Quantum computational complexity, Einstein's equations and accelerated expansion of the Universe

We study the relation between quantum computational complexity and general relativity. The quantum computational complexity is proposed to be quantified by the shortest length of geodesic quantum curves. We examine the complexity/volume duality in a geodesic causal ball in the framework of Fermi normal coordinates and derive the full non-linear Einstein equation. Using insights from the complexity/action duality, we argue that the accelerated expansion of the universe could be driven by the quantum complexity and free from coincidence and fine-tunning problems.Comment: 1+20 pages, 2 figures, references adde

Shear Viscosity to Entropy Density Ratio in Higher Derivative Gravity with Momentum Dissipation

We investigate $\eta/s$ in linear scalar fields modified Gauss-Bonnet theory that breaks translation invariance. We first calculate $\eta/s$ both analytically and numerically and show its relationship with temperature in log-log plot. Our results show that $\eta/s\sim T^2$ at low temperatures. The causality is also considered in this work. We then find that causality violation still happens in the presence of the linear scalar field and we suggest there is a Gauss-Bonnet coupling dependent lower limit for the effective mass of the graviton. If the effective mass of the graviton is big enough, then there will be no causality violation and hence no constraints for the Gauss-Bonnet coupling.Comment: 21 pages, 5 figures, revised version, references added, to appear in PR

Polarization of Quasars: Electron Scattering in the Broad Absorption Line Region

It is widely accepted that the broad absorption line region (BALR) exists in most (if not all) quasars with a small covering factor. Recent works showed that the BALR is optically thick to soft and even medium energy X-rays, with a typical hydrogen column density of a few 10$^{23}$ to $>$ 10$^{24}$ cm$^{-2}$. The electron scattering in the thick absorber might contribute significantly to the observed continuum polarization for both BAL QSOs and non-BAL QSOs. In this paper, we present a detailed study of the electron scattering in the BALR by assuming an equatorial and axisymmetric outflow model. Monte-Carlo simulations are performed to correct the effect of the radiative transfer. Assuming an average covering factor of 0.2 of the BALR, which is consistent with observations, we find the electron scattering in the BALR with a column density of $\sim$ 4 $\times$ 10$^{23}$ cm$^{-2}$ can successfully produce the observed average continuum polarization for both BAL QSOs and non-BAL QSOs. The observed distribution of the continuum polarization of radio quiet quasars (for both BAL QSOs and non-BAL QSOs) is used to constrain the dispersal distribution of the BALR. We find that, to match the observations, the maximum continuum polarization produced by the BALR (while viewed edge-on) peaks at $P$ = 0.34%, which is much smaller than the average continuum polarization of BAL QSOs ($P$ = 0.93%). The discrepancy can be explained by a selection bias that the BAL with larger covering factor, and thus producing larger continuum polarization, is more likely to be detected. A larger sample of radio quiet quasars with accurate measurement of the continuum polarization will give better constraints to the distribution of the BALR properties.Comment: 17 pages,9 figures, accepted by AP

Polarization of Quasars: Rotated and Funnel-shaped Outflow

Polarization is a useful probe to investigate the geometries and dynamics of outflow in BAL QSOs. We perform a Monte-Carlo simulation to calculate the polarization produced by resonant and electron scattering in BALR. We find: 1)A rotated and funnel-shaped outflow is preferred to explain many observed polarization features. 2)The resonant scattering can contribute a significant part of NV emission line in some QSOs.Comment: 2 pages,2 figures;Conference proceedings to appear in "The Central Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP

Sound modes and stability of momentum dissipated black branes in holography

We systematically investigate the sound modes of momentum dissipated holographic systems. In particular, we focus on the Einstein-linear axions and the Einstein-Maxwell-dilaton-axion theories in four-dimensional bulk spacetime dimensions. The sound velocities of the two theories are computed respectively and the sound attenuation of the Einstein-Maxwell-axion theory is also calculated analytically. We also obtain numeral dispersion relations in the two theories which match with our analytical results. Our results show that the sound velocity of the Einstein-Maxwell-dilaton theory with additional linear axion fields is equivalent to that of 2 + 1 - dimensional Banados-Teitelboim-Zanelli black holes. It allowed us to compare our solution of the Einstein-linear axions theory with that of systems without translational invariance from another method. After the computation on the sound velocity, we calculate the quasinormal modes of scalar-type fluctuations in the Einstein-Maxwell-dilaton-axion theory. The results show that a dynamical instability is observed under the condition that the null energy condition is violated.Comment: 22 pages, 6 figure

