Absorption cross section in de Sitter space

We study the wave equation for a minimally coupled massive scalar in three-dimensional de Sitter space. We compute the absorption cross section to investigate its cosmological horizon in the southern diamond. Although the absorption cross section is not defined exactly, we can be determined it from the fact that the low-energy $s(j=0)$-wave absorption cross section for a massless scalar is given by the area of the cosmological horizon. On the other hand, the low-temperature limit of $j\not=0$-mode absorption cross section is useful for extracting information surrounding the cosmological horizon. Finally we mention a computation of the absorption cross section on the CFT-side using the dS/CFT correspondence.Comment: 13 pages, version to appear in MPL

Dynamical Cobordisms in General Relativity and String Theory

We describe a class of time-dependent solutions in string- or M-theory that are exact with respect to alpha-prime and curvature corrections and interpolate in physical space between regions in which the low energy physics is well-approximated by different string theories and string compactifications. The regions are connected by expanding "domain walls" but are not separated by causal horizons, and physical excitations can propagate between them. As specific examples we construct solutions that interpolate between oriented and unoriented string theories, and also between type II and heterotic theories. Our solutions can be weakly curved and under perturbative control everywhere and can asymptote to supersymmetric at late times.Comment: 35 pages, 5 figures, LaTeX v2: reference adde

On fluctuations of closed string tachyon solitons

We discuss fluctuations on solitons in the dilaton/graviton/tachyon system using the low energy effective field theory approach. It is shown that closed string solitons are free of tachyons in this approximation, regardless of the exact shape of the tachyon potential.Comment: 13 pages, 1 figure, uses JHEP3.cl

Challenging the Authority of Jesus: Mark 11:27-33 and Mediterranean Notions of Honour and Shame

Primjena različitih novih metoda u proučavanju Novog zavjeta obilježava kraj dvadesetog stoljeća. Modeli tumačenja preuzeti iz područja kulturne antropologije pokazali su se posebno plodnim za znanstvenike koji žele steći bolje razumijevanje svijeta ranih kršćana. Na početku gotovo svakog udžbenika o kulturnoj pozadini Novog zavjeta čitatelj se susreće s poglavljem o mediteranskom poimanju časti i sramote. Čast se redovito prepoznaje kao najvažnija vrednota ili dobro drevnog svijeta. Kulturna važnost časti služi, zauzvrat, da u Evanđeljima objasni znatan dio Isusovih sporova s njegovim protivnicima. Moja je namjera pozorno ispitati važan susret između Isusa i njegovih neprijatelja opisan u Evanđelju po Marku 11, 27-33. Istaknut ću način na koji primarna vrednota časti u mediteranskom društvu objašnjava ključne točke ovog vrlo napetog dijaloga.The final quarter of the twentieth century has been characterized by the employment of a variety of new methodologies in NT study. Models imported from the field of cultural anthropology have proved particularly fruitful for scholars interested in gaining a better understanding of the world of the early Christians. Near the beginning of almost every introductory textbook dealing with the cultural background of the NT, the reader encounters a chapter addressing Mediterranean sensibilities concerning honour and shame. Honor is consistently identified as the single most important value or "good" in the ancient world. The cultural centrality of honor serves, in turn, to explain much about Jesus\u27 interactions with his antagonists in the Gospel narratives. My intent here is to carefully examine a single important encounter between Jesus and his adversaries narrated in Mark 11:27-33. I will highlight the way in which the pivotal value of honour in Mediterranean society illuminates the text at crucial points in the course of the highly charged dialogue

Inflation Assisted by Heterotic Axions

We explore the possibility of obtaining inflation in weakly coupled heterotic string theory, where the model dependent axions are responsible for driving inflation. This model can be considered as a certain extrapolation of $m^{2}\phi^{2}$-inflation, and is an attempt to explicitly realize the so called N-flation proposal in string theory. The instanton generated potential for the axions essentially has two parameters; a natural mass scale $M$ and the string coupling $g_{s}$. For isotropic compactifications leading to of order $\mathcal{O} (10^4)$ axions in the four dimensional spectrum we find that with $(M, g_{s})\simeq(M_{GUT}, 0.5)$ the observed temperature fluctuations in the CMB are correctly reproduced. We assume an initially random distribution for the vevs of the axions. The spectral index, $n_{s}$, is generically more red than for $m^{2}\phi^{2}$-inflation. The greater the vevs, the more red the spectral index becomes. Allowing for a wide range of vevs 55 $e$-foldings from the end of inflation, we find $0.946\lesssim n_{s} \lesssim 0.962$. The tensor-to-scalar ratio, $r$, is more sensitive to the vevs, but typically smaller than in $m^{2}\phi^{2}$-inflation. Furthermore, in the regime where the leading order theory is valid, $r$ is bounded by $r < 0.10$. The spectral index and the tensor-to-scalar ratio are correlated. For example, $n_{s}\simeq 0.951$ corresponds to $r\simeq 0.036$.Comment: 1+21 pages, 2 figures, v2: Typos corrected, v3: Typos, very minor corrections, reference added, to appear in JCA

On the origin of thermal string gas

We investigate decaying D-branes as the origin of the thermal string gas of string gas cosmology. We consider initial configurations of low-dimensional branes and argue that they can time evolve to thermal string gas. We find that there is a range in the weak string coupling and fast brane decay time regimes, where the initial configuration could drive the evolution of the dilaton to values, where exactly three spacelike directions grow large.Comment: 16 pages, 4 figures, v2: references adde

Non-birational twisted derived equivalences in abelian GLSMs

In this paper we discuss some examples of abelian gauged linear sigma models realizing twisted derived equivalences between non-birational spaces, and realizing geometries in novel fashions. Examples of gauged linear sigma models with non-birational Kahler phases are a relatively new phenomenon. Most of our examples involve gauged linear sigma models for complete intersections of quadric hypersurfaces, though we also discuss some more general cases and their interpretation. We also propose a more general understanding of the relationship between Kahler phases of gauged linear sigma models, namely that they are related by (and realize) Kuznetsov's `homological projective duality.' Along the way, we shall see how `noncommutative spaces' (in Kontsevich's sense) are realized physically in gauged linear sigma models, providing examples of new types of conformal field theories. Throughout, the physical realization of stacks plays a key role in interpreting physical structures appearing in GLSMs, and we find that stacks are implicitly much more common in GLSMs than previously realized.Comment: 54 pages, LaTeX; v2: typo fixe

Charge and Current in the Quantum Hall Matrix Model

We extend the quantum Hall matrix model to include couplings to external electric and magnetic fields. The associated current suffers from matrix ordering ambiguities even at the classical level. We calculate the linear response at low momenta -- this is unambigously defined. In particular, we obtain the correct fractional quantum Hall conductivity, and the expected density modulations in response to a weak and slowly varying magnetic field. These results show that the classical quantum Hall matrix models describe important aspects of the dynamics of electrons in the lowest Landau level. In the quantum theory the ordering ambiguities are more severe; we discuss possible strategies, but we have not been able to construct a good density operator, satisfying the pertinent lowest Landau level commutator algebra.Comment: 12 pages, no figures; a logical error below the proposed density operator (46) in version 1 is corrected, and the claim that this density operator satisfy the magnetic algebra (2) is withdrawn. Some formulations have been changed and a few misprints correcte