Papers

Seemingly different physical systems may exhibit universal behavior. For instance, certain properties of water vapor close to its boiling point or the behavior of a magnet close to the temperature in which it looses its magnetization are identical near the phase transition.

In a recent paper E. Shaverin and A. Yarom studied a different notion of universality. Using the gauge-gravity correspondence they considered various types of generalized viscosities which characterize conformal fluids that can be described by a black hole dual. Generically, these viscosities are not universal and depend on the particle content of the theory. However, a certain linear combination of these generalized viscosities does seem to exhibit universal behavior. In particular, it has been shown that within the context of theories whose kinetic terms have two derivatives, this linear combination does not depend on the particle content of the theory. In their recent work the Shaverin and Yarom have shown that the linear combination of viscosities vanishes even beyond two derivative gravity, at least in a subset of possible extensions of the gravitational action. This is in contrast to the behavior of the ratio of the shear viscosity to entropy density which was thought to be universal but has been found to deviate from its universal behavior once the gravitational action includes perturbative terms which involve more than two derivatives.

Five-dimensional gauge theories are in general not well-defined as quantum field theories. The Yang-Mills coupling has dimension of mass^(-1/2), making these theories non-renormalizable, which means that we cannot consistently remove the UV cutoff used to regularize loop amplitudes. Physically the theories become strongly coupled in the UV, where they must be replaced by some “UV complete” theory. In some cases, however, Seiberg argued that there exist supersymmetric UV fixed points described by strongly interacting five-dimensional superconformal field theories, although these CFT’s do not seem to admit a Lagrangian description.

In view of the AdS/CFT duality it is natural to wonder whether these 5d CFT’s admit 6d AdS gravity duals. This case of the AdS/CFT correspondence has been the least studied. It is complicated by the fact that the amount of supersymmetry is reduced relative to the maximal supersymmetry cases in other dimensions, making it harder to find 6d AdS solutions by dimensional reduction of supergravity. The first explicit example was found by Brandhuber and Oz, who made use of a brane construction in string theory to relate the fixed point CFT of a particular 5d gauge theory based on the gauge group USp(2N) to a ten-dimensional geometry with a non-compact 6d AdS component and a compact 4-sphere.

Recently, O. Bergman and D. Rodriguez-Gomez have generalized this case to three classes of 5d supersymmetric gauge theories involving products of symplectic and unitary groups, and argued that they are dual to geometries where the compact part is replaced by an orbifold of a 4-sphere. These new examples allow for several non-trivial checks of the duality, by comparing various wrapping modes of branes on compact cycles with operators in the 5d CFT that carry the corresponding global symmetry charges. This also opens the door to a number of further generalizations related to the possibility of turning on various fluxes on the cycles.

 

It is usually taken for granted that the scheme we use to characterize distances will have no effect on our measurement. More formally, physical systems are invariant under coordinate transformations. As a result, the energy-momentum tensor is conserved. In a seminal paper, Alvarez-Gaume and Witten have shown that coordinate transformations can become anomalous: due to quantum effects invariance under coordinate transformations is broken and the energy-momentum tensor is no longer conserved. This breaking of coordinate invariance is called a gravitational anomaly.

In a recent publication, A. Yarom together with K. Jensen from the University of Victoria and R. Loganayagam from Harvard studied the impact of gravitational anomalies on thermodynamic behavior. Quite surprisingly, these anomalies affect the thermodynamic behavior of the system in a strikingly pronounced way than has been anticipated; the response of the equilibrated system to particular perturbations is controlled by the strength of gravitational anomalies. Gravitational anomalies, one of the most subtle effects of quantum mechanics, may play an important role in characterizing thermodynamic equilibria of various physical systems.