We analyze the large frequency behavior of the spectral densities that govern the generalized Langevin diffusion process for a heavy quark in the context of the gauge/gravity duality. The bare Langevin correlators obtained from the trailing string solution have a singular short-distance behavior. We argue that the proper dressed spectral functions are obtained by subtracting the zero-temperature correlators. The dressed spectral functions have a sufficiently fast fall-off at large frequency so that the Langevin process is well defined and the dispersion relations are satisfied. We identify the cases in which the subtraction does not modify the associated low-frequency transport coefficients. These include conformal theories and the non-conformal, non-confining models. We provide several analytic and numerical examples in conformal and non-conformal holographic backgrounds.
We present a new, two-parameter family of string solutions corresponding to the holographic duals of specific 1/8-BPS Wilson loops on S^2 in N = 4 supersymmetric Yang-Mills theory. The solutions are obtained using the dressing method on the known longitude solution in the context of the auxiliary sigma-model on S^3 put forth in arXiv:0905.0665[hep-th]. We verify that the regularized area of the worldsheets are consistent with expectations.
We study the nuclear symmetry energy of dense matter using holographic QCD. To this end, we consider two flavor branes with equal quark masses in a D4/D6/D6 model. We find that at all densities the symmetry energy monotonically increases. At small densities, it exhibits a power law behavior with the density, $E_{\rm sym} \sim ρ^{1/2}$.
We study the structure of the higher-curvature gravitational densities that are induced from holographic renormalization in AdS$_{d+1}$. In a braneworld construction, such densities define a d-dimensional higher-curvature gravitational theory on the brane, which in turn is dual to a (d-1)-dimensional CFT living at its boundary. We show that this CFT$_{d-1}$ satisfies a holographic c-theorem in general dimensions (different than the g-theorem of holographic boundary CFTs), since at each and every order the higher-curvature densities satisfy c-theorems on their own. We find that, in these densit…
We show that in field theories with a holographic dual the retarded Green's function of a conserved current can be represented as a convergent sum over the quasinormal modes. We find that the zero-frequency conductivity is related to the sum over quasinormal modes and their high-frequency asymptotics via a sum rule. We derive the asymptotics of the quasinormal mode frequencies and their residues using the phase-integral (WKB) approach and provide analytic insight into the existing numerical observations concerning the asymptotic behavior of the spectral densities.
The holographic principle implies that quantum field theory overcounts the number of independent degrees of freedom in quantum gravity. An argument due to Cohen, Kaplan, and Nelson (CKN) suggests that the number of degrees of freedom well-described by QFT is even smaller than required by holographic bounds, and CKN interpreted this result as indicative of a correlation between the UV and IR cutoffs on QFT. Here we consider an alternative interpretation in which the QFT degrees of freedom are depleted as a function of scale. We use a simple recipe to estimate the impact of depleted densities of…
We study a bottom-up holographic model of large-Nc Yang-Mills theory, in which conformal invariance is broken through the introduction of a dilaton potential on the gravity side. We use the model to calculate the spectral densities of the shear and bulk channels at finite temperature. In the shear channel, we compare our results to those derived in strongly coupled N=4 Super Yang-Mills theory as well as in weakly coupled ordinary Yang-Mills. In the bulk channel, where the conformal result is trivial, we make comparisons with both perturbative and lattice QCD. In both channels, we pay particula…
We investigate the infrared behavior of the spectrum of scalar-dressed, asymptotically Anti de Sitter (AdS) black brane (BB) solutions of effective holographic models. These solutions describe scalar condensates in the dual field theories. We show that for zero charge density the ground state of these BBs must be degenerate with the AdS vacuum, must satisfy conformal boundary conditions for the scalar field and it is isolated from the continuous part of the spectrum. When a finite charge density is switched on, the ground state is not anymore isolated and the degeneracy is removed. Depending o…
We investigate the behavior of energy momentum tensor correlators in holographic $\mathcal{N}=4$ super Yang-Mills plasma, taking finite coupling corrections into account. In the thermal limit we determine the flow of quasinormal modes as a function of the 't Hooft coupling. Then we use a specific model of holographic thermalization to study the deviation of the spectral densities from their thermal limit in an out-of-equilibrium situation. The main focus lies on the thermalization pattern with which the plasma constituents approach their thermal distribution as the coupling constant decreases …
