Papers by winfried zimdahl
arXiv (Cornell University), 2015
We discuss the possibility to implement a viscous cosmological model, attributing to the dark mat... more We discuss the possibility to implement a viscous cosmological model, attributing to the dark matter component a behaviour described by bulk viscosity. Since bulk viscosity implies negative pressure, this rises the possibility to unify the dark sector. At the same time, the presence of dissipative effects may alleviate the so called small scale problems in the $\Lambda$CDM model. While the unified viscous description for the dark sector does not lead to consistent results, the non-linear behaviour indeed improves the situation with respect to the standard cosmological model.
arXiv (Cornell University), Dec 3, 2015
We discuss the possibility to implement a viscous cosmological model, attributing to the dark mat... more We discuss the possibility to implement a viscous cosmological model, attributing to the dark matter component a behaviour described by bulk viscosity. Since bulk viscosity implies negative pressure, this rises the possibility to unify the dark sector. At the same time, the presence of dissipative effects may alleviate the so called small scale problems in the ΛCDM model. While the unified viscous description for the dark sector does not lead to consistent results, the non-linear behaviour indeed improves the situation with respect to the standard cosmological model.
The European Physical Journal C, 2020
Models with non-gravitational interactions between the dark matter and dark energy components are... more Models with non-gravitational interactions between the dark matter and dark energy components are an alternative to the standard cosmological scenario. These models are characterized by an interaction term, and a frequently used parameterization is $$Q = 3\xi H \rho _{x}$$ Q = 3 ξ H ρ x , where H is the Hubble parameter and $$\rho _{x}$$ ρ x is the dark energy density. Although recent analyses have reported that this particular scenario provides a potential solution to the $$H_{0}$$ H 0 and $$\sigma _{8}$$ σ 8 tensions for negative values of the interaction parameter $$\xi $$ ξ , we show here that such an interval of values of $$\xi $$ ξ leads to a violation of the Weak Energy Condition for the dark matter density, which is accompanied by unphysical instabilities of matter perturbations. Using current observational data we also show that the inclusion of the physical prior $$\xi \ge 0$$ ξ ≥ 0 in the statistical analysis alters the parameter selection for this model and discards it a...
Journal of Cosmology and Astroparticle Physics, 2016
We construct a cosmological scalar-tensor-theory model in which the Brans-Dicke type scalar Φ ent... more We construct a cosmological scalar-tensor-theory model in which the Brans-Dicke type scalar Φ enters the effective (Jordan-frame) Hubble rate as a simple modification of the Hubble rate of the ΛCDM model. This allows us to quantify differences between the background dynamics of scalar-tensor theories and general relativity (GR) in a transparent and observationally testable manner in terms of one single parameter. Problems of the mapping of the scalar-field degrees of freedom on an effective fluid description in a GR context are discused. Data from supernovae, the differential age of old galaxies and baryon acoustic oscillations are shown to strongly limit potential deviations from the standard model.
The European Physical Journal C, 2017
Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characte... more Both scalar fields and (generalized) Chaplygin gases have been widely used separately to characterize the dark sector of the universe. Here we investigate the cosmological background dynamics for a mixture of both these components and quantify the fractional abundances that are admitted by observational data from supernovae of type Ia and from the evolution of the Hubble rate. Moreover, we study how the growth rate of (baryonic) matter perturbations is affected by the dark-sector perturbations. 1130628 as well as by CNPq (Brazil) and FAPES (Brazil). Sadly, shortly after this paper was started Prof. Sergio del Campo, unexpectedly, passed away. JF, RH and WZ dedicate this paper to his Memory.
Journal of Cosmology and Astroparticle Physics, 2017
An interaction between dark matter and dark energy, proportional to the product of their energy d... more An interaction between dark matter and dark energy, proportional to the product of their energy densities, results in a scaling behavior of the ratio of these densities with respect to the scale factor of the Robertson-Walker metric. This gives rise to a class of cosmological models which deviate from the standard model in an analytically tractable way. In particular, it becomes possible to quantify the role of potential dark-energy perturbations. We investigate the impact of this interaction on the structure formation process. Using the (modified) CAMB code we obtain the CMB spectrum as well as the linear matter power spectrum. It is shown that the strong degeneracy in the parameter space present in the background analysis is considerably reduced by considering Planck data. Our analysis
Monthly Notices of the Royal Astronomical Society, 2016
A suitable nonlinear interaction between dark matter with an energy density ρ M and dark energy w... more A suitable nonlinear interaction between dark matter with an energy density ρ M and dark energy with an energy density ρ X is known to give rise to a non-canonical scaling ρ M ∝ ρ X a −ξ where ξ is a parameter which generally deviates from ξ = 3. Here we present a covariant generalization of this class of models and investigate the corresponding perturbation dynamics. The resulting matter power spectrum for the special case of a time-varying Lambda model is compared with data from the SDSS DR9 catalogue. We find a best-fit value of ξ = 3.25 which corresponds to a decay of dark matter into the cosmological term. Our results are compatible with the ΛCDM model at the 2σ confidence level.
Annalen der Physik, 1973
ABSTRACT By use of SOMMERFELD expansions T3-corrections of the degenerate electron gas specific h... more ABSTRACT By use of SOMMERFELD expansions T3-corrections of the degenerate electron gas specific heat are obtained. Comparisons with the results of GELL-MANN [2] and KIRKWOOD and COWAN [6] are made.