Observing the inhomogeneity in the holographic models of superconductors

We study the gravity duals of striped holographic superconductors in the AdS black hole and AdS soliton backgrounds. We show the dependences of the condensation and the critical temperature/critical chemical potential on the inhomogeneity in these two different spacetimes. By exploring the dynamics of the normal phase through the scalar field perturbation, we argue that the pair susceptibility and the conductivity can be possible phenomenological indications to disclose the property of inhomogeneity.Comment: 16 page

d-Wave holographic superconductors with backreaction in external magnetic fields

We study the d-wave holographic superconductors (the d-wave model proposed in [arXiv:1003.2991[hep-th]]) immersed in constant external magnetic fields by using the analytic matching method and numerical computation. In the probe limit, we calculate the spatially dependent condensate solution in the presence of the magnetism and find that the expression for the upper critical magnetic field satisfies the relation given in the Ginzburg-Landau theory. The result shows that the upper critical field gradually increases to its maximum value $B_{c2}$ at absolute zero temperature T=0, while vanishing at the critical temperature $T=T_c$. Moving away from the probe limit, we investigate the effect of spacetime backreaction on the critical temperature and the upper critical magnetic field. The magnetic fields as well as the electric fields acting as gravitational sources reduce the critical temperature of the superconductor and actually result in a dyonic black hole solution to the leading order. We obtain the expression for the upper critical magnetic field up to $\mathcal{O}(\kappa^2)$ order. The analytic result is consistent with the numerical findings.Comment: 23 pages, 7 figures. typos corrected. to appear in JHEP. arXiv admin note: text overlap with arXiv:1105.433

Recurrent outbursts revealed in 3XMM J031820.8-663034

3XMM J031820.8-663034, first detected by ROSAT in NGC 1313, is one of a few known transient ultraluminous X-ray sources (ULXs). In this paper, we present decades of X-ray data of this source from ROSAT, XMM-Newton, Chandra and the Neil Gehrels Swift Observatory. We find that its X-ray emission experienced four outbursts since 1992, with a typical recurrent time $\sim$ 1800 days, an outburst duration $\sim 240-300$ days, and a nearly constant peak X-ray luminosity $\sim 1.5 \times 10^{39}$ erg/s. The upper limit of X-ray luminosity at the quiescent state is $\sim 5.6 \times 10^{36}$ erg/s, and the total energy radiated during one outburst is $\sim 10^{46}$ erg. The spectra at the high luminosity states can be described with an absorbed disk black-body, and the disk temperature increases with the X-ray luminosity. We compare its outburst properties with other known transient ULXs including ESO 243-49 HLX-1. As its peak luminosity only marginally puts it in the category of ULXs, we also compare it with normal transient black hole binaries. Our results suggest that the source is powered by an accreting massive stellar-mass black hole, and the outbursts are triggered by the thermal-viscous instability.Comment: 9 pages, 7 figurs, 3 tables, accepted for publication in Ap

Interpretation of the newly observed Σb(6097)±\Sigma_b(6097)^{\pm} and Ξb(6227)−\Xi_b(6227)^- states as the PP-wave bottom baryons

The strong decays of the $P$-wave $\Sigma_b$, $\Xi_b'$ and $\Omega_b$ baryons are investigated with a constituent quark model in the $j$-$j$ coupling scheme. The results show that the newly observed $\Sigma_b(6097)$ and $\Xi_b(6227)$ states by the LHCb collaboration can be assigned as the $\lambda$-mode $P$-wave singly bottom baryons. Given the heavy quark symmetry, both the $\Sigma_b(6097)$ and $\Xi_b(6227)$ states favor the light spin $j=2$ states with spin-parity numbers $J^P=3/2^-$ or $J^P=5/2^-$. More $P$-wave singly bottom baryons are most likely to be observed in future experiments for their relatively narrow width.Comment: 7 pages. arXiv admin note: text overlap with arXiv:1709.0426

Violation of the viscosity/entropy bound in translationally invariant non-Fermi liquids

The shear viscosity is an important characterization of how a many-body system behaves like a fluid. We study the shear viscosity in a strongly interacting solvable model, consisting of coupled Sachdev-Ye-Kitaev (SYK) islands. As temperature is lowered, the model exhibits a crossover from an incoherent metal with local criticality to a marginal fermi liquid. We find that while the ratio of shear viscosity to entropy density in the marginal Fermi liquid regime satisfies a Kovtun-Son-Starinets (KSS) like bound, it can strongly violate the KSS bound in a robust temperature range of the incoherent metal regime, implying a nearly perfect fluidity of the coupled local critical SYK model. Furthermore, this model also provides the first translationally invariant example violating the KSS bound with known gauge-gravity correspondence.Comment: 10 pages, 2 figures; more details added as appendix; and minor correction