Yuri Shtanov

Braneworld models of dark energy

We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop effects induced by matter fields residing on the brane. Spatially flat braneworld models can enter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall–Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the luminosity distance in LCDM. Whereas models with dL ≤ dL(LCDM) imply w = p/ρ ≥ −1 and have frequently been discussed in the literature, models with dL > dL(LCDM) have traditionally been ignored, perhaps because, within the general-relativistic framework, the luminosity distance has this property only if the equation of state of matter is strongly negative (w > t0, where t0 is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.

Universe reheating after inflation

We study the problem of scalar particle production after inflation by a rapidly oscillating inflaton field. We use the framework of the chaotic inflation scenario with quartic and quadratic inflaton potentials. Particular attention is paid to parametric resonance phenomena which take place in the presence of the quickly oscillating inflaton field. We have found that in the region of applicability of perturbation theory the effects of parametric resonance are crucial, and estimates based on first order Born approximation often underestimate the particle production. In the case of the quartic inflaton potential

Non-metric gravity: II. Spherically symmetric solution, missing mass and redshifts of quasars

V(\varphi) = \lambda \varphi^4

Recycling the universe using scalar fields

, the particle production process is very efficient even for small values of coupling constants. The reheating temperature of the universe in this case is

Cosmic mimicry: Is LCDM a braneworld in disguise?

\left[\lambda\, \log\, (1/\lambda) \right]^{- 1}

Pilot wave quantum cosmology

times larger than the corresponding estimates based on first order Born approximation. In the case of the quadratic inflaton potential the reheating process depends crucially on the type of coupling between the inflaton and the other scalar field and on the magnitudes of the coupling constants. If the inflaton coupling to fermions and its linear (in inflaton field) coupling to scalar fields are suppressed, then, as previously discussed by Kofman, Linde and Starobinsky (see e.g. Ref. 13), the inflaton field will eventually decouple from the rest of the matter, and the residual inflaton oscillations may provide the (cold) dark matter of the universe. In the case of the quadratic inflaton potential we obtain the lowest and the highest possible bounds on the effective energy density of the inflaton field when it freezes out.

Gravitational instability on the brane: The role of boundary conditions

We continue the study of the non-metric theory of gravity introduced by Krasnov (2006 Preprint hep-th/0611182) and obtain its general spherically symmetric vacuum solution. It respects the analog of the Birkhoff theorem, i.e. the vacuum spherically symmetric solution is necessarily static. As in general relativity, the spherically symmetric solution is seen to describe a black hole. The exterior geometry is essentially the same as in the Schwarzschild case, with power-law corrections to the Newtonian potential. The behaviour inside the black-hole region is different from the Schwarzschild case in that the usual spacetime singularity gets replaced by a singular surface of a new type, where all basic fields of the theory remain finite but metric ceases to exist. The theory does not admit arbitrarily small black holes: for small objects, the curvature on the would-be horizon is so strong that non-metric modifications prevent the horizon from being formed. The theory allows for modifications of gravity of a very interesting nature. We discuss three physical effects, namely (i) correction to Newtons law in the neighborhood of the source, (ii) renormalization of effective gravitational and cosmological constants at large distances from the source and (iii) additional redshift factor between spatial regions of different curvature. The first two effects can be responsible, respectively, for the observed anomaly in the acceleration of the Pioneer spacecraft and for the alleged missing mass in spiral galaxies and other astrophysical objects. The third effect can be used to propose a non-cosmological explanation of high redshifts of quasars and gamma-ray bursts.

Linearized gravity on the Randall–Sundrum two-brane background with curvature terms in the action for the branes

We examine the behavior of a closed oscillating universe filled with a homogeneous scalar field and find that, contrary to naive expectations, such a universe expands to larger volumes during successive expansion epochs. This intriguing behavior introduces an arrow of time in a system which is time reversible. The increase in the maximum size of the universe is closely related to the work done on or by the scalar field during one complete oscillatory cycle which, in turn, is related to the asymmetry in the scalar field equation of state during expansion and collapse. Our analysis shows that scalar fields with polynomial potentials

Braneworld cosmological solutions and their stability

V(\ensuremath{\varphi})=\ensuremath{\lambda}{\ensuremath{\varphi}}^{q},