Hydrodynamics and collision-dominated transport are crucial to understand the slow dynamics of many correlated quantum liquids. The ratio eta/s of the shear viscosity eta to the entropy density s is uniquely suited to determine how strongly the excitations in a quantum fluid interact. We determine eta/s in clean undoped graphene using a quantum kinetic theory. As a result of the quantum critica...

The cosmological memory effect is a permanent change in the relative separation of test particles located Friedmann-Lema\'{\i}tre-Robertson-Walker (FLRW) spacetime due to passage gravitational waves. In case spatially flat FLRW filled with perfect fluid general relativity, it known that only tensor perturbations contribute while scalar and vector do not. this paper, we show context scalar-tenso...

We investigate barotropic perfect fluid cosmologies which admit an isotropic singularity. From the General Vorticity Result of Scott, it is known that these cosmologies must be irrotational. In this paper we prove, using two different methods, that if we make the additional assumption that the perfect fluid is shear-free, then the fluid flow must be geodesic. This then implies that the only she...