281 research outputs found
Massless and massive graviton spectra in anisotropic dilatonic braneworld cosmologies
We consider a braneworld model in which an anisotropic brane is embedded in a
dilatonic background. We solve the background solutions and study the behavior
of the perturbations when the universe evolves from an inflationary Kasner
phase to a Minkowski phase. We calculate the massless mode spectrum, and find
that it does not differ from what expected in standard four-dimensional
cosmological models. We then evaluate the spectrum of both light
(ultrarelativistic) and heavy (nonrelativistic) massive modes, and find that,
at high energies, there can be a strong enhancement of the Kaluza-Klein
spectral amplitude, which can become dominant in the total spectrum. The
presence of the dilaton, on the contrary, decrease the relative importance of
the massive modes.Comment: 18 pages, 4 figures, Typos correction
Mpemba effect and phase transitions in the adiabatic cooling of water before freezing
An accurate experimental investigation on the Mpemba effect (that is, the
freezing of initially hot water before cold one) is carried out, showing that
in the adiabatic cooling of water a relevant role is played by supercooling as
well as by phase transitions taking place at 6 +/- 1 oC, 3.5 +/- 0.5 oC and 1.3
+/- 0.6 oC, respectively. The last transition, occurring with a non negligible
probability of 0.21, has not been detected earlier. Supported by the
experimental results achieved, a thorough theoretical analysis of supercooling
and such phase transitions, which are interpreted in terms of different
ordering of clusters of molecules in water, is given.Comment: revtex, 4 pages, 2 figure
Small Sample Issues for Microarray-Based Classification
In order to study the molecular biological differences between normal and diseased tissues,
it is desirable to perform classification among diseases and stages of disease using
microarray-based gene-expression values. Owing to the limited number of microarrays
typically used in these studies, serious issues arise with respect to the design, performance
and analysis of classifiers based on microarray data. This paper reviews some fundamental
issues facing small-sample classification: classification rules, constrained classifiers, error
estimation and feature selection. It discusses both unconstrained and constrained classifier
design from sample data, and the contributions to classifier error from constrained
optimization and lack of optimality owing to design from sample data. The difficulty with
estimating classifier error when confined to small samples is addressed, particularly
estimating the error from training data. The impact of small samples on the ability to
include more than a few variables as classifier features is explained
On the seismic analysis and design of offshore wind turbines
Offshore wind farms are a collection of offshore wind turbines (OWTs) and are currently being installed in seismically active regions. An OWT consists of a long slender tower with a top-heavy fixed mass (Nacelle) together with a heavy rotating mass (Hub and blades) and is always exposed to variable environmental wind and wave loads. For dynamic analysis, an OWT can also be seen as an inverted pendulum (with over 25%–50% of the total mass concentrated in the upper 3rd of the tower), yet it is not granted that their seismic response is dominated by the first mode. Guidelines for the design of such special structures are not explicitly mentioned in current codes of practice. The aim of this technical note is to identify the design issues and provide a rational background for the seismic analysis. Where feasible, further research work that is needed is also identified and discussed.•Considerations for seismic design.•Design return period.•Types of seismic analysis.•Selection of input motion
T-duality for open strings in the presence of backgrounds and noncommutativity
We investigate the effect of T-duality on noncommutativity. Starting with
open strings ending on a D2-brane wrapped on a torus in the presence of a
Kalb Ramond field, we consider Buscher transformations on the coordinates and
background. We find that the dual system is commutative. We also study
alternative transformations that can preserve noncommutativity.Comment: In this V3: more references included. Version to appear in J.Phys.
Numerical simulations of the decay of primordial magnetic turbulence
We perform direct numerical simulations of forced and freely decaying 3D
magnetohydrodynamic turbulence in order to model magnetic field evolution
during cosmological phase transitions in the early Universe. Our approach
assumes the existence of a magnetic field generated either by a process during
inflation or shortly thereafter, or by bubble collisions during a phase
transition. We show that the final configuration of the magnetic field depends
on the initial conditions, while the velocity field is nearly independent of
initial conditions.Comment: 10 pages, 6 figures, references added, PRD accepte
Teleology and Realism in Leibniz's Philosophy of Science
This paper argues for an interpretation of Leibniz’s claim that physics requires both mechanical and teleological principles as a view regarding the interpretation of physical theories. Granting that Leibniz’s fundamental ontology remains non-physical, or mentalistic, it argues that teleological principles nevertheless ground a realist commitment about mechanical descriptions of phenomena. The empirical results of the new sciences, according to Leibniz, have genuine truth conditions: there is a fact of the matter about the regularities observed in experience. Taking this stance, however, requires bringing non-empirical reasons to bear upon mechanical causal claims. This paper first evaluates extant interpretations of Leibniz’s thesis that there are two realms in physics as describing parallel, self-sufficient sets of laws. It then examines Leibniz’s use of teleological principles to interpret scientific results in the context of his interventions in debates in seventeenth-century kinematic theory, and in the teaching of Copernicanism. Leibniz’s use of the principle of continuity and the principle of simplicity, for instance, reveal an underlying commitment to the truth-aptness, or approximate truth-aptness, of the new natural sciences. The paper concludes with a brief remark on the relation between metaphysics, theology, and physics in Leibniz
Space/Time Noncommutativity in String Theories without Background Electric Field
The appearance of space/time non-commutativity in theories of open strings
with a constant non-diagonal background metric is considered. We show that,
even if the space-time coordinates commute, when there is a metric with a
time-space component, no electric field and the boundary condition along the
spatial direction is Dirichlet, a Moyal phase still arises in products of
vertex operators. The theory is in fact dual to the non-commutatitive open
string (NCOS) theory. The correct definition of the vertex operators for this
theory is provided. We study the system also in the presence of a field. We
consider the case in which the Dirichlet spatial direction is compactified and
analyze the effect of these background on the closed string spectrum. We then
heat up the system. We find that the Hagedorn temperature depends in a
non-extensive way on the parameters of the background and it is the same for
the closed and the open string sectors.Comment: 18 pages, JHEP styl
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