Alexey S. Koshelev

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.

Time evolution in superstring field theory on non-BPS brane 1. Rolling tachyon and energy-momentum conservation

We derive equations of motion for the tachyon field living on an unstable non-BPS D-brane in the level truncated open cubic superstring field theory in the first non-trivial approximation. We construct a special time dependent solution to this equation which describes a rolling tachyon. It starts from the perturbative vacuum and approaches one of stable vacua in infinite time. We investigate conserved energy functional and show that its different parts dominate in different stages of the evolution. We show that the pressure for this solution has its minimum at zero time and goes to minus energy at infinite time.We derive equations of motion for the tachyon field living on an unstable non-BPS D-brane in the level truncated open cubic superstring field theory in the first non-trivial approximation. We construct a special time dependent solution to this equation which describes the rolling tachyon. It starts from the perturbative vacuum and approaches one of stable vacua in infinite time. We investigate conserved energy functional and show that its different parts dominate in different stages of the evolution. We show that the pressure for this solution has its minimum at zero time and goes to minus energy at infinite time.

Stable bounce and inflation in non-local higher derivative cosmology

One of the greatest problems of primordial inflation is that the inflationary space-time is past-incomplete. This is mainly because Einsteins GR suffers from a space-like Big Bang singularity. It has recently been shown that ghost-free, non-local higher-derivative ultra-violet modifications of Einsteins gravity may be able to resolve the cosmological Big Bang singularity via a non-singular bounce. Within the framework of such non-local cosmological models, we are going to study both sub- and super-Hubble perturbations around an inflationary trajectory which is preceded by the Big Bounce in the past, and demonstrate that the inflationary trajectory has an ultra-violet completion and that perturbations do not suffer from any pathologies.

Two-loop diagrams in noncommutative ϕ44 theory

Abstract Explicit two-loop calculations in noncommutative ϕ 4 4 theory are presented. It is shown that the model is two-loop renormalizable.

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.

On bouncing solutions in non-local gravity

A non-local modified gravity model with an analytical function of the d’Alembert operator, is considered. This model has been recently proposed as a possible way of resolving the singularities problem in cosmology. We present exact bouncing solution, which is simpler compared to the already known one in this model, in the sense it does not require an additional matter to satisfy all gravitational equations.

Stable analytic bounce in non-local Einstein–Gauss–Bonnet cosmology

We consider the most general quadratic in curvature stringy-motivated non-local action for the modified Einsteins gravity. We present exact analytic cosmological solutions including the bouncing ones and develop the relevant techniques for the further study of these types of models. We also elaborate on the perturbation formalism and argue that the found bouncing solution is stable during the bounce phase.

Cosmological signature of tachyon condensation

We consider the dynamics of the open string tachyon condensation in a framework of the cubic fermionic String Field Theory including a non-minimal coupling with closed string massless modes, the graviton and the dilaton. Coupling of the open string tachyon and the dilaton is motivated by the open String Field Theory in a linear dilaton background and the flat space-time. We note that the dilatonic gravity provides several restrictions on the tachyon condensation and show explicitly that the influence of the dilaton on the tachyon condensation is essential and provides a significant effect: oscillations of the Hubble parameter and the state parameter become of a cosmological scale. We give an estimation for the period of these oscillations (0.1-1.0) Gyr and note a good agreement of this period with the observed oscillations with a period (0.15-0.65) Gyr in a distribution of quasar spectra.

Analysis of scalar perturbations in cosmological models with a non-local scalar field

We develop the cosmological perturbations formalism in models with a single non-local scalar field originating from the string field theory description of the rolling tachyon dynamics. We construct the equation for the energy density perturbations of the non-local scalar field in the presence of arbitrary potential and formulate the local system of equations for perturbations in the linearized model when both simple and double roots of the characteristic equation are present. We carry out the general analysis related to the curvature and entropy perturbations and consider the most specific example of perturbations when important quantities in the model become complex.

Stringy dark energy model with cold dark matter

Abstract Cosmological consequences of adding the cold dark matter (CDM) to the exactly solvable stringy dark energy (DE) model are investigated. The model is motivated by the consideration of our Universe as a slowly decaying D3-brane. The decay of this D-brane is described in the string field theory framework. Stability conditions of the exact solution with respect to small fluctuations of the initial value of the CDM energy density are found. Solutions with large initial value of the CDM energy density attracted by the exact solution without CDM are constructed numerically. In contrast to the ΛCDM model the Hubble parameter in the model is not a monotonic function of time. For specific initial data the DE state parameter w DE is also not monotonic function of time. For these cases there are two separate regions of time where w DE being less than −1 is close to −1.