I review holographic models for (dense and cold) nuclear matter, neutron stars, and their mergers. I start by a brief general discussion on current knowledge of cold QCD matter and neutron stars, and go on discussing various approaches to model cold nuclear and quark matter by using gauge/gravity duality, pointing out their strengths and weaknesses. Then I focus on recent results for a complex bottom-up holographic framework (V-QCD), which also takes input from lattice QCD results, effective field theory, and perturbative QCD. Dense nuclear matter is modeled in V-QCD through a homogeneous non-…
In this paper the cosmological evolution of a holographic dark energy model with a non-linear interaction between the dark energy and dark matter components in a FRW type flat universe is analysed. In this context, the deceleration parameter $q$ and the equation state $w_Λ$ are obtained. We found that, as the square of the speed of sound remains positive, the model is stable under perturbations since early times; it also shows that the evolution of the matter and dark energy densities are of the same order for a long period of time, avoiding the so--called coincidence problem. We have also mad…
We use a top-down holographic model for strongly interacting quark matter to study the properties of neutron stars. When the corresponding Equation of State (EoS) is matched with state-of-the-art results for dense nuclear matter, we consistently observe a first order phase transition at densities between two and seven times the nuclear saturation density. Solving the Tolman-Oppenheimer-Volkov equations with the resulting hybrid EoSs, we find maximal stellar masses in the excess of two solar masses, albeit somewhat smaller than those obtained with simple extrapolations of the nuclear matter EoS…
We establish a holographic bottom-up model which covers both the baryonic and quark matter phases in cold and dense QCD. This is obtained by including the baryons using simple approximation schemes in the V-QCD model, which also includes the backreaction of the quark matter to the dynamics of pure Yang-Mills. We examine two approaches for homogeneous baryon matter: baryons as a thin layer of noninteracting matter in the holographic bulk, and baryons with a homogeneous bulk gauge field. We find that the second approach exhibits phenomenologically reasonable features. At zero temperature, the va…
Probe branes with finite worldvolume electric flux in the background created by a stack of Dp branes describe holographically strongly interacting fundamental matter at finite density. We identify two quantities whose leading low temperature behavior is independent of the dimensionality of the probe branes: specific heat and DC conductivity. This behavior can be inferred from the dynamics of the fundamental strings which provide a good description of the probe branes in the regime of low temperatures and finite densities. We also comment on the speed of sound on the branes and the temperature …
In this work I explore theoretical and phenomenological implications of chemical potentials and charge densities inside a strongly coupled thermal plasma, using the gauge/gravity correspondence. Strong coupling effects discovered in this model theory are interpreted geometrically and may be taken as qualitative predictions for heavy ion collisions at RHIC and LHC. In particular I examine the thermodynamics, spectral functions, transport coefficients and the phase diagram of the strongly coupled plasma. For example stable mesons, which are the analogs of the QCD Rho-mesons, are found to survive…
We study the Fermi level structure of (2+1)-dimensional strongly interacting electron systems in external magnetic field using the AdS/CFT correspondence. The gravity dual of a finite density fermion system is a Dirac field in the background of the dyonic AdS-Reissner-Nordstrom black hole. In the probe limit the magnetic system can be reduced to the non-magnetic one, with Landau-quantized momenta and rescaled thermodynamical variables. We find that at strong enough magnetic fields, the Fermi surface vanishes and the quasiparticle is lost either through a crossover to conformal regime or throug…
I argue that scattering theory for massless particles in Minkowski space should be reformulated as a mapping between past and future representations of an algebra of densities on the conformal boundary. These densities are best thought of as living on the momentum space light cone dual to null infinity, which describes the simultaneous eigenstates of the BMS generators. The currents describe the flow of other quantum numbers through the holographic screen at infinity. They are operator valued measures on the momentum light cone, with non-zero support at $P = 0$, which is necessary to describe …
