The traces of anisotropic dark energy in light of Planck

We study a dark energy model with non-zero anisotropic stress, either linked to the dark energy density or to the dark matter density. We compute approximate solutions that allow to characterise the behaviour of the dark energy model and to assess the stability of the perturbations. We also determine the current limits on such an anisotropic stress from the cosmic microwave background data by the Planck satellite, and derive the corresponding constraints on the modified growth parameters like the growth index, the effective Newton's constant and the gravitational slip.Comment: 29 pages, 10 figures; matches version accepted for publication in JCA

Early dark energy from zero-point quantum fluctuations

We examine a cosmological model with a dark energy density of the form $\rho_{DE}(t)=\rho_X(t)+\rho_Z(t)$, where $\rho_X$ is the component that accelerates the Hubble expansion at late times and $\rho_Z(t)$ is an extra contribution proportional to $H^2(t)$. This form of $\rho_Z(t)$ follows from the recent proposal that the contribution of zero-point fluctuations of quantum fields to the total energy density should be computed by subtracting the Minkowski-space result from that computed in the FRW space-time. We discuss theoretical arguments that support this subtraction. By definition, this eliminates the quartic divergence in the vacuum energy density responsible for the cosmological constant problem. We show that the remaining quadratic divergence can be reabsorbed into a redefinition of Newton's constant only under the assumption that the energy-momentum tensor of vacuum fluctuations is conserved in isolation. However, in the presence of an ultra-light scalar field $X$ with $m_X<H_0$, as typical of some dark energy models, the gravity effective action depends both on the gravitational field and on the $X$ field. In this case general covariance only requires the conservation of the total energy-momentum tensor, including both the classical term $T^X_{\mu\nu}$ and the vacuum expectation value of T_{\mu\nu}. If there is an exchange of energy between these two terms, there are potentially observable consequences. We construct an explicit model with an interaction between $\rho_X$ and $\rho_Z$ and we show that the total dark energy density $\rho_{DE}(t)=\rho_X(t)+\rho_Z(t)$ always remains a finite fraction of the critical density at any time, providing a specific model of early dark energy. We discuss the implication of this result for the coincidence problem and we estimate the model parameters by means of a full likelihood analysis using current CMB, SNe Ia and BAO data.Comment: 7 pages, 3 figures; v3: improved discussion, references adde

Komplexe Anlagen einfach simuliert : Past, Present, Future

Das Institut für Angewandte Simulation hat ein Vorgehenskonzept entwickelt, welches die detaillierte Simulation von komplexen Anlagen auf einfache Art und Weise ermöglicht. Dadurch kann die Simulation vom Anlagenverkäufer über den Anlagenplaner bis hin zum Anlagenbetreiber effizient eingesetzt werden. Im Rahmen verschiedener Simulationsstudien wurde das Konzept erfolgreich getestet und optimiert. Gegenwärtiges Highlight ist ein Simulationsbaukasten, der nun weltweit im Einsatz ist

Cosmological Ohm's law and dynamics of non-minimal electromagnetism

The origin of large-scale magnetic fields in cosmic structures and the intergalactic medium is still poorly understood. We explore the effects of non-minimal couplings of electromagnetism on the cosmological evolution of currents and magnetic fields. In this context, we revisit the mildly non-linear plasma dynamics around recombination that are known to generate weak magnetic fields. We use the covariant approach to obtain a fully general and non-linear evolution equation for the plasma currents and derive a generalised Ohm law valid on large scales as well as in the presence of non-minimal couplings to cosmological (pseudo-)scalar fields. Due to the sizeable conductivity of the plasma and the stringent observational bounds on such couplings, we conclude that modifications of the standard (adiabatic) evolution of magnetic fields are severely limited in these scenarios. Even at scales well beyond a Mpc, any departure from flux freezing behaviour is inhibited.Comment: 24 pages, 2 figures; matches version published in JCA

Theoretical Priors On Modified Growth Parametrisations

Next generation surveys will observe the large-scale structure of the Universe with unprecedented accuracy. This will enable us to test the relationships between matter over-densities, the curvature perturbation and the Newtonian potential. Any large-distance modification of gravity or exotic nature of dark energy modifies these relationships as compared to those predicted in the standard smooth dark energy model based on General Relativity. In linear theory of structure growth such modifications are often parameterised by virtue of two functions of space and time that enter the relation of the curvature perturbation to, first, the matter over-density, and second, the Newtonian potential. We investigate the predictions for these functions in Brans-Dicke theory, clustering dark energy models and interacting dark energy models. We find that each theory has a distinct path in the parameter space of modified growth. Understanding these theoretical priors on the parameterisations of modified growth is essential to reveal the nature of cosmic acceleration with the help of upcoming observations of structure formations.Comment: 10 pages, 5 figure

Kooperieren? Ja! Aber wie? : Chancen und Herausforderungen bei der Entwicklung einer Kooperationsplattform für urbane Güterlogistik in der Stadt Zürich

Die urbane Güterlogistik ist angesichts zunehmender Volumen und steigender Kundenanforderungen eine grosse Herausforderung. Eine quantitative Studie zeigt, dass eine Kooperation von Transporteuren nicht nur die Gesamtkosten senken, sondern auch die gefahrenen Kilometer reduzieren würde. Ein Geschäftsmodell basierend auf einer virtuellen Plattform zur Ermöglichung solcher Kooperation könnte also die Nachhaltigkeit fördern. Die Herausforderungen für ein derartiges Geschäftsmodell sind derzeit noch gross, doch könnten neue rechtliche Rahmenbedingungen und die zunehmende Digitalisierung der Märkte die Attraktivität solcher Plattformen erhöhen

Simulationsgestützte Optimierung der Produktionsreihenfolge

Die USM U. Schärer Söhne AG produziert seit 50 Jahren modulare Möbelbausysteme in zeitlosem Design. In Münsingen hat die USM eine hochmoderne Lackieranlage in Betrieb genommen. Das Institut für Angewandte Simulation hat schon während des Planungsprozesses eine Simulation der neuen Anlage aufgebaut. Damals diente sie dazu, den Nachweis für die geplante Leistung zu liefern. Heute ist die Simulation Kern eines Produktionsplanungstools, das eine innovative hybride Optimierungsmethode einsetzt