16412418
Expanding (3+1)-dimensional universe from a lorentzian matrix model for superstring theory in (9+1) dimensions.
We reconsider the matrix model formulation of type IIB superstring theory in (9+1)-dimensional space-time. Unlike the previous works in which the Wick rotation was used to make the model well defined, we regularize the Lorentzian model by introducing infrared cutoffs in both the spatial and temporal directions. Monte Carlo studies reveal that the two cutoffs can be removed in the large-N limit and that the theory thus obtained has no parameters other than one scale parameter. Moreover, we find that three out of nine spatial directions start to expand at some "critical time," after which the space has SO(3) symmetry instead of SO(9).
14066902
Classification of the BPS states in Bagger-Lambert Theory
We classify, in a group theoretical manner, the BPS configura tions in the multiple M2-brane theory recently proposed by Bagger and Lambert. We present three types of BPS equations preserving various fractions of supersymmetries: in the first type we have const ant fields and the interactions are purely algebraic in nature; in the second type the equations are inv ariant under spatial rotation SO(2), and the fields can be time-dependent; in the third class the equat ions are invariant under boost SO(1;1) and provide the eleven-dimensional generalizations of the Nahm equations. The BPS equations for different number of supersymmetries exhibit the division a lgebra structures: octonion, quarternion or complex.
118275742
Supersymmetry non-renormalization theorem from a computer and the AdS/CFT correspondence
We perform Monte Carlo calculation of correlation functions in 4d N=4 super Yang-Mills theory on R*S^3 in the planar limit. In order to circumvent the well-known problem of lattice SUSY, we adopt the idea of a novel large-N reduction, which reduces the calculation to that of corresponding correlation functions in the plane-wave matrix model or the BMN matrix model. This model is a 1d gauge theory with 16 supersymmetries, which can be simulated in a manner similar to the recent studies of the D0-brane system. We study two-point and three-point functions of chiral primary operators at various coupling constant, and find that they agree with the free theory results up to overall constant factors. The ratio of the overall factors for two-point and three-point functions agrees with the prediction of the AdS/CFT correspondence.
119232936
Existence of a critical point in the phase diagram of the ideal relativistic neutral Bose gas
We explore the phase transitions of the ideal relativistic neutral Bose gas confined in a cubic box, without assuming the thermodynamic limit nor continuous approximation. While the corresponding non-relativistic canonical partition function is essentially a one-variable function depending on a particular combination of temperature and volume, the relativistic canonical partition function is genuinely a two-variable function of them. Based on an exact expression for the canonical partition function, we performed numerical computations for up to 105 particles. We report that if the number of particles is equal to or greater than a critical value, which amounts to 7616, the ideal relativistic neutral Bose gas features a spinodal curve with a critical point. This enables us to depict the phase diagram of the ideal Bose gas. The consequent phase transition is first order below the critical pressure or second order at the critical pressure. The exponents corresponding to the singularities are 1/2 and 2/3, respectively. We also verify the recently observed 'Widom line' in the supercritical region.
44214844
Abelian decomposition of Einstein's theory: Reformulation of general relativity
We propose a reformulation of general relativity by making the Abelian decomposition of Einstein's theory. Based on the view that Einstein's theory can be interpreted as a gauge theory of Lorentz group, we decompose the Einstein's gravitational connection (the gauge potential of Lorentz group $\vGm_\mu$) into the restricted connection made of the potential of the maximal Abelian subgroup $H$ of Lorentz group $G$ and the valence connection made of $G/H$ part of the potential which transforms covariantly under Lorentz group. With this decomposition we show that the Einstein's theory can be decomposed into the restricted part made of the restricted connection which has the full Lorentz gauge invariance and the valence part made of the valence connection which plays the role of gravitational source of the restricted gravity. We show that there are two different Abelian decomposition of Einstein's theory, because Lorentz group has two maximal Abelian subgroups. In this decomposition the role of the space-time metric $g_\mn$ is replaced by a four-index metric tensor $\vg_\mn$ which transforms covariantly under the Lorentz group, and the metric-compatibility condition $\nabla_\alpha g_\mn=0$ of the connection is replaced by the gauge covariant condition $D_\mu \vg^\mn$ which tells that $\vg_\mn$ is invariant under the parallel transport along the $\pro_\mu$-direction defined by $\vGm_\mu$. We discuss the physical implications of the Abelian decomposition. In particular, we argue that the decomposition implies the existence of a restricted theory of gravitation which has the full general invariance but has less physical degrees of freedom.
119097342
Torsion as a dynamic degree of freedom of quantum gravity
The gauge approach to gravity based on the local Lorentz group with a general independent affine connection A?cd is developed. We consider SO(1, 3) gauge theory with a Lagrangian quadratic in curvature as a simple model of quantum gravity. The torsion is proposed to represent a dynamic degree of freedom of quantum gravity at scales above the Planckian energy. The Einstein?Hilbert theory is induced as an effective theory due to quantum corrections of torsion via generating a stable gravito-magnetic condensate. We conjecture that torsion possesses an intrinsic quantum nature and can be confined.
