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Reheating temperature in a sudden-decay modulus model

Physical scope

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 ℏ=c=kB=1 and the inherited FLRW coordinate convention of FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter: t and a have length dimension, with dimensionless spatial coordinates and γ. Thus [H]=[m]=[MP]=[Γ]=[T]=mass, [ρϕ]=[ρr]=[pr]=mass4, and g∗ is dimensionless. Let J⊆R 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 M=J×Σ, a∈C∞(J,(0,∞)), metric g=−dt2+a(t)2γ and H=a˙/a. Write πJ:M→J for the first-factor projection; in these units the smooth comoving unit timelike vector field is n=∂t. Let MP=(8πGN)−1/2>0 be the reduced four-dimensional Planck mass and let Γ>0 be a constant decay rate of mass dimension one. Let ϕ∈C∞(J,R) be a scalar time profile and set the spacetime scalar to Φ:=ϕ∘πJ, so homogeneity means that Φ is independent of x∈Σ. 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 G11=1, scalar coupling K=1, and constant m>0 of mass dimension one, with potential V(Φ)=m2Φ2/2, and comoving density profile ρϕ(t)=(ϕ˙(t)2+m2ϕ(t)2)/2. For the pre-decay radiation component, choose smooth profiles ρ~r,p~r∈C∞(J,R) and define the perfect-fluid fields on M by ρr=ρ~r∘πJ and pr=p~r∘πJ. These factorization identities specify homogeneity; use the same comoving velocity field n=∂t in the perfect-fluid matter model, so Trab=(ρr+pr)nanb+prgab. Here radiation means pr=ρr/3 with ρr≥0; its rest energy density is ρr. This equation of state is part of the model data, although the temporal stress calculation below depends only on the rest density. Let ρtot(t):=ρϕ(t)+ρ~r(t) 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 f with a left limit at td, write f(td−):=lim⁡t↑tdf(t). A sudden-decay model here supplies a time td∈J at which H(td−)=Γ and instantaneously transfers all of ρtot(td−) into an ideal, immediately thermalized radiation bath with vanishing chemical potentials. Let g∗:(0,∞)→(0,∞) be the specified dimensionless effective number of relativistic degrees of freedom, so the adopted bath density is ρrad(T)=π2g∗(T)T4/30. Any positive reheating temperature TRH satisfying ρrad(TRH)=ρtot(td−) then obeys

TRH=(90π2g∗(TRH))1/4ΓMP.

The equation is implicit if g∗ 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 Γ.

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 Γ, the supplied sudden-decay/total-energy-matching prescription, and the adopted equilibrium radiation-density relation with g∗(T)>0.

[F1]

Adopted two-derivative multi-scalar cosmology EFT: adopt the direct Einstein equation Gμν=MP−2Tμν with zero cosmological term; any nonzero additional matter sector must have its stress tensor and couplings stated.

[F9]

Adopted two-derivative multi-scalar cosmology EFT: in the adopted natural-unit normalization, MP2=(8πGN)−1.

[F2]

Ricci, scalar and Einstein tensors of the FLRW metric in product charts: in the spatially flat FLRW case, the temporal Einstein component is G00=3a˙2/a2=3H2 when c=1.

[F3]

Four-dimensional Einstein-frame multi-scalar cosmology EFT data: MP>0 is the reduced Planck mass of mass dimension one and the scalar EFT is in Einstein frame with ℏ=c=1; this item additionally fixes kB=1 so temperature is measured in energy units.

[F4]

The Friedmann, acceleration and continuity equations of an FLRW model datum: the Hubble function is H=a˙/a on the cosmic-time interval; the definition of the Friedmann system alone does not assert its physical derivation.

[F5]

The scalar-field matter model of the classical gravitational framework: for the homogeneous canonical scalar with K=1, c=1, and g00=−1, the covariant 00 component of its candidate stress tensor is ϕ˙2/2+V(ϕ).

[F6]

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.

[F7]

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 ε,p and a smooth future-directed unit timelike field n, with Tab=(ε+p)nanb+pgab; here ε=ρr and p=pr. FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter defines the FLRW comoving field as c−1∂t; since c=1 here, n=∂t, so n0=1 and g00=−1. Substitution gives T00=ε=ρr, independently of the pressure.

[F10]

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.

[F11]

FLRW model data: warped product metric, cosmic time, comoving congruence, scale factor and Hubble parameter: in product coordinates for the FLRW metric, g00=−c2, which is −1 in the units fixed here.

[A1]

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 pr=ρr/3; no averaged-pressureless approximation or controlled averaging family is required for the scalar, and the residual radiation fraction need not be zero.

[A2]

At the supplied td, impose H(td−)=Γ and instantaneously set ρrad(TRH)=ρtot(td−); the conversion is complete, with no unthermalized hidden products or additional entropy source.

[A3]

Adopt ρrad(T)=π2g∗(T)T4/30 for the specified ideal relativistic equilibrium bath, with T>0 and g∗(T)>0. No value, uncertainty, or microscopic prediction for g∗ or Γ is asserted.

Proof

1.1givenF5F6F7F10F11A1algebra

Put V(ϕ)=m2ϕ2/2 on the pre-decay branch. The tensor formula in [F5], with K=1, c=1, the homogeneous field, and g00=−1 from [F11], gives T00ϕ=ϕ˙2−g00(−ϕ˙2/2+V)=ϕ˙2/2+V=ρϕ; [F6] confirms this candidate is the scalar action's Hilbert tensor. For the radiation fluid, [F7] gives Tr00=(ρr+pr)(n0)2+prg00=(ρr+pr)−pr=ρr since n0=1 and g00=−1; lowering both indices leaves T00r=ρr. By [A1] and [F10] the total source is the stated additive matter model, so T00=ρϕ+ρr=ρtot. This tensor computation does not require an averaged equation of state for the scalar and specifies no decay channel.

2.1F1F2F3F4F9step 1.1algebra

The direct Einstein equation [F1] and exact flat-FLRW tensor component [F2], with k=0 and c=1, give 3H2=8πGNρtot. By [F9], 8πGN=MP−2, hence ρtot=3MP2H2. This uses the spatially flat branch and zero cosmological term explicitly; it does not use an equation of state for the combined source.

3.1A2step 2.1algebra

Evaluate step 2.1 at the supplied pre-decay event. Since H(td−)=Γ by [A2], ρtot(td−)=3MP2Γ2.

4.1A2A3step 3.1algebra

By [A2], the entire pre-decay energy becomes the thermal bath, and [A3] gives π2g∗(TRH)TRH4/30=3MP2Γ2. Multiplication by 30/(π2g∗(TRH)), permitted because g∗(TRH)>0, yields TRH4=90MP2Γ2/(π2g∗(TRH)).

5.1A3F3step 4.1algebra∎

Since TRH, MP, Γ and g∗(TRH) are positive, taking the positive fourth root of step 4.1 gives the displayed equation. Its dimensions agree: g∗ is dimensionless and ΓMP has mass dimension one, the dimension of TRH in the adopted kB=1 convention. No inversion of the possibly temperature-dependent function g∗ was used, so no uniqueness claim follows.

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