We review progress in the hydrodynamic description of heavy-ion collisions, focusing on recent developments in modeling the fluctuating initial state and event-by-event viscous hydrodynamic simulations. We discuss how hydrodynamics can be used to extract information on fundamental properties of quantum-chromo-dynamics from experimental data, and review successes and challenges of the hydrodynamic framework. arXiv:1301.5893v1 [nucl-th]

Hydrodynamic transport coefBcients may be evaluated from first principles in a weakly coupled scalar field theory at an arbitrary temperature. In a theory with cubic and quartic interactions, the infinite class of diagrams which contributes to the leading weak coupling behavior is identified and summed. The resulting expression may be reduced to a single linear integral equation, which is shown to be identical to the corresponding result obtained from a linearized Boltzmann equation describing effective thermal excitations with temperature-dependent masses and scattering amplitudes. The effective Boltzmann equation is valid even at very high temperature where the thermal lifetime and mean free path are short compared to the Compton wavelength of the fundamental particles.Numerical results for the shear and the bulk viscosities are presented.

It has been suggested that a scalar field with negative kinetic energy, or "ghost," could be the source of the observed late-time cosmological acceleration. Naively, such theories should be ruled out by the catastrophic quantum instability of the vacuum. We derive phenomenological bounds on the Lorentz-violating ultraviolet cutoff Λ which must apply to low-energy effective theories of ghosts, in order to keep the instability at unobservable levels. Assuming only that ghosts interact at least gravitationally, we show that Λ < ∼ 3 MeV for consistency with the cosmic gamma ray background. We also show that theories of ghosts with a Lorentz-conserving cutoff are completely excluded. PACS numbers: 98.80.Cq, 98.70.Vc The present accelerated expansion of the universe seems to be an experimental fact, now that data from distant type Ia supernovae [1] have been corroborated by those from the cosmic microwave background [2]. Although the simplest explanation is a cosmological constant Λ of order (10 −3 eV) 4 , this tiny energy scale is so far below the expected "natural" size for a cosmological constant, that alternative explanations have been vigorously pursued. A common approach has been to assume that the true value of Λ is zero, due to an unknown mechanism, and to propose new physics which would explain why the present-day vacuum energy differs from zero by the small observed amount.The most popular idea has been quintessence, in which the universe is gradually approaching the zero of the vacuum energy by the slow rolling of an extremely weakly coupled scalar field. More recently, some less conventional alternatives have been considered, including "phantom matter," which is essentially quintessence with a wrong-sign kinetic term [3]. These models are motivated by the supernova data, which suggest that the dark energy equation of state violates the weak energy condition by having p < −ρ [4].A serious problem with phantom matter, which is overlooked in the literature that attempts to apply it to cosmology, is that such theories are not quantum mechanically viable, either because they violate conservation of probability, or they have unboundedly negative energy density and lead to the absence of a stable vacuum state. Whether a ghost carries negative norm and positive energy, or vice versa, is a choice which is made during the quantization procedure. This choice exists because the iǫ prescription for defining the propagator near its poles is not unique, and not specified by the Lagrangian itself. The momentum space propagator for a ghost can have either of the two formsIn the first form in (1), the imaginary part of the propagator has the opposite sign relative to that of a positive norm particle. This will cause the optical theorem to be violated, leading to a nonunitary theory. That is, this choice gives a theory with no probabilistic interpretation. It is therefore unphysical and should be dismissed.On the other hand, if the second form in (1) is chosen, unitarity is maintained. The price to be paid is that the pole...

We present results for the elliptic and triangular flow coefficients v(2) and v(3) in Au+Au collisions at √s=200  AGeV using event-by-event D=3+1 viscous hydrodynamic simulations. We study the effect of initial state fluctuations and finite viscosities on the flow coefficients v(2) and v(3) as functions of transverse momentum and pseudorapidity. Fluctuations are essential to reproduce the measured centrality dependence of elliptic flow. We argue that simultaneous measurements of v(2) and v(3) can determine η/s more precisely.

We revisit the analysis of the drag a massive quark experiences and the wake it creates at a temperature T while moving through a plasma using a gravity dual that captures the renormalisation group runnings in the dual gauge theory. Our gravity dual has a black hole and seven branes embedded via Ouyang embedding, but the geometry is a deformation of the usual conifold metric. In particular the gravity dual has squashed two spheres, and a small resolution at the IR. Using this background we show that the drag of a massive quark receives corrections that are proportional to powers of log T when compared with the drag computed using AdS/QCD correspondence. The massive quarks map to fundamental strings in the dual gravity theory. We use the perturbation produced by these strings to compute the wake and compare with the results obtained using AdS/QCD correspondence. We also study the shear viscosity in the theory with running couplings, analyze the viscosity to entropy ratio and compare the result with the bound derived from AdS backgrounds. In the presence of higher order curvature square corrections from the back-reactions of the embedded D7 branes, we argue the possibility of the entropy to viscosity bound being violated. Finally, we show that our set-up could in-principle allow us to study a family of gauge theories at the boundary by cutting off the dual geometry respectively at various points in the radial direction. All these gauge theories can have well defined UV completions, and more interestingly, we demonstrate that any thermodynamical quantities derived from these theories would be completely independent of the cut-off scale and only depend on the temperature at which we define these theories. Such a result would justify the holographic renormalisabilities of these theories which we, in turn, also demonstrate. We give physical interpretations of these results and compare them with more realistic scenarios.