Schwarzschild Geometry Emerging from Matrix Models

We demonstrate how various geometries can emerge from Yang-Mills type matrix models with branes, and consider the examples of Schwarzschild and Reissner-Nordstroem geometry. We provide an explicit embedding of these branes in R^{2,5} and R^{4,6}, as well as an appropriate Poisson resp. symplectic structure which determines the non-commutativity of space-time. The embedding is asymptotically flat with asymptotically constant \theta^{\mu\nu} for large r, and therefore suitable for a generalization to many-body configurations. This is an illustration of our previous work arXiv:1003.4132, where we have shown how the Einstein-Hilbert action can be realized within such matrix models.Comment: 21 pages, 1 figur

Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?

The recently discovered coreshine effect can aid in exploring the core properties and in probing the large grain population of the ISM. We discuss the implications of the coreshine detected from the molecular cloud core L1506C in the Taurus filament for the history of the core and the existence of a primitive ISM component of large grains becoming visible in cores. The coreshine surface brightness of L1506C is determined from IRAC Spitzer images at 3.6 micron. We perform grain growth calculations to estimate the grain size distribution in model cores similar in gas density, radius, and turbulent velocity to L1506C. Scattered light intensities at 3.6 micron are calculated for a variety of MRN and grain growth distributions to compare with the observed coreshine. For a core with the overall physical properties of L1506C, no detectable coreshine is predicted for an MRN size distribution. Extending the distribution to grain radii of about 0.65 $\mu$m allows to reproduce the observed surface brightness level in scattered light. Assuming the properties of L1506C to be preserved, models for the growth of grains in cores do not yield sufficient scattered light to account for the coreshine within the lifetime of the Taurus complex. Only increasing the core density and the turbulence amplifies the scattered light intensity to a level consistent with the observed coreshine brightness. The grains could be part of primitive omni-present large grain population becoming visible in the densest part of the ISM, could grow under the turbulent dense conditions of former cores, or in L1506C itself. In the later case, L1506C must have passed through a period of larger density and stronger turbulence. This would be consistent with the surprisingly strong depletion usually attributed to high column densities, and with the large-scale outward motion of the core envelope observed today.Comment: 6 pages, 6 figures, accepted for publication in Astronomy & Astrophysic

Fermions on spontaneously generated spherical extra dimensions

We include fermions to the model proposed in hep-th/0606021, and obtain a renormalizable 4-dimensional SU(N) gauge theory which spontaneously generates fuzzy extra dimensions and behaves like Yang-Mills theory on M^4 \times S^2. We find a truncated tower of fermionic Kaluza-Klein states transforming under the low-energy gauge group, which is found to be either SU(n), or SU(n_1) x SU(n_2) x U(1). The latter case implies a nontrivial U(1) flux on S^2, leading to would-be zero modes for the bifundamental fermions. In the non-chiral case they may pair up to acquire a mass, and the emerging picture is that of mirror fermions. We discuss the possible implementation of a chirality constraint in 6 dimensions, which is nontrivial at the quantum level due to the fuzzy nature of the extra dimensions.Comment: 34 pages. V2: references added, minor corrections V3: discussion added, final versio

Fuzzy Extra Dimensions: Dimensional Reduction, Dynamical Generation and Renormalizability

We examine gauge theories defined in higher dimensions where theextra dimensions form a fuzzy (finite matrix) manifold. First we reinterpret these gauge theories as four-dimensional theories with Kaluza-Klein modes and then we perform a generalized \`a la Forgacs-Manton dimensional reduction. We emphasize some striking features emerging in the later case such as (i) the appearance of non-abelian gauge theories in four dimensions starting from an abelian gauge theory in higher dimensions, (ii) the fact that the spontaneous symmetry breaking of the theory takes place entirely in the extra dimensions and (iii) the renormalizability of the theory both in higher as well as in four dimensions. Then reversing the above approach we present a renormalizable four dimensional SU(N) gauge theory with a suitable multiplet of scalar fields, which via spontaneous symmetry breaking dynamically develops extra dimensions in the form of a fuzzy sphere. We explicitly find the tower of massive Kaluza-Klein modes consistent with an interpretation as gauge theory on $M^4 \times S^2$, the scalars being interpreted as gauge fields on $S^2$. Depending on the parameters of the model the low-energy gauge group can be of the form $SU(n_1) \times SU(n_2) \times U(1)$.Comment: 18 pages, Based on invited talks presented at various conferences, Minor corrections, Acknowledgements adde

