Vitaly Vanchurin

Bio: Vitaly Vanchurin is an academic researcher from University of Minnesota. The author has contributed to research in topics: Cosmic string & Tensor. The author has an hindex of 17, co-authored 59 publications receiving 1554 citations. Previous affiliations of Vitaly Vanchurin include Ludwig Maximilian University of Munich & Stanford University.

Vector Inflation

Abstract: We propose a scenario where inflation is driven by non-minimally coupled massive vector fields In an isotropic homogeneous universe these fields behave in presicely the same way as a massive minimally coupled scalar field Therefore our model is very similar to the model of chaotic inflation with scalar field For vector fields the isotropy of expansion is achived either by considering a triplet of orthogonal vector fields or for the expense of $N$ randomly oriented vector fields In the last case the substantial anisotropy of the expansion of order $1/\sqrt{N}$ survives until the end of inflation The lightest vector fields might also force the late time acceleration of the Universe

Scaling of cosmic string loops

Abstract: We study the spectrum of loops as a part of a complete network of cosmic strings in flat spacetime. After a long transient regime, characterized by production of small loops at the scale of the initial conditions, it appears that a true scaling regime takes over. In this final regime the characteristic length of loops scales as $0.1t$, in contrast to earlier simulations which found tiny loops. We expect the expanding-universe behavior to be qualitatively similar. If this expectation is correct, then the large loop sizes have important cosmological implications. In particular, the nucleosynthesis bound then becomes $G\ensuremath{\mu}\ensuremath{\lesssim}{10}^{\ensuremath{-}7}$, much tighter than that obtained from earlier analyses.

Cosmic string loops in the expanding universe

Abstract: We study the production of loops in the cosmic string network in the expanding background by means of a numerical simulation exact in the flat-spacetime limit and first order in the expansion rate. We find an initial regime characterized by production of small loops at the scale of the initial correlation length, but later we see the emergence of a scaling regime of loop production. This qualitatively agrees with earlier expectations derived from the results of flat-spacetime simulations. In the final scaling regime we find that the characteristic length of loops scales as {approx}0.1t in both radiation and matter eras.

Attractor scenarios and superluminal signals in k-essence cosmology

Abstract: Cosmological scenarios with $k$-essence are invoked in order to explain the observed late-time acceleration of the Universe. These scenarios avoid the need for fine-tuned initial conditions (the ``coincidence problem'') because of the attractorlike dynamics of the $k$-essence field $\ensuremath{\phi}$. It was recently shown that all $k$-essence scenarios with Lagrangians $p=L(X){\ensuremath{\phi}}^{\ensuremath{-}2}$, where $X\ensuremath{\equiv}\frac{1}{2}{\ensuremath{\phi}}_{,\ensuremath{\mu}}{\ensuremath{\phi}}^{,\ensuremath{\mu}}$, necessarily involve an epoch where perturbations of $\ensuremath{\phi}$ propagate faster than light (the ``no-go theorem''). We carry out a comprehensive study of attractorlike cosmological solutions (``trackers'') involving a $k$-essence scalar field $\ensuremath{\phi}$ and another matter component. The result of this study is a complete classification of $k$-essence Lagrangians that admit asymptotically stable tracking solutions, among all Lagrangians of the form $p=K(\ensuremath{\phi})L(X)$. Using this classification, we select the class of models that describe the late-time acceleration and avoid the coincidence problem through the tracking mechanism. An analogous ``no-go theorem'' still holds for this class of models, indicating the existence of a superluminal epoch. In the context of $k$-essence cosmology, the superluminal epoch does not lead to causality violations. We discuss the implications of superluminal signal propagation for possible causality violations in Lorentz-invariant field theories.

