Koichiro Kobayashi

Equations of Motion in Double Field Theory: From particles to scale factors

In double field theory, the equation of motion for a point particle in the background field is considered. We find that the motion is described by a geodesic flow in the doubled geometry. Inspired by the analysis on the particle motion, we consider a modified model of quantum string cosmology, which includes two scale factors.

“Critical” Cosmology in Higher Order Gravity

We construct the higher order terms of curvatures in Lagrangians of the scale factor in the D-dimensional Robertson-Walker metric, which are linear in the second derivative of the scale factor with respect to cosmic time. It is shown that they are composed of the Lovelock tensors at the first step; iterative construction yields arbitrarily high order terms. The relation to the former work on higher order gravity is discussed. Despite the absence of scalar degrees of freedom in cosmological models which come from our Lagrangian, it is shown that an inflationary behavior of the scale factor can be found. The application to the thick brane solutions is also studied.

Classical and quantum cosmology of multigravity

Recently, a multigraviton theory on a simple closed circuit graph corresponding to the discretization of S1 compactification of the Kaluza–Klein (KK) theory has been considered. In the present paper, we extend this theory to that on a general graph and study what modes of particles are included. Furthermore, we generalize it in a possible nonlinear theory based on the vierbein formalism and study classical and quantum cosmological solutions in the theory. We found that scale factors in a solution for this theory repeat acceleration and deceleration.

HOSOTANI MECHANISM IN HIGHER DIMENSIONAL LEE-WICK THEORY

Hosotani mechanism in higher-dimensional Lee-Wick theory is investigated. The symmetry breaking mechanism proposed by Hosotani is studied at one-loop level through a toy model in this theory. We find that the phase diagram of symmetry and masses of fields are modified from the original one if masses of Lee-Wick particles are in the same order of the inverse of the compactification scale.

Flux vacua in DBI type Einstein–Maxwell theory

We study compactification of extra dimensions in a theory of Dirac–Born–Infeld type gravity. We investigate the solution for Minkowski spacetime with an S2 extra space as well as that for de Sitter spacetime (S4) with an S2 extra space. They are derived by the effective potential method in the presence of the magnetic flux on the extra sphere. We also consider the higher-dimensional generalization of the solutions. We find that, in a certain model, the radius of the extra space has a minimum value independent of the higher-dimensional Newton constant.

Magnetic Monopoles in Multivector Boson Theories

A classical solution for a magnetic monopole is found in a specific multivector boson theory. We consider the model whose gauge group is broken by sigma model fields (a la dimensional deconstruction) and further spontaneously broken by an adjoint scalar (a la triplet Higgs mechanism). In this multivector boson theory, we find the solution for the monopole whose mass is , where is the common gauge coupling constant and is the vacuum expectation value of the triplet Higgs field, by using a variational method with the simplest set of test functions.

Simple models in supersymmetric quantum mechanics on a graph

We study some types of dimensionally-deconstructed models for supersymmetric (Euclidean) quantum mechanics, or zero-dimensional field theory. In these models, we assign bosonic and fermionic variables to vertices and the edges of a graph. We investigate a discrete version of the Gaussian model and the Wess?Zumino-type model on a graph. The topological index as a multiple integral is discussed on these models. In addition, we propose simple examples of supersymmetric extensions of the Lee?Wick model and the Galileon model. A model with two supersymmetries is also provided and generalization to ?local? supersymmtric models is examined.

GRAPH-THEORY INDUCED GRAVITY AND STRONGLY-DEGENERATE FERMIONS IN A SELF-CONSISTENT EINSTEIN UNIVERSE

We study UV-finite theory of induced gravity. We use scalar fields, Dirac fields and vector fields as matter fields whose one-loop effects induce the gravitational action. To obtain the mass spectrum that satisfies the UV-finiteness condition, we use a graph-based construction of mass matrices. The existence of a self-consistent static solution for an Einstein universe is shown in the presence of degenerate fermions.

Fermion spectrum and localization on kinks in a deconstructed dimension

We study the deconstructed scalar theory having nonlinear interactions and being renormalizable. It is shown that the kink-like configurations exist in such models. The possible forms of Yukawa coupling are considered. We find the degeneracy in mass spectrum of fermions coupled to the nontrivial scalar configuration.