Field theoretical methods in cosmology (original) (raw)
Dissipation via particle production in scalar field theories
Physical Review D, 1995
Hartree approximation and clearly exhibit dissipative effects related to the thresholds to particle production. The asymptotic dynamics depends on the coupling and initial conditions but does not seem to lead to exponential relaxation. The effect of dissipation by Goldstone bosons is studied non-perturbatively in the large N limit in an O(N) theory. Dissipation produced by Goldstone bosons dramatically changes the picture of the phase transition. We find the remarkable result that for ``slow-roll'' initial conditions (with the expectation value of the field initially near the origin) the final value of the expectation value of the scalar field is very close to its initial value. We find that the minima of the effective action depend on the initial conditions. We provide extensive numerical analysis of the dynamics.
Asymptotic dynamics in scalar field theory: Anomalous relaxation
Physical Review D, 1998
We analyze the dynamics of dissipation and relaxation in the unbroken and broken symmetry phases of scalar theory in the nonlinear regime for large initial energy densities, and after linear unstabilities (parametric or spinodal) are shut-off by the quantum backreaction. A new time scale emerges that separates the linear from the non-linear regimes. This scale is non- perturbative in the coupling and initial amplitude. The non-perturbative evolution is studied in the large the N limit for the O(N) vector model. A combination of numerical analysis and the multitime scale analysis reveals the presence of unstable bands in the nonlinear regime. These are associated with power law growth of quantum fluctuations, that result in power law relaxation and dissipation with non-universal and non-perturbative dynamical anomalous exponents.We find substantial particle production during this non-linear evolu- tion which is of the same order as that in the linear regime. The expectation value of the scalar field vanishes asymptotically transferring all of the initial energy into produced particles via the non-linear resonances in the unbroken symmetry phase.The effective mass for the quantum modes tends asympto- tically to a constant plus oscillating O(1/t) terms. This slow decay causes the power behaviour in the modes which become free for t=infty. We derive a simple expression for the equation of state for the fluid of produced particles that interpolates between radiation-type and dust-type equations according to the initial value of the order parameter for unbroken symmetry. For broken symmetry the produced particles are massless Goldstone bosons with a radiation-type equation of state. We find the onset of a novel form of dynamical Bose condensation in the collisionless regime in the absence of thermalization.
2005
We study the symmetry breaking pattern of an O(4) symmetric model of scalar fields, with both charged and neutral fields, interacting with a photon bath. Nagasawa and Brandenberger argued that in favourable circumstances the vacuum manifold would be reduced from S^3 to S^1. Here it is shown that a selective condensation of the neutral fields, that are not directly coupled to photons, can be achieved in the presence of a minimal ``external'' dissipation, i.e. not related to interactions with a bath. This should be relevant in the early universe or in heavy-ion collisions where dissipation occurs due to expansion.
Nonequilibrium dynamics in quantum field theory at high density: The “tsunami”
Physical Review D, 2001
The dynamics of a dense relativistic quantum fluid out of thermodynamic equilibrium is studied in the framework of the ⌽ 4 scalar field theory in the large N limit. The time evolution of a particle distribution in momentum space ͑the tsunami͒ is computed. The effective mass felt by the particles in such a high density medium equals the tree level mass plus the expectation value of the squared field. The case of negative tree level squared mass is particularly interesting. In such a case dynamical symmetry restoration as well as dynamical symmetry breaking can happen. Furthermore, the symmetry may stay broken with a vanishing asymptotic squared mass showing the presence of out of equilibrium Goldstone bosons. We study these phenomena and identify the set of initial conditions that lead to each case. We compute the equation of state which turns out to depend on the initial state. Although the system does not thermalize, the equation of state for asymptotically broken symmetry is of radiation type. We compute the correlation functions at equal times. The two point correlator for late times is the sum of different terms. One stems from the initial particle distribution. Another term accounts for the out of equilibrium Goldstone bosons created by spinodal unstabilities when the symmetry is asymptotically broken. Both terms are of the order of the inverse of the coupling for distances where causal signals can connect the two points. The contribution of the out of equilibrium Goldstones exhibits scaling behavior in a generalized sense.
