Horizons Inside Classical Lumps

Hopefully tex-able version.Comment: 13 pages, UMHEP-37

Chaplygin-like gas and branes in black hole bulks

We explore the possibility to locate a brane in black hole bulks. We study explicitly the cases of BHTZ and Schwarzschild-anti de Sitter (AdS) black holes. Our result is that in these cases branes cannot be supported by brane tension alone and it is necessary to introduce other forms of matter on the brane. We find classes of perfect fluid solutions obeying to peculiar state equations. For the case of BHTZ bulk geometry the state equation takes exactly the form of a ``Chaplygin gas'', which is relevant in the brane context. In the Schwarzschild-AdS case we find new state equations which reduce to the Chaplygin form when the brane is located near the horizon.Comment: 11 pages, 1 figur

Variable Modified Chaplygin Gas and Accelerating Universe

In this letter, I have proposed a model of variable modified Chaplygin gas and shown its role in accelerating phase of the universe. I have shown that the equation of state of this model is valid from the radiation era to quiessence model. The graphical representations of statefinder parameters characterize different phase of evolution of the universe. All results presented in the letter concerns the case $k=0$.Comment: 7 Latex pages, 5 figures, revtex styl

The 51^{51}Cr neutrino source and Borexino: a desirable marriage

Exposure to a $^{51}$Cr neutrino source as that used in Gallex will provide an excellent overall performance test of Borexino, which should collect about 1400 source induced events, with an initial rate of about 35 counts per day. This will be particularly important if MSW-small-angle turns out to be the solution of the solar neutrino problem. In addition, if an independent, accurate calibration is available, one will have an interesting experiment on neutrino properties: as an example, a neutrino magnetic moment of the order $5\cdot10^{-11}\mu_B$could be detected/excluded at the 90\% C.L.Comment: 7 pages, RevTeX, plus 3 postscripts figures, tarred, compresse

Some Simple Mixing and Mass Matrices for Neutrinos

We argue that the accumulated neutrino data, including recent results from KamLAND and K2K, point to a neutrino mixing matrix with (V_{11}, V_{21}, V_{31}; V_{21}, V_{22}, V_{32}; V_{13}, V_{23}, V_{33}) = (-2/\sqrt{6}, 1/\sqrt{6}, 1/\sqrt{6}; 1/\sqrt{3}, 1/\sqrt{3}, 1/\sqrt{3}; 0, 1/\sqrt{2}, -1/\sqrt{2}). We propose some simple neutrino mass matrices which predict such a mixing matrix.Comment: RevTex 9 pages, no figures. Some new references adde

Dynamical Cosmological Constant from A Very Recent Phase Transition

Observation indicates that the expansion of the Universe is accelerating and favours a dynamical cosmological constant, \Lambda(t). We consider the possibility that this is due to a scalar field which has undergone a very recent phase transition. We study a simple class of model, corresponding to a \phi^4 potential with a time-dependent mass squared term. For the models considered the phase transition occurs at a red shift z \leq 1.2. The evolution of the equation of state \omega_{\phi} and energy density \rho_{\phi} with time is distinct from existing dynamical \Lambda models based on slowly rolling fields, with \omega_{\phi} and \rho_{\phi} rapidly changing in a characteristic way following the transition. The \phi energy density is composed of a time-dependent vacuum energy and coherently oscillating condensate component with a negative pressure. The condensate component will collapse to form non-topological soliton lumps, '\phi-axitons', which smoothly populate the Universe.Comment: 13 pages LaTeX, 10 figures, final versio

Structure and Nuclear Composition of General Relativistic, Magnetohydrodynamic Outflows from Neutrino-Cooled Disks

We compute the structure and degree of neutronization of general relativistic magnetohydrodynamic (GRMHD) outflows originating from the inner region of neutrino-cooled disks. We consider both, outflows expelled from a hydrostatic disk corona and outflows driven by disk turbulence. We show that in outflows driven thermally from a static disk the electron fraction quickly evolves to its equilibrium value which is dominated by neutrino capture. Those outflows are generally proton rich and, under certain conditions, can be magnetically dominated. They may also provide sites for effective production of 56Ni. Centrifugally driven outflows and outflows driven by disk turbulence, on the other hand, can preserve the large in-disk neutron excess. Those outflows are, quite generally, subrelativistic by virtue of the large mass flux driven by the additional forces.Comment: 16 pages, 3 figures, submitted to New Astronom

First-order framework and generalized global defect solutions

This work deals with defect structures in models described by scalar fields. The investigations focus on generalized models, with the kinetic term modified to allow for a diversity of possibilities. We develop a new framework, in which we search for first-order differential equations which solve the equations of motion. The main issue concerns the introduction of a new function, which works like the superpotential usually considered in the standard situation. We investigate the problem in the general case, with an arbitrary number of fields, and we present several explicit examples in the case of a single real scalar field.Comment: 8 pages, 6 figures; version to appear in PL

Can the Chaplygin gas be a plausible model for dark energy?

In this note two cosmological models representing the flat Friedmann Universe filled with a Chaplygin fluid, with or without dust, are analyzed in terms of the recently proposed "statefinder" parameters. Trajectories of both models in the parameter plane are shown to be significantly different w.r.t. "quiessence" and "tracker" models. The generalized Chaplygin gas model with an equation of state of the form $p = -A/\rho^{\alpha}$ is also analyzed in terms of the statefinder parameters.Comment: 6 pages, 2 figure

Scale-invariance of galaxy clustering

Some years ago we proposed a new approach to the analysis of galaxy and cluster correlations based on the concepts and methods of modern statistical Physics. This led to the surprising result that galaxy correlations are fractal and not homogeneous up to the limits of the available catalogs. The usual statistical methods, which are based on the assumption of homogeneity, are therefore inconsistent for all the length scales probed so far, and a new, more general, conceptual framework is necessary to identifythe real physical properties of these structures. In the last few years the 3-d catalogs have been significatively improved and we have extended our methods to the analysis of number counts and angular catalogs. This has led to a complete analysis of all the available data that we present in this review. The result is that galaxy structures are highly irregular and self-similar: all the available data are consistent with each other and show fractal correlations (with dimension $D \simeq 2$) up to the deepest scales probed so far (1000 \hmp) and even more as indicated from the new interpretation of the number counts. The evidence for scale-invariance of galaxy clustering is very strong up to 150 \hmp due to the statistical robustness of the data but becomes progressively weaker (statistically) at larger distances due to the limited data. In These facts lead to fascinating conceptual implications about our knowledge of the universe and to a new scenario for the theoretical challenge in this field.Comment: Latex file 165 pages, 106 postscript figures. This paper is also available at http://www.phys.uniroma1.it/DOCS/PIL/pil.html To appear in Physics Report (Dec. 1997