111 research outputs found
Quantum Einstein Gravity
We give a pedagogical introduction to the basic ideas and concepts of the
Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum
approach based upon the effective average action, we summarize the state of the
art of the field with a particular focus on the evidence supporting the
existence of the non-trivial renormalization group fixed point at the heart of
the construction. As an application, the multifractal structure of the emerging
space-times is discussed in detail. In particular, we compare the continuum
prediction for their spectral dimension with Monte Carlo data from the Causal
Dynamical Triangulation approach.Comment: 87 pages, 13 figures, review article prepared for the New Journal of
Physics focus issue on Quantum Einstein Gravit
The Kahler Cone as Cosmic Censor
M-theory effects prevent five-dimensional domain-wall and black-hole
solutions from developing curvature singularities. While so far this analysis
was performed for particular models, we now present a model-independent proof
that these solutions do not have naked singularities as long as the Kahler
moduli take values inside the extended Kahler cone. As a by-product we obtain
information on the regularity of the Kahler-cone metric at boundaries of the
Kahler cone and derive relations between the geometry of moduli space and
space-time.Comment: 21 pages, 1 figure. Improved discussion of the relation between
Kahler moduli and five-dimensional scalars. No changes in the conclusion
Renormalization Group Flow of Quantum Gravity in the Einstein-Hilbert Truncation
The exact renormalization group equation for pure quantum gravity is used to
derive the non-perturbative \Fbeta-functions for the dimensionless Newton
constant and cosmological constant on the theory space spanned by the
Einstein-Hilbert truncation. The resulting coupled differential equations are
evaluated for a sharp cutoff function. The features of these flow equations are
compared to those found when using a smooth cutoff. The system of equations
with sharp cutoff is then solved numerically, deriving the complete
renormalization group flow of the Einstein-Hilbert truncation in . The
resulting renormalization group trajectories are classified and their physical
relevance is discussed. The non-trivial fixed point which, if present in the
exact theory, might render Quantum Einstein Gravity nonperturbatively
renormalizable is investigated for various spacetime dimensionalities.Comment: 58 pages, latex, 24 figure
D-instantons and twistors
Finding the exact, quantum corrected metric on the hypermultiplet moduli
space in Type II string compactifications on Calabi-Yau threefolds is an
outstanding open problem. We address this issue by relating the
quaternionic-Kahler metric on the hypermultiplet moduli space to the complex
contact geometry on its twistor space. In this framework, Euclidean D-brane
instantons are captured by contact transformations between different patches.
We derive those by recasting the previously known A-type D2-instanton
corrections in the language of contact geometry, covariantizing the result
under electro-magnetic duality, and using mirror symmetry. As a result, we are
able to express the effects of all D-instantons in Type II compactifications
concisely as a sum of dilogarithm functions. We conclude with some comments on
the relation to microscopic degeneracies of four-dimensional BPS black holes
and to the wall-crossing formula of Kontsevich and Soibelman, and on the form
of the yet unknown NS5-brane instanton contributions.Comment: 47 pages, 1 figure, uses JHEP3.cl
Phase Space Analysis of Quintessence Cosmologies with a Double Exponential Potential
We use phase space methods to investigate closed, flat, and open
Friedmann-Robertson-Walker cosmologies with a scalar potential given by the sum
of two exponential terms. The form of the potential is motivated by the
dimensional reduction of M-theory with non-trivial four-form flux on a
maximally symmetric internal space. To describe the asymptotic features of
run-away solutions we introduce the concept of a `quasi fixed point.' We give
the complete classification of solutions according to their late-time behavior
(accelerating, decelerating, crunch) and the number of periods of accelerated
expansion.Comment: 46 pages, 5 figures; v2: minor changes, references added; v3: title
changed, refined classification of solutions, 3 references added, version
which appeared in JCA
Renormalization Group Flow in Scalar-Tensor Theories. II
We study the UV behaviour of actions including integer powers of scalar
curvature and even powers of scalar fields with Functional Renormalization
Group techniques. We find UV fixed points where the gravitational couplings
have non-trivial values while the matter ones are Gaussian. We prove several
properties of the linearized flow at such a fixed point in arbitrary dimensions
in the one-loop approximation and find recursive relations among the critical
exponents. We illustrate these results in explicit calculations in for
actions including up to four powers of scalar curvature and two powers of the
scalar field. In this setting we notice that the same recursive properties
among the critical exponents, which were proven at one-loop order, still hold,
in such a way that the UV critical surface is found to be five dimensional. We
then search for the same type of fixed point in a scalar theory with minimal
coupling to gravity in including up to eight powers of scalar curvature.
Assuming that the recursive properties of the critical exponents still hold,
one would conclude that the UV critical surface of these theories is five
dimensional.Comment: 14 pages. v.2: Minor changes, some references adde
Manejo reprodutivo em gado de corte.
ABSTRACT: The reproductive efficiency is one of the major factors contributing to the success of beef cattle production systems. To achieve satisfatory reproductive performance, it is necessary an adequate herd management, particulary the adoption of breeding season, adequate cow nutrition and adequate body condition score, evaluation of the fertility of the bulls and adoption of suitable sire: cow ratio. Moreover, other practices can enhance female reproductive performance, such as separation of calves from their dams for a period (Shang), suckling restriction, exposition of females to bulls, and selection of the most fertile animals.bitstream/CPAC-2009/27469/1/doc_134.pd
Flux Compactifications: Stability and Implications for Cosmology
We study the dynamics of the size of an extra-dimensional manifold stabilised
by fluxes. Inspecting the potential for the 4D field associated with this size
(the radion), we obtain the conditions under which it can be stabilised and
show that stable compactifications on hyperbolic manifolds necessarily have a
negative four-dimensional cosmological constant, in contradiction with
experimental observations. Assuming compactification on a positively curved
(spherical) manifold we find that the radion has a mass of the order of the
compactification scale, M_c, and Planck suppressed couplings. We also show that
the model becomes unstable and the extra dimensions decompactify when the
four-dimensional curvature is higher than a maximum value. This in particular
sets an upper bound on the scale of inflation in these models: V_max \sim M_c^2
M_P^2, independently of whether the radion or other field is responsible for
inflation. We comment on other possible contributions to the radion potential
as well as finite temperature effects and their impact on the bounds obtained.Comment: 16 pages, 1 figure, LaTeX; v2: typos fixed and references adde
From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework
Using the Einstein-Hilbert approximation of asymptotically safe quantum
gravity we present a consistent renormalization group based framework for the
inclusion of quantum gravitational effects into the cosmological field
equations. Relating the renormalization group scale to cosmological time via a
dynamical cutoff identification this framework applies to all stages of the
cosmological evolution. The very early universe is found to contain a period of
``oscillatory inflation'' with an infinite sequence of time intervals during
which the expansion alternates between acceleration and deceleration. For
asymptotically late times we identify a mechanism which prevents the universe
from leaving the domain of validity of the Einstein-Hilbert approximation and
obtain a classical de Sitter era.Comment: 47 pages, 17 figure
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