Abstract
We numerically simulate gravitational shock wave collisions in a holographic model dual to a non-conformal four-dimensional gauge theory. We find two novel effects associated to the non-zero bulk viscosity of the resulting plasma. First, the hydrodynamization time increases. Second, if the bulk viscosity is large enough then the plasma becomes well described by hydrodynamics before the energy density and the average pressure begin to obey the equilibrium equation of state. We discuss implications for the quark-gluon plasma created in heavy ion collision experiments.
Highlights
JHEP01(2017)026 by the desire to mimic detailed properties of Quantum Chromodynamics (QCD) but by simplicity: the UV fixed point guarantees that holography is on its firmest footing, since the bulk metric is asymptotically AdS; the IR fixed point guarantees that the solutions are regular in the interior; and turning on a source for a relevant operator is the simplest way to break conformal invariance
Eqs. (3.6) imply that the hydrodynamic viscous correction to the equilibrium pressure is controlled by the bulk viscosity alone, since
This is illustrated in figure 3 by the fact that hydrodynamics provides an excellent prediction for Pat thyd, whereas at this time Pand Peq still differ by 18%
Summary
The decrease of the number of degrees of freedom along the flow is reflected in the fact that LIR < L. Out of equilibrium the average pressure is not determined by the energy density because the scalar expectation value V fluctuates independently. The equilibrium pressure for this case is shown in figure 1. As expected, both at high and low energies the physics becomes approximately conformal and Peq asymptotes to E/3. The bulk viscosity-to-entropy ratio as a function of temperature is shown in figure 2(top) In this case approximate conformal invariance implies that ζ/s → 0 at high and low temperatures. Ζ/s attains a maximum at T = 0.22Λ, reflecting the fact that the theory is maximally non-conformal around the scale set by the source
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