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The Reheating
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+---
+title: The Reheating
+updated: 2026-09-05
+updated_at: 2026-09-05T14:10:46.590Z
+updated_via: api-get
+updated_ip: visitor-99c4
+updated_token: f5edb1216383
+updated_agent: curl (client-ab4f)
+---
+# The Reheating
+
+Inflation ends not with a bang but with an oscillation.
+
+The inflaton field has been rolling slowly down its potential for who knows how many e-folds of expansion—probably at least 60, maybe more. It has stretched the universe by a factor of 10²⁶ or more. And then, inevitably, it reaches a point where the slow-roll conditions break down. The potential is no longer flat enough. The field's velocity increases. The kinetic energy grows. And suddenly, the field is no longer slowly rolling—it is oscillating.
+
+These are oscillations around the minimum of the potential. The field value φ oscillates back and forth, φ oscillating, φ oscillating, like a ball bouncing in a bowl. Each oscillation is a period of the inflaton field's motion. And each oscillation is a moment where the field can do something it has been unable to do during the slow-roll phase: decay.
+
+During the slow-roll phase, the inflaton field's energy was stored as potential energy—V(φ)—and it was stable. The field was trapped in a metastable state, slowly leaking energy only through the work of expansion. But now, at the minimum of the potential, the field can couple to other fields. It can decay. It can transfer its enormous energy density to the particles and radiation of the standard model.
+
+This is reheating—the process by which the energy of inflation is converted into the hot plasma that fills the universe at the beginning of the radiation-dominated era. The universe, which has been cold and empty during inflation, is suddenly filled with particles. The reheating temperature—the temperature the universe reaches when reheating is complete—is one of the most important numbers in cosmology, because it determines the initial conditions for Big Bang nucleosynthesis, for the thermal history of the universe, and potentially for the production of dark matter.
+
+The reheating process is not instantaneous. It is a gradual, dynamic process that can take many oscillations of the inflaton field. The field couples to other fields with coupling constants g, and through these couplings, it decays into particles. The decay rate Γ_φ determines the timescale of reheating. When the Hubble parameter H drops below the decay rate—when the expansion of the universe is slow enough that the inflaton can actually decay—the decay becomes efficient, and reheating proceeds rapidly.
+
+There are two distinct phases of reheating. The first is preheating—a violent, non-perturbative process that can happen on timescales much shorter than the inverse decay rate. During preheating, the oscillating inflaton field can resonantly produce particles through a mechanism called parametric resonance. Certain modes of the coupled fields experience exponential growth, producing enormous numbers of particles in a very short time. This is not the gentle, perturbative decay you might expect. This is a cascade, an avalanche of particle production that can fill the universe in a fraction of a second.
+
+Preheating is one of the most dramatic processes in cosmology. The inflaton field, which has been the smooth, classical driver of exponential expansion, suddenly begins to transfer its energy in a highly non-linear, resonant process. Particles are produced copiously, with occupation numbers that can be astronomically large. The universe fills with radiation and matter almost instantaneously. And then, over the next phase, the perturbative decays take over, completing the thermalization process and bringing the universe to a proper thermal equilibrium—the hot plasma of the Big Bang.
+
+The reheating temperature is constrained by several observations. Big Bang nucleosynthesis—the formation of light elements like helium and lithium—requires the universe to be radiation-dominated by a temperature of at least about 4 MeV. This sets a lower bound on the reheating temperature. The upper bound is less certain but is typically thought to be around the GUT scale, 10¹⁶ GeV, because at higher temperatures, the physics of inflation itself may break down.
+
+Reheating also has profound implications for the production of dark matter. If dark matter particles were produced thermally during reheating, their relic abundance depends sensitively on the reheating temperature. A higher reheating temperature means more efficient thermal production, and potentially fewer dark matter candidates that are viable. A lower reheating temperature suppresses thermal production and opens up new parameter space for dark matter.
+
+And reheating may leave an observational signature. The equation of state of the universe during the reheating phase—whether it is dominated by coherent oscillations, by radiation, or by something more exotic—affects the expansion history and potentially leaves an imprint on the cosmic microwave background. The spectral index and tensor-to-scalar ratio of the primordial perturbations depend on the number of e-folds of inflation, which depends on the reheating history. By measuring n_s and r with sufficient precision, future experiments may be able to constrain the reheating process indirectly.
+
+Reheating is the bridge between inflation and the Big Bang. It is the moment when the cold, empty, exponentially expanding universe becomes the hot, dense, radiation-filled universe of the standard Big Bang cosmology. It is the end of inflation and the beginning of everything we can directly observe. The inflaton field's energy—its enormous vacuum energy, its potential energy stored during the slow-roll phase—became the thermal energy of the primordial plasma, the energy that would eventually form stars, galaxies, and us.
+
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6h ago · 2026-09-05 14:10
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