17464160
Dynamical Friction on Star Clusters near the Galactic Center
Numerical simulations of the dynamical friction suffered by a star cluster near the Galactic center have been performed with a parallelized tree code. Gerhard has suggested that dynamical friction, which causes a cluster to lose orbital energy and spiral in toward the Galactic center, may explain the presence of a cluster of very young stars in the central parsec, where star formation might be prohibitively difficult because of strong tidal forces. The clusters modeled in our simulations have an initial total mass of 105-106 M? and initial Galactocentric radii of 2.5-30 pc. We have identified a few simulations in which dynamical friction indeed brings a cluster to the central parsec, although this is only possible if the cluster either is very massive (~106 M?) or is formed near the central parsec (5 pc). In both cases, the cluster should have an initially very dense core (>106 M? pc-3). The initial core collapse and segregation of massive stars into the cluster core, which typically happens on a much shorter timescale than that characterizing the dynamical inspiral of the cluster toward the Galactic center, can provide the requisite high density. Furthermore, because it is the cluster core that is most likely to survive the cluster disintegration during its journey inward, this can help account for the observed distribution of presumably massive He I stars in the central parsec.
17518170
An Extended Star Formation History for the Galactic Center from Hubble Space Telescope NICMOS Observations
We present Hubble Space Telescope (HST) Near-Infrared Camera and Multiobject Spectrometer (NICMOS) observations as evidence that continuous star formation has created much of the central stellar cusp of the Galaxy. The data are the deepest ever obtained for a Galactic center population, being more than 50% complete for mF205W < 19.3, or initial stellar masses 2 M☉. We use Geneva and Padova stellar evolution models to produce synthetic luminosity functions for burst and continuous star formation scenarios, finding that the observations are fitted best by continuous star formation at a rate that is consistent with the recent star formation activity that produced the three massive young clusters in the central 50 pc. Further, it is not possible to fit the observations with ancient burst models, such as would be appropriate for an old population like that in Baade's window or NGC 6528.
118368147
Counterterms in gravity in the light-front formulation and a D = 2 conformal-like symmetry in gravity
A bstractIn this paper we discuss gravity in the light-front formulation (light-cone gauge) and show how possible counterterms arise. We find that Poincaré invariance is not enough to find the three-point counterterms uniquely. Higher-spin fields can intrude and mimic three-point higher derivative gravity terms. To select the correct term we have to use the remaining reparametrization invariance that exists after the gauge choice. We finally sketch how the corresponding programme for N = 8 Supergravity should work.
17082200
N-flation: Multifield inflationary dynamics and perturbations
We carry out numerical investigations of the dynamics and perturbations in the Nflation model of Dimopoulos et al. (2005). This model features large numbers of scalar fields with different masses, which can cooperate to drive inflation according to the assisted inflation mechanism. We extend previous work to include random initial conditions for the scalar fields, and explore the predictions for density perturbations and the tensor-to-scalar ratio. The tensor-to-scalar ratio depends only on the number of e-foldings and is independent of the number of fields, their masses, and their initial conditions. It therefore always has the same value as for a single massive field. By contrast, the scalar spectral index has significant dependence on model parameters. While normally multi-field inflation models make predictions for observable quantities which depend also on the unknown field initial conditions, we find evidence of a ‘thermodynamic’ regime whereby the predicted spectral index becomes independent of initial conditions if there are enough fields. Only in parts of parameter space where the mass spectrum of the fields is extremely densely packed is the model capable of satisfying the tight observational constraints from WMAP3 observations.
118561263
Extremal solutions of the S3 model and nilpotent orbits of G2(2)
We study extremal black hole solutions of the S3 model (obtained by setting S=T=U in the STU model) using group theoretical methods. Upon dimensional reduction over time, the S3 model exhibits the pseudo-Riemannian coset structure $ {{G} \left/ {{\tilde{K}}} \right.} $ with G = G2(2) and $ \tilde{K} = {\text{S}}{{\text{O}}_0}\left( {2,2} \right) $. We study nilpotent $ \tilde{K} $-orbits of G2(2) corresponding to non-rotating single-center extremal solutions. We find six such distinct $ \tilde{K} $-orbits. Three of these orbits are supersymmetric, one is non-supersymmetric, and two are unphysical. We write general solutions and discuss examples in all four physical orbits. We show that all solutions in supersymmetric orbits when uplifted to five-dimensional minimal supergravity have single-center Gibbons-Hawking space as their four-dimensional Euclidean hyper-Kähler base space. We construct hitherto unknown extremal (supersymmetric as well as non-supersymmetric) pressureless black strings of minimal five-dimensional supergravity and briefly discuss their relation to black rings.