In this note, we construct simple stochastic toy models for holographic gauge theories in which distributions of energy on a collection of sites evolve by a master equation with some specified transition rates. We build in only energy conservation, locality, and the standard thermodynamic requirement that all states with a given energy are equally likely in equilibrium. In these models, we investigate the qualitative behavior of the dynamics of the energy distributions for different choices of the density of states for the individual sites. For typical field theory densities of states (\log(ρ(…
We present a holographic model of QCD with a first order chiral restoration phase transition with chemical potential, mu. The first order behaviour follows from allowing a discontinuity in the dual description as the quarks are integrated out below their constituent mass. The model predicts a deconfined yet massive quark phase at intermediate densities (350 MeV< mu <500 MeV), above the nuclear density phase, which has a very stiff equation of state and a speed of sound close to one. We also include a holographic description of a colour superconducting condensate in the chirally restored vacuum…
In Hoyos et al. (arXiv:1603.02943) a holographic D3/D7 system was used to describe a deconfined yet massive quark phase of QCD at finite density, concluding that the equation of state of such a phase was not stiff enough to support exotic dense stars. That analysis used a hard quark mass to represent the dynamical mass and assumed a conformal gauge background. Here we phenomenologically adjust the D3/D7 system to include a running anomalous dimension for the quark condensate. This introduces a dynamical mechanism for chiral symmetry breaking yet the model still has a deconfined massive phase a…
We continue our recently proposed holographic description of single-particle correlation functions for four-dimensional chiral fermions with Lifshitz scaling at zero chemical potential, paying particular attention to the dynamical exponent z = 2. We present new results for the spectral densities and dispersion relations at non-zero momenta and temperature. In contrast to the relativistic case with z = 1, we find the existence of a quantum phase transition from a non-Fermi liquid into a Fermi liquid in which two Fermi surfaces spontaneously form, even at zero chemical potential. Our findings sh…
We demonstrate that a transition from decelerated to accelerated cosmic expansion arises as a pure interaction phenomenon if pressureless dark matter is coupled to holographic dark energy whose infrared cutoff scale is set by the Hubble length. In a spatially flat universe the ratio of the energy densities of both components remains constant through this transition, while it is subject to slow variations for non-zero spatial curvature. The coincidence problem is dynamized and reformulated in terms of the interaction rate. An early matter era is recovered since for negligible interaction at hig…
The Langevin diffusion process of a relativistic heavy quark in a non-conformal holographic setup is discussed. The bulk geometry is a general, five-dimensional asymptotically AdS black hole. The heavy quark is described by a trailing string attached to a flavor brane, moving at constant velocity. From the equations describing linearized fluctuations of the string world-sheet, the correlation functions defining a generalized Langevin process are constructed via the AdS/CFT prescription. In the local limit, analytic expressions for the Langevin diffusion and friction coefficients are obtained i…
The behavior of holographic CFTs is constrained by the existence of a bulk dual geometry. For example, in (2+1)-dimensional holographic CFTs living on a static spacetime with compact spatial slices, the vacuum energy must be nonpositive, certain averaged energy densities must be nonpositive, and the spectrum of scalar operators is bounded from below by the Ricci scalar of the CFT geometry. Are these results special to holographic CFTs? Here we show that for perturbations about appropriate backgrounds, they are in fact universal to all CFTs, as they follow from the universal behavior of two- an…
We argue that given holographic CFT$_1$ in some state with a dual spacetime geometry M, and given some other holographic CFT$_2$, we can find states of CFT$_2$ whose dual geometries closely approximate arbitrarily large causal patches of M, provided that CFT$_1$ and CFT$_2$ can be non-trivially coupled at an interface. Our CFT$_2$ states are "dressed up as" states of CFT$_1$: they are obtained from the original CFT$_1$ state by a regularized quench operator defined using a Euclidean path-integral with an interface CFT$_1$ CFT$_2$ and CFT$_1$. Our results are consistent with the idea that the p…
In this note we consider a relativistic heavy quark which moves in the quark-gluon plasmas. By using the holographic methods, we analyze the Langevin diffusion process of this relativistic heavy quark. This heavy quark is described by a trailing string attached to a flavor brane and moving at constant velocity. The fluctuations of this string are related to the thermal correlators and the correlation functions are precisely the kinds of objects that we compute in the gravity dual picture. We obtain the action of the trailing string in hyperscaling violation backgrounds and we then find the equ…