Direct evidence of dust growth in L183 from MIR light scattering

Theoretical arguments suggest that dust grains should grow in the dense cold parts of molecular clouds. Evidence of larger grains has so far been gathered in near/mid infrared extinction and millimeter observations. Interpreting the data is, however, aggravated by the complex interplay of density and dust properties (as well as temperature for thermal emission). We present new Spitzer data of L183 in bands that are sensitive and insensitive to PAHs. The visual extinction AV map derived in a former paper was fitted by a series of 3D Gaussian distributions. For different dust models, we calculate the scattered MIR radiation images of structures that agree agree with the AV map and compare them to the Spitzer data. The Spitzer data of L183 show emission in the 3.6 and 4.5 micron bands, while the 5.8 micron band shows slight absorption. The emission layer of stochastically heated particles should coincide with the layer of strongest scattering of optical interstellar radiation, which is seen as an outer surface on I band images different from the emission region seen in the Spitzer images. Moreover, PAH emission is expected to strongly increase from 4.5 to 5.8 micron, which is not seen. Hence, we interpret this emission to be MIR cloudshine. Scattered light modeling when assuming interstellar medium dust grains without growth does not reproduce flux measurable by Spitzer. In contrast, models with grains growing with density yield images with a flux and pattern comparable to the Spitzer images in the bands 3.6, 4.5, and 8.0 micron.Comment: 13 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

Emergent Geometry and Gravity from Matrix Models: an Introduction

A introductory review to emergent noncommutative gravity within Yang-Mills Matrix models is presented. Space-time is described as a noncommutative brane solution of the matrix model, i.e. as submanifold of \R^D. Fields and matter on the brane arise as fluctuations of the bosonic resp. fermionic matrices around such a background, and couple to an effective metric interpreted in terms of gravity. Suitable tools are provided for the description of the effective geometry in the semi-classical limit. The relation to noncommutative gauge theory and the role of UV/IR mixing is explained. Several types of geometries are identified, in particular "harmonic" and "Einstein" type of solutions. The physics of the harmonic branch is discussed in some detail, emphasizing the non-standard role of vacuum energy. This may provide new approach to some of the big puzzles in this context. The IKKT model with D=10 and close relatives are singled out as promising candidates for a quantum theory of fundamental interactions including gravity.Comment: Invited topical review for Classical and Quantum Gravity. 57 pages, 5 figures. V2,V3: minor corrections and improvements. V4,V5: some improvements, refs adde

Bubbles in galactic haloes

We briefly discuss a possible interconnection of vertical HI structures observed in the Milky Way Galaxy with large scale blow-outs caused by the explosions of multiple clustered SNe. We argue that the observed OB associations can produce only about 60 such events, or approximately one chimney per 3 kpc$^2$ within the solar circle. We also discuss the overall properties of HI shells in nearby face-on galaxies and the distribution of H$\alpha$ and dust in edge-on galaxies. We argue that the presence of dust in galactic haloes may indicate that radiation pressure is the most probable mechanism capable of transporting dust to large heights above the galactic plane. In order to make this possible, the galactic magnetic field must have a strong vertical component. We mention that SNe explosions can initiate the Parker instability which in turn creates large scale magnetic loops with a strong vertical component. Recent observations of nearby edge-on galaxies favour this suggestion.Comment: 11 pages, 4 Figs, Talk at the JENAM, May 29 -- June 3, 2000, Mosco