Higuchi Ghosts and Gradient Instabilities in Bimetric Gravity

Bimetric gravity theories allow for many different types of cosmological solutions, but not all of them are theoretically allowed. In this work we discuss the conditions to satisfy the Higuchi bound and to avoid gradient instabilities in the scalar sector at the linear level. We find that in expanding universes the ratio of the scale factors of the reference and observable metric has to increase at all times. This automatically implies a ghost-free helicity-2 and helicity-0 sector and enforces a phantom dark energy. Furthermore, the condition for the absence of gradient instabilities in the scalar sector will be analyzed. Finally, we discuss whether cosmological solutions can exist, including exotic evolutions like bouncing cosmologies, in which both the Higuchi ghost and scalar instabilities are absent at all times.Comment: 13 pages, 2 figures; version published in PR

Emergent Majorana Mass and Axion Couplings in Superfluids

Axions (in the general sense) may acquire qualitatively new couplings inside superfluids. Their conventional couplings to fermions, in empty space, involve purely imaginary masses; the new couplings involve emergent Majorana masses. The possibility of weak links for axions, recently put forward, is analyzed, rejected, and replaced with a non-local analogue.Comment: 10 pages, no figures. v2: Additional comment on non-local Josephson effects, additional expository remark

Capturing User Interests for Content-based Recommendations

Nowadays, most recommender systems provide recommendations by either exploiting feedback given by similar users, referred to as collaborative filtering, or by identifying items with similar properties, referred to as content-based recommendation. Focusing on the latter, this keynote presents various examples and case studies that illustrate both strengths and weaknesses of content-based recommendatio

Polynomial poly-vector fields

In this text we give a decomposition result on polynomial poly-vector fields generalizing a result on the decomposition of homogeneous Poisson structures. We discuss consequences of this decomposition result in particular for low dimensions and low degrees. We provide the tools to calculate simple cubic Poisson structures in dimension three and quadratic Poisson structures in dimension four. Our decomposition result has a nice effect on the relation between Poisson structures and Jacobi structures.Comment: 20 pages. v4: sections rearranged and formulations clarifie