Sang-Jin Sin

Late time cosmological phase transition and galactic halo as Bose liquid

We consider the ultra light pseudo Nambu-Goldstone boson appearing in the late time cosmological phase transition theories as a dark matter candidate. Since it is almost massless, its nature is more wave like than particle like. Hence we apply quantum mechanics to study how they form the galactic halos. Three predictions are made; (1)the mass profile

A Gravity Dual of RHIC Collisions

\rho\sim r^{-1.6}

A holographic dual of hydrodynamics

, (2)there are ripple-like fine structures in rotation curve, (3) the rotation velocity times ripples wave length is largely galaxy independent. We compare the rotation curves predicted by our theory with the data observed.

Shear viscosity, instability and the upper bound of the Gauss-Bonnet coupling constant

In the context of the AdS/CFT correspondence we discuss the gravity dual of a heavyion-like collision in a variant of N = 4 SYM. We provide a gravity dual picture of the entire process using a model where the scattering process creates initially a holographic shower in bulk AdS. The subsequent gravitational fall leads to a moving black hole that is gravity dual to the expanding and cooling heavy-ion fireball. The front of the fireball cools at the rate of 1/�, while

Viscosity bound, causality violation and instability with stringy correction and charge

We consider a gravity dual description of time dependent, strongly interacting large-Nc = 4 SYM. We regard the gauge theory system as a fluid with shear viscosity. Our fluid is expanding in one direction following the Bjorkens picture that is relevant to RHIC experiments. We obtain the dual geometry at the late time that is consistent with dissipative hydrodynamics. We show that the integration constants that cannot be determined by hydrodynamics are given by looking at the horizon of the dual geometry. Relationship between time dependence of the energy density and bulk singularity is also discussed.

A New Phase at a Finite Quark Density from AdS/CFT

We compute the dimensionality dependence of �/s for charged black branes with Gauss-Bonnet correction. We find that both causality and stability constrain the value of Gauss-Bonnet coupling constant to be bounded by 1/4 in the infinite dimensionality limit. We further show that higher dimensionality stabilize the gravitational perturbation. The stabilization of the perturbation in higher dimensional space-time is a straightforward consequence of the Gauss-Bonnet coupling constant bound.

Holography of radiation and jet quenching

Recently, it has been shown that if we consider the higher derivative correction, the viscosity bound conjectured to be ?/s = 1/4? is violated and so is the causality. In this paper, we consider medium effect and the higher derivative correction simultaneously by adding charge and Gauss-Bonnet terms. We find that the viscosity bound violation is not changed by the charge. However, we find that two effects together create another instability for large momentum regime. We argue the presence of tachyonic modes and show it numerically. We draw a phase diagram relevant to the instability in charge-coupling space.

Baryon-charge Chemical Potential in AdS/CFT

We explore phases of N=2 super Yang-Mills theory at finite quark density by introducing quark chemical potential in a D3-D7 setup. We formulate the thermodynamics of brane embeddings and find that we need to renormalize the finite chemical potential due to the divergence of the thermodynamic potentials and we find that the density versus chemical potential equation of state has rich structure. This yields two distinct first order phase transitions in a small window of quark density. In order words, there is a new first order phase transition in the region of deconfined quarks. In this new phase, the chemical potential is a decreasing function of the density. We suggest that this might be relevant to the difference in sQGP--wQGP phases of QCD.

Coherent/incoherent metal transition in a holographic model

Abstract We study the non-linear propagation of radiation in N = 4 SYM at zero and finite temperature using the refined radius/scale duality in AdS/CFT. We argue that a pulse radiation by a quark at the boundary should be described holographically by a “point like object” passing through the center of the AdS bulk. We find that at finite temperature, the radiation stalls at a distance of 1 / π T with a natural geometric and holographic interpretation. Indeed, the stalling is the holographic analogue of the gravitational in-fall of light towards the black hole in the bulk. We suggest that these results are relevant for jet quenching by a strongly coupled quark–gluon liquid as currently probed in heavy ion colliders at RHIC. In particular, colored jets cannot make it beyond 1 / 3 fm at RHIC whatever their energy.

Shear viscosity and instability from third order Lovelock gravity

We present a closed framework of AdS/CFT with finite U(1)B-charge chemical potential. We show how the gauge-invariant identification of the chemical potential with the bulk gauge field emerges from the standard AdS/CFT dictionary. Physical importance and necessity of the Minkowski embeddings within the present framework is also shown numerically in the D3-D7 systems. We point out that the D3-D7 model with only the black-hole embeddings does not have the low-temperature and lowchemical-potential region in the grand-canonical ensemble, hence it is incomplete. A physical interpretation that explains these numerical results is also proposed.