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The Warm Inflation Early Universe

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VAND-TH-98-12

The Warm In?ation Early Universe

Arjun Berera Department of Physics and Astronomy Vanderbilt University,Nashville,Tennessee 37235,USA E-mail:berera@https://www.wendangku.net/doc/ca18145404.html, A general overview is given of the warm in?ation scenario.Talk presented at PASCOS-98,Northeastern University,March 1998The thermodynamics of in?ationary expansion can be either isentropic 1,2,3or non-isentropic 4,5,6,7.Representing the early universe by a two ?uid mixture of radiation energy density ρr and vacuum energy density ρv ,the in?ationary regime,when the scale factor accelerates ¨R >0,is for ρv >ρr .From the point of view of two ?uid Friedmann cosmology,isentropic in?ationary expan-sion appears as a limiting case within the general regime of non-isentropic of in?ation.In particle physics the vacuum equation of state ρv =?p v is realized by a scalar ?eld with energy density ρ(φ)=˙φ2/2+(?φ)2/2+V (φ),in which the potential energy density dominates V (φ)?12(?φ)2.(1)Most ?eld theory descriptions of in?ation represent the vacuum energy through

a scalar ?eld satisfying eq.(1),with φreferred to as the in?aton.The goal of in?ationary scalar ?eld dynamics is to sustain the vacuum energy su?ciently long for expansion of the scale factor to exceed observational lower bounds and then end the in?ationary epoch by entering the radiation dominated epoch.In the context of dynamical models,isentropic in?ation is referred to as super-cooled in?ation and non-isentropic in?ation is referred to as warm in?ation.

Scalar ?eld models that represent the vacuum energy have no fundamen-tal motivation.They provide a convenient mode for studying the complex dynamics of in?ation.In addition,phase transitions can be represented by such models,and that is another key element to the particle physics picture of the early universe.

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From the point of view of particle physics,a system of interacting?elds will exchange energy at all times,with the in?ationary epoch having no a priori reason for being di?erent.To insist otherwise imposes a sharp division between a expansion regime with negligible energy exchange and then a subsequent reheating period,which is essentially a second Big-Bang.

On the one hand,for a?rst-order phase transition with bubble nucleation kinetics,this is a natural scenario dictated by the dynamics.This conception of Guth’s proved unsuccessful1,8due to the con?icting requirements of having a slow nucleation rate to the true vacuum for obtaining adequate expansion versus a fast rate to allow bubble collisions after expansion,which would reheat the universe.

On the other hand,for continuous transitions,no qualitative feature of the dynamics requires a sharp division between the expansion and heating pe-riods.Such a division can be constructed by requiring the scalar?eld poten-tial to be ultra-?at during the in?ationary expansion period and then sharply cusped to permit a subsequent reheating period that ends in?ation and be-gins the radiation dominated regime.This is the new in?ation picture2,3. While the scalar?eld in on the ultra-?at portion of the potential,it will have weak self-interaction,since such a potential requires this.Also,it will interact weakly with other?elds,since in?ationary expansion will rapidly dilute any pre-existing?eld energy apart from the vacuum energy.An ultra-?at potential is su?cient for satisfying the requirements of in?ation,but it is not necessary.

The general case is to allow the scalar?eld to interact with other?elds during the entire evolution down the potential well.With this follows re-active forces on the in?aton,and they can slow the motion of the in?aton. The generic kinetics of continuous phase transitions for terrestrial systems, Ginzburg-Landau kinetics,is emphatic on the dissipational properties of the order parameter.Cosmological theories of phase transitions assume that the statistical mechanical principles on such large scales are no di?erent from those on terrestrial scales.Thus Ginzburg-Landau kinetics is a viable possibility for the in?aton,provided that appropriate conditions can be realized.In particular the dissipational dynamics of Ginzburg-Landau kinetics assumes the presence of a large heat bath which interacts with the order parameter.For in?ation, this requirement imposes the following self-consistency condition.The in?a-ton must release adequate vacuum energy into the heat bath to compensate for dilution of the heat bath due to in?ationary expansion.Simultaneously the reaction of the heat bath on the in?aton must be su?cient to slow the in?aton’s roll down the potential.The questions are?rst can these require-ments be satis?ed by a sensible phenomenological dynamics and second can this dynamics be derived from quantum?eld theory?

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Consider stochastic evolution for the in?aton governed by the Langevin-like equation

¨φ(t)+ Γ+3˙R(t)

dt =?

1

.(4)

In this limit,the in?aton has a vacuum equation of state sinceρφ≈V(φ) which for a scalar?eld implies pφ=?ρφ.In addition,in a two?uid model composed of the scalar?eld and radiation,by energy conservation

˙ρr=?4

˙R(t)

2π/(3λ)(Γ/m p) andρr(τEI)/ρr(τBI)≈1/(4N2e),where the subscripts BI and EI signify begin and end in?ation respectively.

