Compare · The Origin of Our Universe

Old Inflation vs Warm Inflation

Inflationary Big Bang· within family

	Old Inflation Historical	Warm Inflation Strongly supported
Proposed	1980 / 1981	1995 / 2020
Key figures	Alan Guth	Arjun Berera, Ian Moss, Rudnei Ramos, Mar Bastero-Gil
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.	Warm inflation adds friction to the inflaton field: it transfers energy into a radiation bath as it rolls, so the universe stays hot during inflation and slides straight into the radiation era with no distinct reheating phase.
Predictions	Spatial flatness with Omega close to 1 Homogeneous and isotropic large-scale universe Absence of GUT-scale magnetic monopoles at observable densities	A radiation bath persists throughout inflation, so the handover to the hot Big Bang is smooth, with no separate reheating phase. Density perturbations are sourced largely by thermal fluctuations rather than vacuum fluctuations, which raises the scalar amplitude and lowers the tensor-to-scalar ratio relative to cold inflation on the same potential. A distinct non-Gaussian signature in the primordial perturbations, different in shape and sign from the small non-Gaussianity of cold single-field inflation. Viable inflation on steeper potentials than cold inflation allows, easing the flatness fine-tuning and the tension with swampland-type conjectures.
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.	Deriving the dissipation strength from first-principles quantum field theory is hard: the inflaton must couple strongly enough to thermalize a bath, yet not so strongly that radiative corrections spoil the potential's flatness. Thermal backreaction can destabilize the slow roll, and many early warm-inflation models did not survive a careful treatment of it. Current cosmological data do not require warm inflation over cold inflation. It is a viable alternative, not a preferred one, and its sharpest signature, the non-Gaussian shape, sits below present detection thresholds.
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 dissipation coefficient problem: it is hard to calculate the friction from basic quantum field theory, because coupling the field strongly enough to heat up a radiation bath tends to wrinkle the flat potential the model relies on.
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Old Inflation

1980 / 1981 · Historical

Warm Inflation

1995 / 2020 · Strongly supported

Proposed

1980 / 1981

1995 / 2020

Key figures

Alan Guth

Arjun Berera, Ian Moss, Rudnei Ramos, Mar Bastero-Gil

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.

Warm inflation adds friction to the inflaton field: it transfers energy into a radiation bath as it rolls, so the universe stays hot during inflation and slides straight into the radiation era with no distinct reheating phase.

Predictions

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

A radiation bath persists throughout inflation, so the handover to the hot Big Bang is smooth, with no separate reheating phase.
Density perturbations are sourced largely by thermal fluctuations rather than vacuum fluctuations, which raises the scalar amplitude and lowers the tensor-to-scalar ratio relative to cold inflation on the same potential.
A distinct non-Gaussian signature in the primordial perturbations, different in shape and sign from the small non-Gaussianity of cold single-field inflation.
Viable inflation on steeper potentials than cold inflation allows, easing the flatness fine-tuning and the tension with swampland-type conjectures.

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.

Deriving the dissipation strength from first-principles quantum field theory is hard: the inflaton must couple strongly enough to thermalize a bath, yet not so strongly that radiative corrections spoil the potential's flatness.
Thermal backreaction can destabilize the slow roll, and many early warm-inflation models did not survive a careful treatment of it.
Current cosmological data do not require warm inflation over cold inflation. It is a viable alternative, not a preferred one, and its sharpest signature, the non-Gaussian shape, sits below present detection thresholds.

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 dissipation coefficient problem: it is hard to calculate the friction from basic quantum field theory, because coupling the field strongly enough to heat up a radiation bath tends to wrinkle the flat potential the model relies on.

Reader vote

No votes yet