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Using the Raychaudhuri equation, we associate quantum probability amplitudes (propagators) to equatorial principal ingoing and outgoing null geodesic congruences in the Kerr metric. The expansion scalars diverge at the ring singularity; however, the propagators remain finite, which is an indication that at the quantum level singularities might disappear or, at least, become softened., Comment: 8 pages, 2 figures

Black holes contradict the Nernst-Planck (N/P) version of the 3rd. law of thermodynamics, but agree with its unattainability (U) version. This happens without contradiction, because the N/P and U versions are not equivalent, namely, N/P implies U but U does not imply N/P. So, black holes obey the weaker version of the 3rd. law, but not the stronger one., Comment: 5 pages, 4 figures

Using the Raychaudhuri equation, we show that a quantum probability amplitude (Feynman propagator) can be univocally associated to any timelike or null affinely parametrized geodesic congruence., Comment: 8 pages, 2 figures

Through a M\"obius transformation, we study aspects like topology, ligth cones, horizons, curvature singularity, lines of constant Schwarzschild coordinates $r$ and $t$, null geodesics, and transformed metric, of the spacetime $(SKS/2)^\prime$ that results from: i) the antipode identification in the Schwarzschild-Kruskal-Szekeres ($SKS$) spacetime, and ii) the suppression of the consequent conical singularity. In particular, one obtains a non simply-connected topology: $(SKS/2)^\prime\cong \mathbb{R}^{2*}\times S^2$ and, as expected, bending light cones., Comment: 9 pages, 4 figures

Rhythmic activity in the gamma band (30-100Hz) has been observed in numerous animal species ranging from insects to humans, and in relation to a wide range of cognitive tasks. Various experimental and theoretical studies have investigated this rhythmic activity. The theoretical efforts have mainly been focused on the neuronal dynamics, under the assumption that network connectivity satisfies certain fine-tuning conditions required to generate gamma oscillations. However, it remains unclear how this fine tuning is achieved. Here we investigated the hypothesis that spike timing dependent plasticity (STDP) can provide the underlying mechanism for tuning synaptic connectivity to generate rhythmic activity in the gamma band. We addressed this question in a modeling study. We examined STDP dynamics in the framework of a network of excitatory and inhibitory neuronal populations that has been suggested to underlie the generation of gamma. Mean field Fokker Planck equations for the synaptic weights dynamics are derived in the limit of slow learning. We drew on this approximation to determine which types of STDP rules drive the system to exhibit gamma oscillations, and demonstrate how the parameters that characterize the plasticity rule govern the rhythmic activity. Finally, we propose a novel mechanism that can ensure the robustness of self-developing processes, in general and for rhythmogenesis in particular., Comment: 9 pages with 4 figures

We discuss the wormholes associated with the four-dimensional Schwarzschild (S4), Schwarzschild anti De Sitter (SaDS4), and Reissner-N\"ordstrom (RN4) black holes, in Schwarzschild, isotropic and Kruskal-Szekeres cordinates. The first two coordinate systems are valid outside the horizons, while the third one is used for the interiors. In Schwarzschild coordinates, embedding for SaDS4 exists only for a finite interval of the radial coordinate r, and similar restrictions exist for RN4. The use of the K-S coordinates allows us to give an explicit proof of the pinching-off of the bridges, making them non-traversable. The case of the extreme Reissner-N\"ordstrom (ERN4) is also discussed., Comment: 19 pages, 11 figures

We investigate the prevalence of galactic-scale outflows in post-starburst (PSB) galaxies at high redshift ($1 < z < 1.4$), using the deep optical spectra available in the UKIDSS Ultra Deep Survey (UDS). We use a sample of $\sim40$ spectroscopically confirmed PSBs, recently identified in the UDS field, and perform a stacking analysis in order to analyse the structure of strong interstellar absorption features such as Mg ii ($\lambda2800$ Ang.). We find that for massive ($M_* > 10^{10}\rm\,M_{\odot}$) PSBs at $z > 1$, there is clear evidence for a strong blue-shifted component to the Mg ii absorption feature, indicative of high-velocity outflows ($v_{\rm out}\sim1150\pm160\rm\,km\,s^{-1}$) in the interstellar medium. We conclude that such outflows are typical in massive PSBs at this epoch, and potentially represent the residual signature of a feedback process that quenched these galaxies. Using full spectral fitting, we also obtain a typical stellar velocity dispersion $\sigma_*$ for these PSBs of $\sim200\rm\,km\,s^{-1}$, which confirms they are intrinsically massive in nature (dynamical mass $M_{\rm d}\sim10^{11}\rm\,M_{\odot}$). Given that these high-$z$ PSBs are also exceptionally compact ($r_{\rm e}\sim1$--$2\rm\,kpc$) and spheroidal (Sersic index $n\sim3$), we propose that the outflowing winds may have been launched during a recent compaction event (e.g. major merger or disc collapse) that triggered either a centralised starburst or active galactic nuclei (AGN) activity. Finally, we find no evidence for AGN signatures in the optical spectra of these PSBs, suggesting they were either quenched by stellar feedback from the starburst itself, or that if AGN feedback is responsible, the AGN episode that triggered quenching does not linger into the post-starburst phase., Comment: 13 pages, 6 figures, accepted for publication in MNRAS