Physics of the Dark Universe, 2017
Yes, but only for a parameter value that makes it almost coincide with the standard model. We rec... more Yes, but only for a parameter value that makes it almost coincide with the standard model. We reconsider the cosmological dynamics of a generalized Chaplygin gas (gCg) which is split into a cold dark matter (CDM) part and a dark energy (DE) component with constant equation of state. This model, which implies a specific interaction between CDM and DE, has a ΛCDM limit and provides the basis for studying deviations from the latter. Including matter and radiation, we use the (modified) CLASS code [1] to construct the CMB and matter power spectra in order to search for a gCg-based concordance model that is in agreement with the SNIa data from the JLA sample and with recent Planck data. The results reveal that the gCg parameter α is restricted to |α| 0.05, i.e., to values very close to the ΛCDM limit α = 0. This excludes, in particular, models in which DE decays linearly with the Hubble rate.
Physics Letters A, 1974
A general formula for the Hall conductivity, which is exact in the one-particle approximation, ha... more A general formula for the Hall conductivity, which is exact in the one-particle approximation, has been obtained. Making use of this formula, the Hall coefficient for small polarons has been calculated. On leave of absence from the University of Rostock, Rostock (DDR).
A curvature self-interaction of the cosmic gas is shown to mimic a cosmological constant or other... more A curvature self-interaction of the cosmic gas is shown to mimic a cosmological constant or other forms of dark energy, such as a rolling tachyon condensate or a Chaplygin gas. Any given Hubble rate and deceleration parameter can be traced back to the action of an effective curvature force on the gas particles. This force self-consistently reacts back on the cosmological dynamics. The links between an imperfect fluid description, a kinetic description with effective antifriction forces, and curvature forces, which represent a non-minimal coupling of gravity to matter, are established. PACS numbers: 98.80.Hw, 04.40.Nr, 95.30.Tg, 05.70.Ln
General Relativity and Gravitation, 2000
Searching in the Astrophysics e-Print archive for papers with "Large-scale structure" either in t... more Searching in the Astrophysics e-Print archive for papers with "Large-scale structure" either in the title or in the abstract produces a result of about 200 only for the past year. This figure may provide a purely quantitative impression of the attractive power of this subject. On the other hand, it indicates a rapid evolution of the field which rather likely implies that any review becomes outdated, at least partially, on a rather short timescale. Large-scale structures are inhomogeneities in the matter distribution such as groups and clusters of galaxies, superclusters and voids which extend over scales between roughly a few and a few hundreds of Mpc. Cosmological inhomogeneities are assumed to have developed from initially small fluctuations on a homogeneous and isotropic background by gravitational clumping. This gravitational instability picture was most impressively backed up by the results of the COBE mission 1992 which represents a milestone in cosmology. To understand the presently observed large-scale structures from the physics of the early Universe is one of the main topics in modern cosmology. "Generation of Cosmological Large-Scale Structure" was the focus of the Third Erice School on Particle Astrophysics 1996, still organized by D. N. Schramm and P. Galeotti under the auspices of the NATO Advanced Study Institute. (The two previous schools in 1986 and 1988 were devoted to "Gauge-Theory and the Early Universe" and "Dark Matter in the Universe," respectively.) The larger part of the proceedings (about two third) are eight review articles of about thirty pages each, followed by 17 shorter (about 5 pages each) students' contributions. The reviews comprise both theoretical and observational aspects of cosmology. One of the most exciting developments during the past two decades relies on the concept of inflation which offers the nowadays most popular candidate
Physics of the Dark Universe, 2018
We reconsider the dynamics of the Universe in the presence of interactions in the cosmological da... more We reconsider the dynamics of the Universe in the presence of interactions in the cosmological dark sector. A class of interacting models is introduced via a real function f (r) of the ratio r between the energy densities of the (pressureless) cold dark matter (CDM) and dark energy (DE). The subclass of models for which the ratio r depends only on the scale factor is shown to be equivalent to unified models of the dark sector, i.e. models for which the CDM and DE components can be combined in order to form a unified dark fluid. For specific choices of the function f (r) we recover several models already studied in the literature. We analyse various special cases of this type of interacting models using a suitably modified version of the CLASS
Monthly Notices of the Royal Astronomical Society, 1994
Journal of Cosmology and Astroparticle Physics, 2016
Based on the Lemaître-Tolman-Bondi (LTB) metric we consider two flat inhomogeneous big-bang model... more Based on the Lemaître-Tolman-Bondi (LTB) metric we consider two flat inhomogeneous big-bang models. We aim at clarifying, as far as possible analytically, basic features of the dynamics of the simplest inhomogeneous models and to point out the potential usefulness of exact inhomogeneous solutions as generalizations of the homogeneous configurations of the cosmological standard model. We discuss explicitly partial successes but also potential pitfalls of these simplest models. Although primarily seen as toy models, the relevant free parameters are fixed by best-fit values using the Joint Light-curve Analysis (JLA)-sample data. On the basis of a likelihood analysis we find that a local hump with an extension of almost 2 Gpc provides a better description of the observations than a local void for which we obtain a best-fit scale of about 30 Mpc. Future redshift-drift measurements are discussed as a promising tool to discriminate between inhomogeneous configurations and the ΛCDM model.
Uploads
Papers by winfried zimdahl