A note on classical and quantum unimodular gravity

We discuss unimodular gravity at a classical level, and in terms of its extension into the UV through an appropriate path integral representation. Classically, unimodular gravity is simply a gauge fixed version of General Relativity (GR), and as such it yields identical dynamics and physical predictions. We clarify this and explain why there is no sense in which it can "bring a new perspective" to the cosmological constant problem. The quantum equivalence between unimodular gravity and GR is more of a subtle question, but we present an argument that suggests one can always maintain the equivalence up to arbitrarily high momenta. As a corollary to this, we argue that whenever inequivalence is seen at the quantum level, that just means we have defined two different quantum theories that happen to share a classical limit.Comment: 5 pages; v2: Some clarifying comments added. Version to appear in European Physical Journal

Stability analysis of electric power systems for ‘more electric’ aircraft

This paper presents a comprehensive assessment of small-signal stability for a “more-electric” aircraft power system consisting of a synchronous variable-frequency generator which supplies several power electronic controlled loads via an 18-pulse autotransformer rectifier unit (ATRU) for AC-DC conversion. Functional models for key power system components and loads are derived. Numerical tools employed for the automatic calculation of linearized equations and operating points are described, and the influence of leading design and operational parameter on system stability is evaluated

Status of background-independent coarse-graining in tensor models for quantum gravity

A background-independent route towards a universal continuum limit in discrete models of quantum gravity proceeds through a background-independent form of coarse graining. This review provides a pedagogical introduction to the conceptual ideas underlying the use of the number of degrees of freedom as a scale for a Renormalization Group flow. We focus on tensor models, for which we explain how the tensor size serves as the scale for a background-independent coarse-graining flow. This flow provides a new probe of a universal continuum limit in tensor models. We review the development and setup of this tool and summarize results in the 2- and 3-dimensional case. Moreover, we provide a step-by-step guide to the practical implementation of these ideas and tools by deriving the flow of couplings in a rank-4-tensor model. We discuss the phenomenon of dimensional reduction in these models and find tentative first hints for an interacting fixed point with potential relevance for the continuum limit in four-dimensional quantum gravity.Comment: 28 pages, Review prepared for the special issue "Progress in Group Field Theory and Related Quantum Gravity Formalisms" in "Universe