Predictability crisis in inflationary cosmology and its resolution

Abstract: Models of inflationary cosmology can lead to variation of observable parameters (``constants of nature'') on extremely large scales. The question of making probabilistic predictions for today's observables in such models has been investigated in the literature. Because of the infinite thermalized volume resulting from eternal inflation, it has proved difficult to obtain a meaningful and unambiguous probability distribution for observables, in particular due to the gauge dependence. In the present paper, we further develop the gauge-invariant procedure proposed in a previous work for models with a continuous variation of ``constants.'' The recipe uses an unbiased selection of a connected piece of the thermalized volume as sample for the probability distribution. To implement the procedure numerically, we develop two methods applicable to a reasonably wide class of models: one based on the Fokker-Planck equation of stochastic inflation and the other based on direct simulation of inflationary spacetime. We present and compare results obtained using these methods.

"relative State" Formulation of Quantum Mechanics

Abstract: The task of quantizing general relativity raises serious questions about the meaning of the present formulation and interpretation of quantum mechanics when applied to so fundamental a structure as the space-time geometry itself. This paper seeks to clarify the foundations of quantum mechanics. It presents a reformulation of quantum theory in a form believed suitable for application to general relativity. The aim is not to deny or contradict the conventional formulation of quantum theory, which has demonstrated its usefulness in an overwhelming variety of problems, but rather to supply a new, more general and complete formulation, from which the conventional interpretation can be deduced. The relationship of this new formulation to the older formulation is therefore that of a metatheory to a theory, that is, it is an underlying theory in which the nature and consistency, as well as the realm of applicability, of the older theory can be investigated and clarified.

Cosmology and Fundamental Physics with the Euclid Satellite

Abstract: Euclid is a European Space Agency medium-class mission selected for launch in 2020 within the cosmic vision 2015–2025 program. The main goal of Euclid is to understand the origin of the accelerated expansion of the universe. Euclid will explore the expansion history of the universe and the evolution of cosmic structures by measuring shapes and red-shifts of galaxies as well as the distribution of clusters of galaxies over a large fraction of the sky. Although the main driver for Euclid is the nature of dark energy, Euclid science covers a vast range of topics, from cosmology to galaxy evolution to planetary research. In this review we focus on cosmology and fundamental physics, with a strong emphasis on science beyond the current standard models. We discuss five broad topics: dark energy and modified gravity, dark matter, initial conditions, basic assumptions and questions of methodology in the data analysis. This review has been planned and carried out within Euclid’s Theory Working Group and is meant to provide a guide to the scientific themes that will underlie the activity of the group during the preparation of the Euclid mission.

Loop Quantum Cosmology: A Status Report

Abstract: Loop quantum cosmology (LQC) is the result of applying principles of loop quantum gravity (LQG) to cosmological settings. The distinguishing feature of LQC is the prominent role played by the quantum geometry effects of LQG. In particular, quantum geometry creates a brand new repulsive force which is totally negligible at low spacetime curvature but rises very rapidly in the Planck regime, overwhelming the classical gravitational attraction. In cosmological models, while Einstein's equations hold to an excellent degree of approximation at low curvature, they undergo major modifications in the Planck regime: for matter satisfying the usual energy conditions, any time a curvature invariant grows to the Planck scale, quantum geometry effects dilute it, thereby resolving singularities of general relativity. Quantum geometry corrections become more sophisticated as the models become richer. In particular, in anisotropic models, there are significant changes in the dynamics of shear potentials which tame their singular behavior in striking contrast to older results on anisotropies in bouncing models. Once singularities are resolved, the conceptual paradigm of cosmology changes and one has to revisit many of the standard issues—e.g. the 'horizon problem'—from a new perspective. Such conceptual issues as well as potential observational consequences of the new Planck scale physics are being explored, especially within the inflationary paradigm. These considerations have given rise to a burst of activity in LQC in recent years, with contributions from quantum gravity experts, mathematical physicists and cosmologists. The goal of this review is to provide an overview of the current state of the art in LQC for three sets of audiences: young researchers interested in entering this area; the quantum gravity community in general and cosmologists who wish to apply LQC to probe modifications in the standard paradigm of the early universe. In this review, effort has been made to streamline the material so that each of these communities can read only the sections they are most interested in, without loss of continuity.