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Reheating temperature in a sudden-decay modulus model
An exact algebraic deduction inside the explicitly defined instantaneous-decay prescription: a homogeneous quadratic scalar and a comoving radiation fluid with p_r=rho_r/3 supply the total pre-decay stress, the flat-FLRW Einstein equation holds with zero cosmological term, and all pre-decay energy is instantaneously transferred to an ideal thermal bath at H=Gamma. It does not derive a modulus decay width, thermalization, a continuous-decay correction, a BBN threshold, or a string compactification.
Statement
Use Einstein-frame natural units and the inherited FLRW coordinate convention of FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter: and have length dimension, with dimensionless spatial coordinates and . Thus , , and is dimensionless. Let be a nonempty open cosmic-time interval and take a spatially flat FLRW model datum from the FLRW model-data definition def-gr-flrw-model-data, with , , metric and . Write for the first-factor projection; in these units the smooth comoving unit timelike vector field is . Let be the reduced four-dimensional Planck mass and let be a constant decay rate of mass dimension one. Let be a scalar time profile and set the spacetime scalar to , so homogeneity means that is independent of . In the one-field convention of the scalar EFT and scalar-field matter definitions (def-r5-cosmology-4d-scalar-eft-data and def-gr-scalar-field-matter-model), take target metric , scalar coupling , and constant of mass dimension one, with potential , and comoving density profile . For the pre-decay radiation component, choose smooth profiles and define the perfect-fluid fields on by and . These factorization identities specify homogeneity; use the same comoving velocity field in the perfect-fluid matter model, so . Here radiation means with ; its rest energy density is . This equation of state is part of the model data, although the temporal stress calculation below depends only on the rest density. Let be the total comoving energy density; the scalar and fluid stress tensors are the only sources in the adopted zero-cosmological-term Einstein equation. For any pre-decay quantity with a left limit at , write . A sudden-decay model here supplies a time at which and instantaneously transfers all of into an ideal, immediately thermalized radiation bath with vanishing chemical potentials. Let be the specified dimensionless effective number of relativistic degrees of freedom, so the adopted bath density is . Any positive reheating temperature satisfying then obeys
The equation is implicit if depends on temperature and asserts no existence or uniqueness for that implicit equation. It is exact for this sudden-decay algebra; it gives no continuous-decay or entropy-injection correction and no universal string prediction for .
Statement references
Four-dimensional Einstein-frame multi-scalar cosmology EFT data
Adopted two-derivative multi-scalar cosmology EFT
The Friedmann, acceleration and continuity equations of an FLRW model datum
Ricci, scalar and Einstein tensors of the FLRW metric in product charts
The scalar-field matter model of the classical gravitational framework
First variation, Hilbert tensor and conservation of the scalar-field model
The perfect-fluid matter model: rest energy density, pressure, four-velocity and stress-energy
Facts & Assumptions
Given: The four-dimensional Einstein-frame Planck convention, a spatially flat homogeneous scalar-plus-radiation model with zero cosmological term and additive comoving stress, the positive decay rate , the supplied sudden-decay/total-energy-matching prescription, and the adopted equilibrium radiation-density relation with .
Adopted two-derivative multi-scalar cosmology EFT: adopt the direct Einstein equation with zero cosmological term; any nonzero additional matter sector must have its stress tensor and couplings stated.
Adopted two-derivative multi-scalar cosmology EFT: in the adopted natural-unit normalization, .
Ricci, scalar and Einstein tensors of the FLRW metric in product charts: in the spatially flat FLRW case, the temporal Einstein component is when .
Four-dimensional Einstein-frame multi-scalar cosmology EFT data: is the reduced Planck mass of mass dimension one and the scalar EFT is in Einstein frame with ; this item additionally fixes so temperature is measured in energy units.
The Friedmann, acceleration and continuity equations of an FLRW model datum: the Hubble function is on the cosmic-time interval; the definition of the Friedmann system alone does not assert its physical derivation.
The scalar-field matter model of the classical gravitational framework: for the homogeneous canonical scalar with , , and , the covariant component of its candidate stress tensor is .
First variation, Hilbert tensor and conservation of the scalar-field model: the stated scalar candidate tensor is the Hilbert tensor of the scalar action under the declared compact-variation convention.
The perfect-fluid matter model: rest energy density, pressure, four-velocity and stress-energy defines a perfect-fluid datum by smooth rest-density and pressure fields and a smooth future-directed unit timelike field , with ; here and . FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter defines the FLRW comoving field as ; since here, , so and . Substitution gives , independently of the pressure.
Adopted matter-content assumptions: fields, equations, stress-energy and energy conditions: a model supplies its nongravitational fields or matter and their explicit equations and couplings as model input.
FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter: in product coordinates for the FLRW metric, , which is in the units fixed here.
The field content is model input as in [F10]: a homogeneous canonical scalar and the smooth comoving radiation-fluid data specified in the Statement are the only pre-decay stress sources, their total tensor is the sum of the scalar and perfect-fluid tensors, and the adopted Einstein law applies to that total. The radiation equation of state is ; no averaged-pressureless approximation or controlled averaging family is required for the scalar, and the residual radiation fraction need not be zero.
At the supplied , impose and instantaneously set ; the conversion is complete, with no unthermalized hidden products or additional entropy source.
Adopt for the specified ideal relativistic equilibrium bath, with and . No value, uncertainty, or microscopic prediction for or is asserted.
Proof
Put on the pre-decay branch. The tensor formula in [F5], with , , the homogeneous field, and from [F11], gives ; [F6] confirms this candidate is the scalar action's Hilbert tensor. For the radiation fluid, [F7] gives since and ; lowering both indices leaves . By [A1] and [F10] the total source is the stated additive matter model, so . This tensor computation does not require an averaged equation of state for the scalar and specifies no decay channel.
The direct Einstein equation [F1] and exact flat-FLRW tensor component [F2], with and , give . By [F9], , hence . This uses the spatially flat branch and zero cosmological term explicitly; it does not use an equation of state for the combined source.
Evaluate step 2.1 at the supplied pre-decay event. Since by [A2], .
By [A2], the entire pre-decay energy becomes the thermal bath, and [A3] gives . Multiplication by , permitted because , yields .
Since , , and are positive, taking the positive fourth root of step 4.1 gives the displayed equation. Its dimensions agree: is dimensionless and has mass dimension one, the dimension of in the adopted convention. No inversion of the possibly temperature-dependent function was used, so no uniqueness claim follows.
Depends on
Four-dimensional Einstein-frame multi-scalar cosmology EFT data
Adopted two-derivative multi-scalar cosmology EFT
Adopted matter-content assumptions: fields, equations, stress-energy and energy conditions
The Friedmann, acceleration and continuity equations of an FLRW model datum
FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter
Ricci, scalar and Einstein tensors of the FLRW metric in product charts
The scalar-field matter model of the classical gravitational framework
First variation, Hilbert tensor and conservation of the scalar-field model
The perfect-fluid matter model: rest energy density, pressure, four-velocity and stress-energy