The solutions in7are an existence proof of the warm in?ation regime.A fundamental justi?cation for such a dynamics requires deriving equation(2) from?rst principles and demonstrating the consistency of the limit eq.(3).Eq.

(2)should not be confused with similar looking equations in earlier reheating models9,since in the warm in?ation case,the dissipative term represents fric-tional forces that arise from interaction ofφwith the heat bath.Furthermore, the overdamped limit eq.(3)is equivalent to an adiabatic limit.Under such conditions,eqs.(2)and(3)are an outcome of quantum mechanics both in ?at spacetime10and for the cosmological setting of warm in?ation11.These

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equations have also been derived from a particular quantum?eld theory model in12.

The fundamental origin of dissipation arises from the coupling of the in-?aton to other?elds which comprise the heat bath.In quantum?eld theory this implies the interactionsφ2χ2,φˉψψandφ2A iμA iμ,for coupling to bosons χ,fermionsψand gauge?elds Aμi.For such couplings,φacts as a mass to the respective heat bath?eld and dissipation e?ects are only relevant when the mass it induces is less than or of order the temperature scale T.To enhance dissipative e?ects for large displacements ofφ,the couplings can also be mod-i?ed by shiftingφonly in the relevant interaction term.For example,for the bosonic case the shiftφ2χ2→(φ?M)2χ2.Thus for several heat bath?elds a distributed mass model suggests itself,which for the scalar case is

i g i(φ?M i)2χ2i(6) or similarly a continuous mass model

dμg(μ)(φ?μ)2χ2μ,(7) with the mass spectrum given by g i,g(μ)respectively.Such models could be motivated if the high energy world below the Planck scale and well above the electroweak scale tended away from an organized group theoretical structure towards a random one.

A second requirement of in?ationary models is producing observationally consistent density perturbationsδρ/ρ~10?52,3,13,14,6,15.The general formula forδρ(t)/ρ(t)at horizon entry t f for perturbations produced by a scalar?eld, that exited the horizon at t i during in?ation,is13

δρ

ρ(t f)=

V′(φ(t i))δφ(t i)

1+w

(8)

where w≡p/ρ.In the warm in?ation case11

δφ2(H i?e,t i)= V=1/H3i d3x V′′(φ(t i)),(9)

where H i is the(slowly varying)Hubble parameter at time t i during warm in?ation.In warm in?ationρr?˙φ2/2so that

δρ

4ρr(t i)

.(10)

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In general the presence of non-negligibleρr tends to suppressδρ/ρ.In models that have been examined,regions withδρ/ρ60and density perturbationsδρ/ρ~10?5.

In conclusion,the warm in?ation scenario has been shown to be consistent with observation for certain models and has nice features for treatment by quantum?eld theory.The warm in?ation regime also has interesting possibil-ities for production of large scale cosmic magnetic?elds16and baryogenesis7, since in a sense,the warm in?ation regime is like a radiation dominated regime except with in?ationary expansion.

References

1.A.H.Guth,Phys.Rev.D23,347(1981).

2.A.Albrecht and P.J.Steinhardt,Phys.Rev.Lett.48,1220(1982).

3.A.Linde,Phys.Lett.B108,389(1982).

4.P.Spindel and R.Brout,Phys.Lett.B320,241(1994).

5.H.P.de Oliveria and R.O.Ramos,Phys.Rev.D57,741(1998);A.V.

Nesteruk,R.Maartens,and E.Gunzig,astro-ph/9703137.

6.A.Berera,Phys.Rev.Lett.75,3218(1995).

7.A.Berera,Phys.Rev.D55,3346(1997).

8.A.Guth and E.Weinberg,Nucl.Phys.B212,321(1983).

9.A.Albrecht,P.J.Steinhardt,M.S.Turner and F.Wilczek,Phys.Rev.

Lett.48,1437(1982);A.D.Dolgov and A.D.Linde,Phys.Lett.B 116,329(1982);L.F.Abbott,E.Farhi,and M.B.Wise,Phys.Lett.B 117,29(1982).

10.A.O.Caldeira and A.J.Leggett,Ann.Phys.149,374(1983).

11.A.Berera,Phys.Rev.D54,2519(1996).

12.A.Berera,M.Gleiser and R.O.Ramos,hep-ph/9803394.

13.J.M.Bardeen,Phys.Rev.D22,1882(1980).

14.A.Berera and L.Z.Fang,Phys.Rev.Lett.74,1912(1995).

15.R.Maartens and D.Tilley,Gen.Rel.Grav.30289,1998.

16.P.L.Biermann,H.Falcke,Proceeding of Frontiers in Contemporary

Physics International Lecture and Workshop,Vanderbilt University1997;

A.Berera,talk at same conference.

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