We argue that a great circle in the 7-sphere plays the role of an Einstein-Rosen bridge in Einstein-Podolsky-Rosen-Bohm decays., Comment: 5 pages, 1 figure

We analyse the structure of galaxies with high specific star formation rate (SSFR) in cluster and field environments in the redshift range $0.5

In the context of fiber bundle theory, we show that the existence of the Aharonov-Bohm connection implies the existence and uniqueness of the Dirac connection., Comment: 5 pages

We present an analysis of the structure of post-starburst (PSB) galaxies in the redshift range $0.5 < z < 2$, using a photometrically-selected sample identified in the Ultra Deep Survey (UDS) field. We examine the structure of $\sim80$ of these transient galaxies using radial light $\mu(r)$ profiles obtained from CANDELS $\textit{Hubble Space Telescope}$ near-infrared/optical imaging, and compare to a large sample of $\sim2000$ passive and star-forming galaxies. For each population, we determine their typical structural properties (effective radius $r_{\rm e}$, S\'ersic index $n$) and find significant differences in PSB structure at different epochs. At high redshift ($z > 1$), PSBs are typically massive ($M_* > 10^{10}\rm\,M_{\odot}$), very compact and exhibit high S\'ersic indices, with structures that differ significantly from their star-forming progenitors but are similar to massive passive galaxies. In contrast, at lower redshift ($0.5 < z < 1$), PSBs are generally of low mass ($M_* < 10^{10}\rm\,M_{\odot}$) and exhibit compact but less concentrated profiles (i.e. lower S\'ersic indices), with structures similar to low-mass passive discs. Furthermore, for both epochs we find remarkably consistent PSB structure across the optical/near-infrared wavebands (which largely trace different stellar populations), suggesting that any preceding starburst and/or quenching in PSBs was not strongly centralized. Taken together, these results imply that PSBs at $z > 1$ have been recently quenched during a major disruptive event (e.g. merger or protogalactic collapse) which formed a compact remnant, while at $z < 1$ an alternative less disruptive process is primarily responsible. Our results suggest that high-$z$ PSBs are an intrinsically different population to those at lower redshifts, and indicate different quenching routes are active at different epochs., Comment: 21 pages, 12 figures, accepted for publication in MNRAS

We investigate the relationship between environment and galaxy evolution in the redshift range $0.5 < z < 1.0$. Galaxy overdensities are selected using a Friends-of-Friends algorithm, applied to deep photometric data in the Ultra-Deep Survey (UDS) field. A study of the resulting stellar mass functions reveals clear differences between cluster and field environments, with a strong excess of low-mass rapidly quenched galaxies in cluster environments compared to the field. Cluster environments also show a corresponding deficit of young, low-mass star-forming galaxies, which show a sharp radial decline towards cluster centres. By comparing mass functions and radial distributions, we conclude that young star-forming galaxies are rapidly quenched as they enter overdense environments, becoming post-starburst galaxies before joining the red sequence. Our results also point to the existence of two environmental quenching pathways operating in galaxy clusters, operating on different timescales. Fast quenching acts on galaxies with high specific star-formation rates, operating on timescales shorter than the cluster dynamical time ($ < 1$ Gyr). In contrast, slow quenching affects galaxies with moderate specific star-formation rates, regardless of their stellar mass, and acts on longer timescales ($\gtrsim 1$ Gyr). Of the cluster galaxies in the stellar mass range $9.0 < \log(M_{*}/M_{\odot}) < 10.5$ quenched during this epoch, we find that 73% were transformed through fast quenching, while the remaining 27% followed the slow quenching route., Comment: 17 pages, 10 figures, Accepted to MNRAS

We discuss the Aharonov-Bohm ($A-B$) effect and the Dirac ($D$) monopole of magnetic charge $g={{1}\over{2}}$ in the context of bundle theory, exhibiting a purely geometric relation between them. If $\xi_{A-B}$ and $\xi_D$ are the respective $U(1)$-bundles, we show that $\xi_{A-B}$ is isomorphic to the pull-back of $\xi_D$ induced by the inclusion of the corresponding base spaces $\iota:(D_0^2)^*\to S^2$}. The fact that the $A-B$ effect disappears when the magnetic flux in the solenoid equals an integer times the quantum of flux $\Phi_0={{2\pi}\over{\vert e\vert}}$ associated with the electric charge $\vert e\vert$, reflects here as a consequence of the pull-back by $\iota$ of the Dirac connection in $\xi_D$ to $\xi_{A-B}$, and the Dirac quantization condition. We also show the necessary vanishing in $\xi_{A-B}$ of the pull-back of the Chern class $c_1$ in $\xi_D$., Comment: 8 pages, 3 diagrams. Reasons for replacement: Added: 4 references; section 8 for comments; keywords and PACS numbers. Results are unchanged

We review several aspects of anti-De Sitter (AdS) spaces in different dimensions, and of four dimensional Schwarzschild anti-De Sitter (SAdS) black hole., Comment: 39 pages, 12 figures