40 research outputs found
Geometric flows in Horava-Lifshitz gravity
We consider instanton solutions of Euclidean Horava-Lifshitz gravity in four
dimensions satisfying the detailed balance condition. They are described by
geometric flows in three dimensions driven by certain combinations of the
Cotton and Ricci tensors as well as the cosmological-constant term. The
deformation curvature terms can have competing behavior leading to a variety of
fixed points. The instantons interpolate between any two fixed points, which
are vacua of topologically massive gravity with Lambda > 0, and their action is
finite. Special emphasis is placed on configurations with SU(2) isometry
associated with homogeneous but generally non-isotropic Bianchi IX model
geometries. In this case, the combined Ricci-Cotton flow reduces to an
autonomous system of ordinary differential equations whose properties are
studied in detail for different couplings. The occurrence and stability of
isotropic and anisotropic fixed points are investigated analytically and some
exact solutions are obtained. The corresponding instantons are classified and
they are all globally R x S^3 and complete spaces. Generalizations to
higher-dimensional gravities are also briefly discussed.Comment: 67 pages, 16 figures; more solutions found, 1 extra figure, 1 more
reference added in v2; minor typos corrected in v3 (to appear in JHEP); an
acknowledgement added in v
Gravitational instantons, self-duality and geometric flows
We discuss four-dimensional "spatially homogeneous" gravitational instantons.
These are self-dual solutions of Euclidean vacuum Einstein's equations with
potentially non-vanishing cosmological constant. They are endowed with a
product structure R \times M_3 leading to a natural foliation into
three-dimensional subspaces evolving in Euclidean time. For a large class of
three-dimensional subspaces, the dynamics coincides with the geometric flow on
the three-dimensional homogeneous slice, driven by the Ricci tensor plus an
so(3) gauge connection. The metric on the three-dimensional space is related to
the vielbein of the three-dimensional subspace, while the gauge field is
inherited from the anti-self-dual component of the four-dimensional
Levi--Civita connection.Comment: 14 page
Introduction to Superstring Cosmology
This is a summary of lectures in superstring cosmology given by the author at
the CORFU 2009 School and Workshops "Theory - Cosmology - Phenomenology", Corfu
Institute, Greece, Sept 6-13, 2009. These lectures are based on some recent
developments and ideas, in the framework of superstring theory, concerning the
evolution and structure of the universe in (i) the very early "non-geometric"'
cosmological era, (ii) the intermediate "radiation-like" era and (iii) the late
time cosmological era characterized by the electroweak phase transition.Comment: Summary of lectures in superstring cosmology given by the author at
the CORFU 2009 School and Workshops "Theory - Cosmology - Phenomenology". 6
page
Vector field models of modified gravity and the dark sector
We present a comprehensive investigation of cosmological constraints on the
class of vector field formulations of modified gravity called Generalized
Einstein-Aether models. Using linear perturbation theory we generate cosmic
microwave background and large-scale structure spectra for general parameters
of the theory, and then constrain them in various ways. We investigate two
parameter regimes: a dark-matter candidate where the vector field sources
structure formation, and a dark-energy candidate where it causes late-time
acceleration. We find that the dark matter candidate does not fit the data, and
identify five physical problems that can restrict this and other theories of
dark matter. The dark energy candidate does fit the data, and we constrain its
fundamental parameters; most notably we find that the theory's kinetic index
parameter can differ significantly from its CDM
value.Comment: 16 pages, 11 figure
Moduli stabilisation in early superstring cosmology
We study moduli stabilization by thermal effects in the cosmological context.
The implementation of finite temperature, which spontaneously breaks
supersymmetry, induces an effective potential at one loop level. At the points
where extra massless states appear in the string spectrum, the potential
develops local minima whose depth depends on the temperature. Moduli attracted
to these points acquire dynamical masses which decrease with cosmological
evolution. This makes the coherent scalar oscillations dilute before
nucleosynthesis, and the cosmological moduli problem is avoided. In particular,
we study the effective potential induced by a maximally supersymmetric
heterotic string gas for spacetime dimension D>=4, and a gas of type II strings
compactified on Calabi-Yau three-folds (D=4). In the former case, the local
minima of the potential arise at enhanced gauge symmetry points, which can
stabilize all moduli but the dilaton. In the latter case, the local minima are
reached at the loci where 2-cycles or 3-cycles in the Calabi-Yau space shrink
to zero size, accompanied with either conifold transitions or non Abelian gauge
symmetries. This stabilizes the type II moduli which characterize the
deformation of these shrinking cycles. Moduli stabilization in the dual string
models is also investigated by heterotic/type I S-dualities and type
II/heterotic dualities.Comment: 13+1 pages. Based on a talk given at the "The Seventh International
Conference Quantum Theory and Symmetries", Prague, Czech Republic, on August
7-13, 201
The cosmological behavior of Bekenstein's modified theory of gravity
We study the background cosmology governed by the Tensor-Vector-Scalar theory
of gravity proposed by Bekenstein. We consider a broad family of potentials
that lead to modified gravity and calculate the evolution of the field
variables both numerically and analytically. We find a range of possible
behaviors, from scaling to the late time domination of either the additional
gravitational degrees of freedom or the background fluid.Comment: 10 pages, 8 figures, A few typos corrected in the text and figures.
Version published in PR
Classification of Heterotic Pati-Salam Models
We extend the classification of free fermionic heterotic-string models to
vacua in which the SO(10) GUT symmetry is broken at the string level to the
Pati-Salam subgroup. Using our classification method we recently presented the
first example of a quasi--realistic heterotic-string vacuum that is free of
massless exotic states. Within this method we are able to derive algebraic
expressions for the generalised GSO projections for all sectors that appear in
the models. This facilitates the programming of the entire spectrum analysis in
a computer code. The total number of vacua in the class of models that we
classify is 2^{51} ~ 10^{15}. We perform a statistical sampling in this space
of models and extract 10^{11} distinct configurations with Pati-Salam gauge
group. Our results demonstrate that one in every 10^{6} vacua correspond to a
three generation exophobic model with the required Higgs states, needed to
induce spontaneous breaking to the Standard Model.Comment: 42 pages. 6 figures. Published version. Added acknowledgement
G3-homogeneous gravitational instantons
We provide an exhaustive classification of self-dual four-dimensional
gravitational instantons foliated with three-dimensional homogeneous spaces,
i.e. homogeneous self-dual metrics on four-dimensional Euclidean spaces
admitting a Bianchi simply transitive isometry group. The classification
pattern is based on the algebra homomorphisms relating the Bianchi group and
the duality group SO(3). New and general solutions are found for Bianchi III.Comment: 24 pages, few correction