444 research outputs found
Where in the String Landscape is Quintessence
We argue that quintessence may reside in certain corners of the string
landscape. It arises as a linear combination of internal space components of
higher rank forms, which are axion-like at low energies, and may mix with
4-forms after compactification of the Chern-Simons terms to 4D due to internal
space fluxes. The mixing induces an effective mass term, with an action which
{\it preserves} the axion shift symmetry, breaking it spontaneously after the
background selection. With several axions, several 4-forms, and a low string
scale, as in one of the setups already invoked for dynamically explaining a
tiny residual vacuum energy in string theory, the 4D mass matrix generated by
random fluxes may have ultralight eigenmodes over the landscape, which are
quintessence. We illustrate how this works in simplest cases, and outline how
to get the lightest mass to be comparable to the Hubble scale now, . The shift symmetry protects the smallest mass from
perturbative corrections in field theory. Further, if the ultralight eigenmode
does not couple directly to any sector strongly coupled at a high scale, the
non-perturbative field theory corrections to its potential will also be
suppressed. Finally, if the compactification length is larger than the string
length by more than an order of magnitude, the gravitational corrections may
remain small too, even when the field value approaches .Comment: 8 pages RevTeX; added references, matches published versio
INFLATION-INDEXED BONDS IN THE EUROZONE
The thesis consists in two papers exploiting thorughly the inflation-indexed bond markets
in the Eurozone. In the first paper, after presenting some empirical stylized facts
about the European sovereign inflation-indexed markets we address the effectiveness of
nominal and real rational expectation hypothesis and of inflation-expectation hypothesis.
Then, we document the existence of a liquidity premium and of a default premium
for France, Italy and Germany, moving from a market based measure of inflation.
The second paper is about yield curve modeling and forecasting. We provide a threefactor
yield curve model delivering estimates for nominal term structure of France,
Germany and Italy, from January 2000 to December 2016 and for real term structure of
France and Italy from July 2003 to December 2016. The framework is the latent factor
model with time varying level, slope and curvature. The overall fitting performances
is good and the identification is consistent with many shapes assumed by the term
structure. After the empirical estimation we forecast the yield curve by forecasting the
factors and we compare them with several standard competitors. Lastly, we document
for the first time a significant liquidity issue on short-term real bond spreads and of
a default premium affecting more heavily real spreads as compared to nominal across
various maturities
Goldberger-Wise variations: stabilizing brane models with a bulk scalar
Braneworld scenarios with compact extra-dimensions need the volume of the
extra space to be stabilized. Goldberger and Wise have introduced a simple
mechanism, based on the presence of a bulk scalar field, able to stabilize the
radius of the Randall-Sundrum model. Here, we transpose the same mechanism to
generic single-brane and two-brane models, with one extra dimension and
arbitrary scalar potentials in the bulk and on the branes. The single-brane
construction turns out to be always unstable, independently of the bulk and
brane potentials. In the case of two branes, we derive some generic criteria
ensuring the stabilization or destabilization of the system.Comment: 8 pages, 2 figures. 1 figure and one subsection added. version
published on PR
Higgsflation at the GUT scale in a Higgsless Universe
We revisit inflation in induced gravity. Our focus is on models where the low
scale Planck mass is completely determined by the breaking of the scaling
symmetry in the field theory sector. The Higgs-like field which breaks the
symmetry with a GUT-scale vacuum expectation value has non-minimal couplings to
the curvature, induced by the gravitational couplings of the other light fields
in the theory, so that its expectation value controls the gravitational
strength. This field can drive inflation, and give a low energy universe in
very good agreement with the cosmological observations. The low energy dynamics
of the Standard Model cannot be unitarized by the Higgsflaton, which decouples
from the low energy theory, both because it picks up a large mass and because
its direct couplings to the low energy modes are weakened. Instead, the short
distance behavior of the Standard Model may be regulated by the dynamics of
other light degrees of freedom, such as in Higgsless models.Comment: 12 pages LaTeX, v2: takes into account the revised version of ref.
[26], v3: improved discussion of the origin of the action, version published
on PR
Surface Superconductivity in Niobium for Superconducting RF Cavities
A systematic study is presented on the superconductivity (sc) parameters of
the ultrapure niobium used for the fabrication of the nine-cell 1.3 GHz
cavities for the linear collider project TESLA. Cylindrical Nb samples have
been subjected to the same surface treatments that are applied to the TESLA
cavities: buffered chemical polishing (BCP), electrolytic polishing (EP),
low-temperature bakeout (LTB). The magnetization curves and the complex
magnetic susceptibility have been measured over a wide range of temperatures
and dc magnetic fields, and also for di erent frequencies of the applied ac
magnetic field. The bulk superconductivity parameters such as the critical
temperature Tc = 9.26 K and the upper critical field Bc2(0) = 410 mT are found
to be in good agreement with previous data. Evidence for surface
superconductivity at fields above Bc2 is found in all samples. The critical
surface field exceeds the Ginzburg-Landau field Bc3 = 1.695Bc2 by about 10% in
BCP-treated samples and increases even further if EP or LTB are applied. From
the field dependence of the susceptibility and a power-law analysis of the
complex ac conductivity and resistivity the existence of two different phases
of surface superconductivity can be established which resemble the Meissner and
Abrikosov phases in the bulk: (1) coherent surface superconductivity, allowing
sc shielding currents flowing around the entire cylindrical sample, for
external fields B in the range between Bc2 and Bcohc3, and (2) incoherent
surface superconductivity with disconnected sc domains between Bcohc3 and Bc3.
The coherent critical surface field separating the two phases is found to be
Bcoh c3 = 0.81Bc3 for all samples. The exponents in the power law analysis are
different for BCP and EP samples, pointing to different surface topologies.Comment: 15 pages, 21 figures, DESY-Report 2004-02
Cosmology of a brane radiating gravitons into the extra dimension
We study in a self-consistent way the impact of the emission of bulk
gravitons on the (homogeneous) cosmology of a three-brane embedded in a
five-dimensional spacetime. In the low energy regime, we recover the well known
result that the bulk affects the Friedmann equation only via a radiation-like
term \C/a^4, called dark or Weyl radiation. By contrast, in the high energy
regime, we find that the Weyl parameter \C is no longer constant but instead
grows very rapidly as \C\propto a^4. As a consequence, the value of \C
today is not a free parameter as usually considered but is a fixed number,
which, generically, depends only on the number of relativistic degrees of
freedom at the high/low energy transition. Our estimated amount of Weyl
radiation satisfies the present nucleosynthesis bounds.Comment: 12 page
Bulk gravitons from a cosmological brane
We investigate the emission of gravitons by a cosmological brane into an Anti
de Sitter five-dimensional bulk spacetime. We focus on the distribution of
gravitons in the bulk and the associated production of `dark radiation' in this
process. In order to evaluate precisely the amount of dark radiation in the
late low-energy regime, corresponding to standard cosmology, we study
numerically the emission, propagation and bouncing off the brane of bulk
gravitons.Comment: 27 pages, 5 figures, minor corrections. Final versio
Maximal Temperature in Flux Compactifications
Thermal corrections have an important effect on moduli stabilization leading
to the existence of a maximal temperature, beyond which the compact dimensions
decompactify. In this note, we discuss generality of our earlier analysis and
apply it to the case of flux compactifications. The maximal temperature is
again found to be controlled by the supersymmetry breaking scale, T_{crit} \sim
\sqrt{m_{3/2} M_P}.Comment: 10 pages, 10 figures. v2:comment and references adde
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