Role of dissipation in biasing the vacuum selection in quantum field theory at finite temperature
Physical Review D, 2005
We study the symmetry breaking pattern of an O(4) symmetric model of scalar fields, with both charged and neutral fields, interacting with a photon bath. Nagasawa and Brandenberger argued that in favourable circumstances the vacuum manifold would be reduced from S 3 to S 1. Here it is shown that a selective condensation of the neutral fields, that are not directly coupled to photons, can be achieved in the presence of a minimal "external" dissipation, i.e. not related to interactions with a bath. This should be relevant in the early universe or in heavy-ion collisions where dissipation occurs due to expansion.
A field theory characterization of interacting adiabatic particles in cosmology
2008
We explore the adiabatic particle excitations of an interacting field in a cosmological background. By following the time-evolution of the quantum state corresponding to the particle excitation, we show how the basic properties characterizing the particle propagation can be recovered from the two-point propagators. As an application, we study the backgroundinduced dissipative effects on the propagation of a two-level atom in an expanding universe.
Fluctuations of quantum fields in a classical background and reheating
Physical Review D, 2010
We consider the particle creation process associated with a quantum field χ in a time-dependent, homogeneous and isotropic, classical background. It is shown that the field square χ 2 , the energy density and the pressure of the created particles have large fluctuations comparable to their vacuum expectation values. Possible effects of these fluctuations on the reheating process after inflation are discussed. After determining the correlation length of the fluctuations in two different models, corresponding to the decay in the parametric resonance regime and in the perturbation theory, it is found that these fluctuations should be taken into account in the final thermalization process, in the back-reaction effects and when the formation of primordial black holes is considered. In both models, by comparing quantum and thermal fluctuations with each other it is observed that very quick thermalization after the complete inflaton decay is not always possible even when the interaction rates are large. On the other hand, when the back-reaction effects are included during the preheating stage, the coherence of the inflaton oscillations is shown to be lost because of the fluctuations in χ 2. Finally, we note that a large fluctuation in the energy density may cause a black hole to form and we determine the fraction of total energy density that goes into such primordial black holes in the model of preheating we consider.
The Big Bang nucleosynthesis and finite temperature field theory
Physics Letters B, 1982
We consider electromagnetic corrections at finite temperature and their effect on the nucleosynthesis in the standard Big Bang scenario. This requires discussing the finite, temperature dependent correction to the neutron-proton mass difference as well as making use of a previous result on the temperature correction to the mass of the electron. We find that these corrections do not affect the conventional results of e.g. the helium abundance to any appreciable extent.
Decay of cosmological constant as Bose condensate evaporation. Mod. Phys
2013
We consider the process of decay of symmetric vacuum state as evaporation of a Bose condensate of physical Higgs particles, defined over asymmetric vacuum state. Energy density of their selfinteraction is identified with cosmological constant Λ in the Einstein equation. Λ decay then provides dynamical realization of spontaneous symmetry breaking. The effective mechanism is found for damping of coherent oscillations of a scalar field, leading to slow evaporation regime as the effective mechanism for Λ decay responsible for inflation without special fine-tuning of the microphysical parameters. This mechanism is able to incorporate reheating, generation of proper primordial fluctuations, and nonzero cosmological constant today. PACS: 04.20.Cv; 97.60.Lf; 98.80.Cq; 98.80.Dr; 04.70.Bw; 04.20.Bw Any thing which contributes to the stress-energy tensor as Tµν = ρvacgµν, behaves like a cosmological term Λgµν in the Einstein equation. Developments in particles and quantum field theory, as well...
On a fate of hot quantum field theories
2000
It is argued that for hot quantum fields, the necessary effective perturbation theories may be based on a resummation procedure which, contrarily to the zero temperature case, differs substantially from the one ordinarily in use. Important differences show up in the infrared sector of hot quantum field theories.