Warm inflation

Warm inflation combines accelerated expansion with ongoing particle production. The inflaton, a scalar field whose potential energy drives expansion, interacts with additional degrees of freedom and loses energy through a dissipative process. That energy appears as radiation with a nonzero temperature while inflation continues.¹ In cold inflation, by contrast, interactions are usually negligible during the accelerated phase and radiation is generated mainly afterward during reheating.

The expansion still requires potential-energy domination, so radiation cannot be so abundant that it ends inflation immediately. A common description uses a dissipation coefficient, often written as Γ, alongside the Hubble rate H. The ratio Q = Γ/(3H) distinguishes weak and strong dissipative regimes. Warm inflation is therefore a dynamical framework rather than a single potential or particle model.

Warm inflation changes both the background evolution and the origin of cosmic structure. Dissipation modifies the inflaton equation of motion, while thermal fluctuations can contribute to the primordial curvature perturbations that later seed galaxies and the cosmic microwave background. In suitable regimes, thermal noise is more important than quantum fluctuations, although mixed quantum-thermal behavior is also possible.

The predicted scalar spectrum depends on the inflaton potential, the temperature, the strength and temperature dependence of dissipation, and the interactions that maintain radiation. These dependencies can alter the scalar spectral index, tensor-to-scalar ratio, and possible statistical features relative to cold-inflation predictions. A viable model must keep the radiation bath sufficiently close to a controlled state without destroying slow-roll expansion.

Warm-inflation models require an interaction structure that dissipates inflaton energy without generating an excessively large correction to its potential. One influential class uses a heavy mediator whose mass depends on the inflaton; the mediator can decay into lighter fields, creating a sequence of interactions that yields friction and radiation production.

This construction is technically challenging because thermal corrections can spoil the flat potential needed for inflation. Supersymmetric models have been investigated partly because cancellations may control some radiative corrections, though the required low-energy spectrum and couplings remain model-dependent. Dissipation can also be nonlocal: the friction coefficient may depend on the history of the fields rather than only on their instantaneous values. Consequently, effective descriptions must be checked against the underlying microscopic theory, especially when mediator masses exceed the temperature.

Warm inflation does not require the universe to be in perfect thermal equilibrium. A radiation bath can be characterized by an effective temperature even when some interactions are too slow to equilibrate every species; this distinction matters for calculating dissipation and fluctuation spectra.² Radiation may also be produced intermittently or through several channels, making the transition out of inflation smoother than the idealized reheating picture.

The idea originated in work by Arjun Berera, who proposed that inflationary expansion and radiation production need not be separated into successive eras.¹ Later studies examined strong-dissipation attractors, nonthermal noise, and possible links to baryogenesis. These features are opportunities rather than generic predictions: observational viability depends on a complete model, and current cosmological data constrain classes of warm-inflation scenarios rather than selecting one unique realization.

Inflaton

A hypothetical scalar field whose potential energy drives inflation.

Dissipation coefficient

A parameter describing the rate at which inflaton energy is transferred to other fields.

Reheating

The post-inflationary conversion of stored inflaton energy into radiation and particles.

Curvature perturbation

A fluctuation in the spatial curvature that can later develop into cosmic structure.

Slow roll

A regime in which the inflaton evolves gradually enough for accelerated expansion to persist.