arXiv subjects

Winfried Zimdahl

Publications and source records attributed to Winfried Zimdahl.

66 records · Page 4Linked to original sources

arXiv1998

Covariant cosmological perturbation dynamics in the inflationary universe

The linear cosmological perturbation theory of almost homogeneous and isotropic perfect fluid and scalar field universes is reconsidered and formally simplified. Using the existence of a covariant conserved quantity on large perturbation scales, a closed integral expression for comoving energy density perturbations is obtained for arbitrary equations of state. On this basis we establish a simple relation between fluid energy density perturbations at `reentry' into the horizon and the corresponding scalar field quantities at the first Hubble scale crossing during an early de Sitter phase of a standard inflationary scenario.

gr-qc↗

arXiv1998

Covariant cosmological perturbation dynamics in the large-scale limit

Using the existence of a covariant conserved quantity on large perturbation scales in a spatially flat perfect fluid or scalar field universe, we present a general formula for gauge-invariantly defined comoving energy density perturbations which encodes the entire linear perturbation dynamics in a closed time integral. On this basis we discuss perturbation modes in different cosmological epochs.

gr-qc↗

arXiv1997

Cosmological particle production and generalized thermodynamic equilibrium

With the help of a conformal, timelike Killing-vector we define generalized equilibrium states for cosmological fluids with particle production. For massless particles the generalized equilibrium conditions require the production rate to vanish and the well known ``global'' equilibrium of standard relativistic thermodynamics is recovered as a limiting case. The equivalence between the creation rate for particles with nonzero mass and an effective viscous fluid pressure follows as a consequence of the generalized equilibrium properties. The implications of this equivalence for the cosmological dynamics are discussed, including the possibility of a power-law inflationary behaviour. For a simple gas a microscopic derivation for such kind of equilibrium is given on the basis of relativistic kinetic theory.

gr-qc↗

arXiv1997

Cosmological perturbations in the inflationary Universe

Previously defined covariant and gauge-invariant perturbation variables, representing, e.g., the fractional spatial energy density gradient on hypersurfaces of constant expansion, are used to simplify the linear perturbation analysis of a classical scalar field. With the help of conserved quantities on large scales we establish an exact first-order relation between comoving fluid energy density perturbations at `reentry' into the horizon and corresponding scalar field energy density perturbations at the first Hubble scale crossing during an early de Sitter phase of a standard inflationary scenario.

astro-ph↗

arXiv1997

Cosmological perturbation theory and conserved quantities in the large-scale limit

The linear cosmological perturbation theory of an almost homogeneous and isotropic perfect fluid universe is reconsidered and formally simplified by introducing new covariant and gauge-invariant variables with physical interpretations on hypersurfaces of constant expansion, constant curvature or constant energy density. The existence of conserved perturbation quantities on scales larger than the Hubble scale is discussed. The quantity which is conserved on large scales in a flat background universe may be expressed in terms of the fractional, spatial gradient of the energy density on constant expansion hypersurfaces or, alternatively, with the help of expansion or curvature perturbation variables on hypersurfaces of constant energy density. For nonvanishing background curvature the perturbation dynamics is most suitably described in terms of energy density perturbations on hypersurfaces of constant curvature.

gr-qc↗

arXiv1997

Reacting fluids in the expanding Universe: A new mechanism for entropy production

It is shown that two reacting cosmological fluids, each of them perfect on its own, which exchange energy and momentum without preserving particle numbers, give rise to an entropy producing `reactive' bulk stress of the system as a whole, as soon as the detailed balance between decay and inverse decay processes is perturbed. This demonstrates explicitly that particle generation is dynamically equivalent to an effective bulk pressure. We derive a semiquantitative formula for the corresponding new kinetic coefficient and evaluate the latter for the out-of-equilibrium decay of heavy, nonrelativistic particles into radiation. It turns out that the associated reactive bulk viscosity may be more than one order of magnitude larger than the conventional bulk viscosity, calculated, e.g., in radiative hydrodynamics.

astro-ph↗

arXiv1996

Reheating and causal thermodynamics

The reheating process in inflationary universe models is considered as an out-of-equilibrium mixture of two interacting and reacting fluids, and studied within the framework of causal, irreversible thermodynamics. The evolution of the temperature and the decay rate as determined by causal thermodynamics are estimated at different stages of the process. A simple model is also used to find the perturbations of the expansion rate, including the possibility of damped oscillations.

astro-ph↗

arXiv1996

Collisional equilibrium, particle production and the inflationary universe

Particle production processes in the expanding universe are described within a simple kinetic model. The equilibrium conditions for a Maxwell-Boltzmann gas with variable particle number are investigated. We find that radiation and nonrelativistic matter may be in equilibrium at the same temperature provided the matter particles are created at a rate that is half the expansion rate. Using the fact that the creation of particles is dynamically equivalent to a nonvanishing bulk pressure we calculate the backreaction of this process on the cosmological dynamics. It turns out that the `adiabatic' creation of massive particles with an equilibrium distribution for the latter necessarily implies power-law inflation. Exponential inflation in this context is shown to become inconsistent with the second law of thermodynamics after a time interval of the order of the Hubble time.

gr-qc↗

arXiv1996

Cosmology with bulk pressure

Different cooling rates of interacting fluid components of the expanding cosmic medium give rise to entropy producing bulk stresses. With the help of effective bulk pressures, on the other hand, one may phenomenologically take into account cosmological particle production processes. We report on recent progress in bulk viscous cosmology both on the phenomenological level and on the level of relativistic kinetic theory.

astro-ph↗

arXiv1996

"Understanding" cosmological bulk viscosity

A universe consisting of two interacting perfect fluids with the same 4-velocity is considered. A heuristic mean free time argument is used to show that the system as a whole cannot be perfect as well but neccessarily implies a nonvanishing bulk viscosity. A new formula for the latter is derived and compared with corresponding results of radiative hydrodynamics.

astro-ph↗

arXiv1996

Kinetic theory for particle production

Recently, the phenomenological description of cosmological particle production processes in terms of effective viscous pressures has attracted some attention. Using a simple creation rate model we discuss the question to what extent this approach is compatible with the kinetic theory of a relativistic gas. We find the effective viscous pressure approach to be consistent with this model for homogeneous spacetimes but not for inhomogeneous ones.

gr-qc↗

arXiv1996

Bulk Viscous Cosmology

The full causal Müller-Israel-Stewart (MIS) theory of dissipative processes in relativistic fluids is applied to a flat, homogeneous and isotropic universe with bulk viscosity. It is clarified in which sense the so called truncated version is a reasonable limiting case of the full theory. The possibility of bulk viscosity driven inflationary solutions of the full theory is discussed. As long as the particle number is conserved almost all these solutions exhibit an exponential increase of the temperature. Assuming that the bulk viscous pressure of the MIS theory may also be interpreted as an effective description for particle production processes, the thermodynamical behaviour of the Universe changes considerably. In the latter case the temperature increases at a lower rate or may remain constant during a hypothetical de Sitter stage, accompanied by a substantial growth of the comoving entropy.

astro-ph↗