We show that the Mott transition can be realized in a holographic model of a fermion with bulk mass, $m$, and a dipole interaction of coupling strength $p$. The phase diagram contains gapless, pseudo-gap and gapped phases and the first one can be further divided into four sub-classes. We compare the spectral densities of our holographic model with the Dynamical Mean Field Theory (DMFT) results for Hubbard model as well as the experimental data of Vanadium Oxide materials. Interestingly, single-site and cluster DMFT results of Hubbard model share some similarities with the holographic model of …
We examine the behavior of entanglement entropy of a subsystem $A$ in a fully backreacted holographic model of a $1+1$ dimensional $p$ wave superconductor across the phase transition. For a given temperature, the system goes to a superconducting phase beyond a critical value of the charge density. The entanglement entropy, considered as a function of the charge density at a given temperature, has a cusp at the critical point. In addition, we find that there are three different behaviors in the condensed phase, depending on the subsystem size. For a subsystem size $l$ smaller than a critical si…
We expand the holographic studies of momentum relaxation to include non-relativistic scaling symmetries in the ultraviolet. We do so by constructing black branes with Lifshitz asymptotics dressed with axions which explicitly depend on the boundary directions. Such configurations arise as analytic solutions of the Einstein-Proca theory coupled to massless scalar fields in arbitrary dimensions. Studying linear perturbations on these backgrounds, we conclude that there is a dual Ward identity which accounts for the dissipation of momentum in the system. In addition, we numerically compute the fre…
In phases where translations are spontaneously broken, new gapless degrees of freedom appear in the low energy spectrum (the phonons). At long wavelengths, they couple to small fluctuations of the conserved densities of the system. This mixing is captured by new diffusive transport coefficients, as well as qualitatively different collective modes, such as shear sound modes. We use Gauge/Gravity duality to model such phases and analytically compute the corresponding diffusivities in terms of data {of the dual background black hole solution}. In holographic quantum critical low temperature phase…
The modified holographic Ricci dark energy coupled to interacting relativistic and non-relativistic dark matter is considered in the nonflat Friedmann-Robertson-Walker universe. Through examining the deceleration parameter, one can find that the transition time of the Universe from decelerating to accelerating phase in the interacting holographic Ricci dark energy model is close to that in the $Λ$ cold dark matter model. The evolution of modified holographic Ricci dark energy's state parameter and the evolution of dark matter and dark energy's densities shows that the dark energy holds the dom…
We extend the holographic trailing string picture of a heavy quark to the case of a bulk geometry dual to a confining gauge theory. We compute the classical trailing confining string solution for a static as well as a uniformly moving quark. The trailing string is infinitely extended and approaches a confining horizon, situated at a critical value of the radial coordinate, along one of the space-time directions, breaking boundary rotational invariance. We compute the equations for the fluctuations around the classical solutions, which are used to obtain boundary force correlators controlling t…
We study the magnetic properties of the coloured multiquark states in the quark-gluon plasma where the gluons are deconfined and the chiral symmetry is still broken, using the Sakai-Sugimoto model. There are two possible magnetized multiquark configurations. Both configurations converge to the same configuration at the critical field and temperature before they dissociate altogether either into less coloured multiquarks or into other phases for a fixed density. It is also found that the multiquarks with higher colour charges respond more to the external magnetic field in both the magnetization…
We propose a holographic dual for (pseudo-)conformal cosmological scenario, with a scalar field that forms a moving domain wall in adS_5. The domain wall separates two vacua with unequal energy densities. Unlike in the existing construction, the 5d solution is regular in the relevant space-time domain.
We study the low frequency spectra of complex Sachdev-Ye-Kitaev (SYK) models at general densities. The analysis applies also to SU($M$) magnets with random exchange at large $M$. The spectral densities are computed by numerical analysis of the saddle point equations on the real frequency ($ω$) axis at zero temperature ($T$). The asymptotic low $ω$ behaviors are found to be in excellent agreement with the scaling dimensions of irrelevant operators which perturb the conformally invariant critical states. Of possible experimental interest is our computation of the universal spin spectral weight o…
