I. Ya. Aref’eva

Crossing the w = − 1 barrier in the D3-brane dark energy model

We explore a possibility for the Universe to cross the w=-1 cosmological constant barrier for the dark energy state parameter. We consider the Universe as a slowly decaying D3-brane. The D3-brane dynamics are approximately described by a nonlocal string tachyon interaction and the backreaction of gravity is incorporated in the closed string tachyon dynamics. In a local effective approximation this model contains one phantom component and one usual field with a simple polynomial interaction. To understand cosmological properties of this system we study toy models with the same scalar fields but with modified interactions. These modifications admit polynomial superpotentials. We find restrictions on these interactions under which it is possible to reach w=-1 from below at large time. Explicit solutions with the dark energy state parameter crossing or not crossing the barrier w=-1 at large time are presented.

Exact solution in a string cosmological model

An exact solution to the Friedmann equations with a string inspired phantom scalar matter field is constructed and the absence of the “Big Rip” singularity is shown explicitly. The notable features of the concerned model are a ghost sign of the kinetic term and a special polynomial form of the effective tachyon potential. The constructed solution is stable with respect to small fluctuations of the initial conditions and special deviations of the form of the potential.

The null energy condition and cosmology

Field theories that violate the null energy condition (NEC) are of interest both for the solution of the cosmological singularity problem and for models of cosmological dark energy with the equation of state parameter w < −1. We consider two recently proposed models that violate the NEC. The ghost condensate model requires higher-derivative terms in the action, and this leads to a heavy ghost field and energy unbounded from below. We estimate the rates of particle decay and discuss possible mass limitations to protect the stability of matter in the ghost condensate model. The nonlocal stringy model that arises from a cubic string field theory and exhibits a phantom behavior also leads to energy unbounded from below. In this case, the energy spectrum is continuous, and there are no particle-like excitations. This model admits a natural UV completion because it comes from superstring theory.

Properties of intersecting p-branes in various dimensions

General properties of intersecting extremal p-brane solutions of gravity coupled with dilatons and several different d-form fields in arbitrary space-time dimensions are considered. It is shown that heuristically expected properties of the intersecting p-branes follow from the explicit formulae for solutions. In particular, harmonic superposition and S-duality hold for all p-brane solutions. Generalized T-duality takes place under additional restrictions on the initial theory parameters.

Critical trapped surfaces formation in the collision of ultrarelativistic charges in (A)dS

We study the formation of marginally trapped surfaces in the head-on collision of two ultrarelativistic charges in (A)dS space-time. The metric of ultrarelativistic charged particles in (A)dS is obtained by boosting Reissner-Nordström (A)dS space-time to the speed of light. We show that formation of trapped surfaces on the past light cone is only possible when charge is below certain critical value - situation similar to the collision of two ultrarelativistic charges in Minkowski space-time. This critical value depends on the energy of colliding particles and the value of a cosmological constant. There is richer structure of critical domains in dS case. In this case already for chargeless particles there is a critical value of the cosmological constant only below which trapped surfaces formation is possible. Appearance of arbitrary small nonzero charge significantly changes the physical picture. Critical effect which has been observed in the neutral case does not take place more. If the value of the charge is not very large solution to the equation on trapped surface exists for any values of cosmological radius and energy density of shock waves. Increasing of the charge leads to decrease of the trapped surface area, and at some critical point the formation of trapped surfaces of the type mentioned above becomes impossible.

Stable exact solutions in cosmological models with two scalar fields

We consider the stability of isotropic solutions for two-field models in the Bianchi I metric. We prove that the sufficient conditions for Lyapunov stability in the Friedmann-Robertson-Walker metric ensure the stability under anisotropic perturbations in the Bianchi I metric and also under perturbations of the energy density for cold dark matter. We find sufficient conditions for the Lyapunov stability of isotropic fixed points for the system of Einstein equations. We use the superpotential method to construct stable kink-type solutions and obtain sufficient conditions on the superpotential for the Lyapunov stability of the corresponding exact solutions. We analyze the stability of isotropic kink-type solutions for models related to string field theory.

Non-perturbative vacuum for superstring field theory and supersymmetry breaking

We analyze possible ground states for the covariant string field theory action of the open NSR-superstring. A remarkable feature of this action is that it possesses a rich set of the auxiliary local fields. We found that some of low-lying scalar auxiliary fields acquire non-zero vacuum expectation values providing a new mechanism for supersymmetry breaking. The gauge vector field become massive while the physical spinor remains massless, thus the supersymmetry is broken in the non-perturbative vacuum.

HOLOGRAPHIC ESTIMATION OF MULTIPLICITY AND THE COLLISION OF MEMBRANES IN MODIFIED AdS5 SPACES

The quark-gluon plasma formed as a result of heavy-ion collisions is currently investigated actively both theoretically and experimentally. According to the holographic approach, forming a quark-gluon plasma in the four-dimensional world is associated with creating black holes in a five-dimensional anti-de Sitter space. The multiplicity of particles produced in heavy-ion collisions is then determined by the entropy of the five-dimensional black hole, which is estimated by the area of the trapped surface. To fit the experimental data for multiplicity Kiritsis and Taliotis have proposed to consider black holes formation in modified

Formation time of quark–gluon plasma in heavy-ion collisions in the holographic shock wave model

{AdS}_5

Gauge equivalence of Tachyon solutions in the cubic Neveu—Schwarz string field theory

spaces with different