47 research outputs found
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
, where is the component that
accelerates the Hubble expansion at late times and is an extra
contribution proportional to . This form of 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
with , as typical of some dark energy models, the gravity
effective action depends both on the gravitational field and on the field.
In this case general covariance only requires the conservation of the total
energy-momentum tensor, including both the classical term 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 and
and we show that the total dark energy density
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