856 research outputs found
Testing and comparing tachyon inflation to single standard field inflation
We compare the standard single scalar field inflationary predictions with
those of an inflationary phase driven by a tachyon field. A slow-roll formalism
is defined for tachyon inflation and we derive the spectra of scalar and tensor
perturbations as well as the consistency relations. At lowest order the
predictions of standard and tachyon inflation are the same. Higher order
deviations are present and their observational relevance is discussed. We
discuss the observational consequences of some typical inflationary tachyon
potentials and compare them with recent data. All the models predict a negative
and very small running of the scalar spectral index, and they consistently lie
within the 1 contour of the data set. However, the regime of blue
scalar spectral index and large gravity waves is not explored by these models.Comment: Proceedings of the 10th Marcel Grossmann Meeting, Rio de Janeiro,
July 2003, 6 pages, 1 figur
Spontaneous Breakdown of Lorentz Invariance in IIB Matrix Model
We study the IIB matrix model, which is conjectured to be a nonperturbative
definition of superstring theory, by introducing an integer deformation
parameter `nu' which couples to the imaginary part of the effective action
induced by fermions. The deformed IIB matrix model continues to be well-defined
for arbitrary `nu', and it preserves gauge invariance, Lorentz invariance, and
the cluster property. We study the model at `nu' = infinity using a
saddle-point analysis, and show that ten-dimensional Lorentz invariance is
spontaneously broken at least down to an eight-dimensional one. We argue that
it is likely that the remaining eight-dimensional Lorentz invariance is further
broken, which can be checked by integrating over the saddle-point
configurations using standard Monte Carlo simulation.Comment: 12 pages, latex, no figures, references added, version to appear in
JHE
A geometrical approach to nonlinear perturbations in relativistic cosmology
We give a pedagogical review of a covariant and fully non-perturbative
approach to study nonlinear perturbations in cosmology. In the first part,
devoted to cosmological fluids, we define a nonlinear extension of the
uniform-density curvature perturbation and derive its evolution equation. In
the second part, we focus our attention on multiple scalar fields and present a
nonlinear description in terms of adiabatic and entropy perturbations. In both
cases, we show how the formalism presented here enables one to easily obtain
equations up to second, third and higher orders.Comment: 16 pages; invited review article for Classical and Quantum Gravity
issue on non-linear cosmolog
Effective Field Theory of Cosmological Perturbations
The effective field theory of cosmological perturbations stems from
considering a cosmological background solution as a state displaying
spontaneous breaking of time translations and (adiabatic) perturbations as the
related Nambu-Goldstone modes. With this insight, one can systematically
develop a theory for the cosmological perturbations during inflation and, with
minor modifications, also describe in full generality the gravitational
interactions of dark energy, which are relevant for late-time cosmology. The
formalism displays a unique set of Lagrangian operators containing an
increasing number of cosmological perturbations and derivatives. We give an
introductory description of the unitary gauge formalism for theories with
broken gauge symmetry---that allows to write down the most general
Lagrangian---and of the Stueckelberg "trick"---that allows to recover gauge
invariance and to make the scalar field explicit. We show how to apply this
formalism to gravity and cosmology and we reproduce the detailed analysis of
the action in the ADM variables. We also review some basic applications to
inflation and dark energy.Comment: 27 pages, references added, matches published version as special
issue article in Classical and Quantum Gravit
The 2007 Personal Income Tax Reform in Italy: Effects on Potential Equity, Horizontal Inequity and Re-ranking
According to Kakwani and Lambert (1998), an equitable income tax should respect three axioms related to each taxpayer’s tax liability, average tax rate and post-tax income: whenever taxation determines unequal tax treatments among equals or modifies pre-tax ordering, it influences the potential vertical effect of the tax through three types of inequity. Following the authors’ measurement system, we investigate changes in axiom violations due to the 2007 Italian personal income tax reform, that introduced significant changes in the tax structure. Our microsimulation model uses as input data those provided by the Bank of Italy in its Survey on Households Income and Wealth in the year 2006; estimates of the distribution of taxpayers are very close to the Ministry of Finance official statistics. The analysis considers both the individual and equivalent household gross income distribution and evaluates the decomposition with and without surtaxes. Main findings suggest that both in the 2006 and 2007 tax system most of the overall violations concern the axiom demanding the average tax rate to be a non decreasing function with respect to the gross income; the axiom requiring richer taxpayers to pay higher tax liabilities than poorer ones and the axiom requiring the tax to do not introduce re-rankings in the pre-tax income order present minor violations. The 2007 reform enhances both the potential redistributive effect, that is the one that could be obtained without axiom violations, and the axiom violations: the net result is a small positive variation of the actual redistributive effect. These phenomena appear more relevant for taxpayers than those for equivalent households. For what concerns taxpayers, the 2007 reform has modified also the composition of the three axiom violations, that remains almost the same whenever equivalent households are considered. Finally, focusing on each decile of the income distribution, regressivities are concentrated in the bottom five deciles of the income distribution both for taxpayers and equivalent households.Personal Income Tax, Redistributive Effect, Horizontal Inequity, Reranking, Microsimulation Models
Prediction of RNA pseudoknots by Monte Carlo simulations
In this paper we consider the problem of RNA folding with pseudoknots. We use
a graphical representation in which the secondary structures are described by
planar diagrams. Pseudoknots are identified as non-planar diagrams. We analyze
the non-planar topologies of RNA structures and propose a classification of RNA
pseudoknots according to the minimal genus of the surface on which the RNA
structure can be embedded. This classification provides a simple and natural
way to tackle the problem of RNA folding prediction in presence of pseudoknots.
Based on that approach, we describe a Monte Carlo algorithm for the prediction
of pseudoknots in an RNA molecule.Comment: 22 pages, 14 figure
Improved RNA pseudoknots prediction and classification using a new topological invariant
We propose a new topological characterization of RNA secondary structures
with pseudoknots based on two topological invariants. Starting from the classic
arc-representation of RNA secondary structures, we consider a model that
couples both I) the topological genus of the graph and II) the number of
crossing arcs of the corresponding primitive graph. We add a term proportional
to these topological invariants to the standard free energy of the RNA
molecule, thus obtaining a novel free energy parametrization which takes into
account the abundance of topologies of RNA pseudoknots observed in RNA
databases.Comment: 9 pages, 6 figure
- …