · Historical

Modern Slow-Roll Inflation

1983 / 2018 · Strongly supported

Proposed

1982

1983 / 2018

Key figures

Andrei Linde, Andreas Albrecht, Paul Steinhardt

Andrei Linde, Alexei Starobinsky, many others

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.

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.

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

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

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.

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.

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 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.

Reader vote

No votes yet