2,300 research outputs found
Homogeneous and isotropic big rips?
We investigate the way big rips are approached in a fully inhomogeneous
description of the space-time geometry. If the pressure and energy densities
are connected by a (supernegative) barotropic index, the spatial gradients and
the anisotropic expansion decay as the big rip is approached. This behaviour is
contrasted with the usual big-bang singularities. A similar analysis is
performed in the case of sudden (quiescent) singularities and it is argued that
the spatial gradients may well be non-negligible in the vicinity of pressure
singularities.Comment: 8 page
Assisting pre-big bang phenomenology through short-lived axions
We present the results of a detailed study of how isocurvature axion
fluctuations are converted into adiabatic metric perturbations through axion
decay, and discuss the constraints on the parameters of pre-big bang cosmology
needed for consistency with present CMB-anisotropy data. The large-scale
normalization of temperature fluctuations has a non-trivial dependence both on
the mass and on the initial value of the axion. In the simplest, minimal models
of pre-big bang inflation, consistency with the COBE normalization requires a
slightly tilted (blue) spectrum, while a strictly scale-invariant spectrum
requires mild modifications of the minimal backgrounds at large curvature
and/or string coupling.Comment: 14 pages, latex, 1 figure included using epsfig. A few typos
corrected, two references added, the figure slightly improved. To appear in
Phys. Lett.
Constraints on pre-big bang parameter space from CMBR anisotropies
The so-called curvaton mechanism --a way to convert isocurvature
perturbations into adiabatic ones-- is investigated both analytically and
numerically in a pre-big bang scenario where the role of the curvaton is played
by a sufficiently massive Kalb--Ramond axion of superstring theory. When
combined with observations of CMBR anisotropies at large and moderate angular
scales, the present analysis allows us to constrain quite considerably the
parameter space of the model: in particular, the initial displacement of the
axion from the minimum of its potential and the rate of evolution of the
compactification volume during pre-big bang inflation. The combination of
theoretical and experimental constraints favours a slightly blue spectrum of
scalar perturbations, and/or a value of the string scale in the vicinity of the
SUSY-GUT scale.Comment: 63 pages in Latex style with 14 figures include
Kink-antikink, trapping bags and five-dimensional Gauss-Bonnet gravity
Five-dimensional Gauss-Bonnet gravity, with one warped extra-dimension,
allows classes of solutions where two scalar fields combine either in a
kink-antikink system or in a trapping bag configuration. While the
kink-antikink system can be interpreted as a pair of gravitating domain walls
with opposite topological charges, the trapping bag solution consists of a
domain wall supplemented by a non-topological defect. In both classes of
solutions, for large absolute values of the bulk coordinate (i.e. far from the
core of the defects), the geometry is given by five-dimensional anti-de Sitter
space.Comment: 8 pages, 2 figure
Electric-magnetic duality and the conditions of inflationary magnetogenesis
The magnetogenesis scenarios triggered by the early variation of the gauge
coupling are critically analyzed. In the absence of sources, it is shown that
the electric and magnetic power spectra can be explicitly computed by means of
electric-magnetic duality transformations. The remnants of a pre-inflationary
expansion and the reheating process break explicitly electric-magnetic duality
by inducing Ohmic currents. The generation of large-scale magnetic field and
the physical distinction between electric and magnetic observables stems, in
this class of models, from the final value reached by the conductivity of the
plasma right after inflation. Specific numerical examples are given. The
physical requirements of viable magnetogenesis scenarios are spelled out.Comment: 25 pages, 9 figure
A giant radio halo in the low luminosity X-ray cluster Abell 523
Radio halos are extended and diffuse non-thermal radio sources present at the
cluster center, not obviously associated with any individual galaxy. A strong
correlation has been found between the cluster X-ray luminosity and the halo
radio power. We observe and analyze the diffuse radio emission present in the
complex merging structure Abell 523, classified as a low luminosity X-ray
cluster, to discuss its properties in the context of the halo total radio power
versus X-ray luminosity correlation. We reduced VLA archive observations at 1.4
GHz to derive a deep radio image of the diffuse emission, and compared radio,
optical, and X-ray data. Low-resolution VLA images detect a giant radio halo
associated with a complex merging region. The properties of this new halo agree
with those of radio halos in general discussed in the literature, but its radio
power is about a factor of ten higher than expected on the basis of the cluster
X-ray luminosity. Our study of this giant radio source demonstrates that radio
halos can also be present in clusters with a low X-ray luminosity. Only a few
similar cases have so far been found . This result suggests that this source
represent a new class of objects, that cannot be explained by classical radio
halo models. We suggest that the particle reacceleration related to merging
processes is very efficient and/or the X-ray luminosity is not a good indicator
of the past merging activity of a cluster.Comment: 5 pages, 6 figures, Astronomy and Astrophysics Letter in pres
Fluid phonons and inflaton quanta at the protoinflationary transition
Quantum and thermal fluctuations of an irrotational fluid are studied across
the transition regime connecting a protoinflationary phase of decelerated
expansion to an accelerated epoch driven by a single inflaton field. The
protoinflationary inhomogeneities are suppressed when the transition to the
slow roll phase occurs sharply over space-like hypersurfaces of constant energy
density. If the transition is delayed, the interaction of the quasi-normal
modes related, asymptotically, to fluid phonons and inflaton quanta leads to an
enhancement of curvature perturbations. It is shown that the dynamics of the
fluctuations across the protoinflationary boundaries is determined by the
monotonicity properties of the pump fields controlling the energy transfer
between the background geometry and the quasi-normal modes of the fluctuations.
After corroborating the analytical arguments with explicit numerical examples,
general lessons are drawn on the classification of the protoinflationary
transition.Comment: 30 pages, 3 figure
The nature of the giant diffuse non-thermal source in the A3411-A3412 complex
VLA deep radio images at 1.4 GHz in total intensity and polarization reveal a
diffuse non-thermal source in the interacting clusters A3411 - A3412. Moreover
a small-size low power radio halo at the center of the merging cluster A3411 is
found. We present here new optical and X-ray data and discuss the nature and
properties of the diffuse non-thermal source. We suggest that the giant diffuse
radio source is related to the presence of a large scale filamentary structure
and to multiple mergers in the A3411-A3412 complex.Comment: 7 pages, 6 figures, accepted for publication in MNRA
The role of venture capital and private equity for innovation and development of SMEs : evidence from Italian puzzle
Numerous studies have discussed that even if fundamental for innovation and
economic growth, SMEs are often financially more constrained than large firms.
Therefore, venture capitalists are often the only available sources of financing to
small and young companies. Through the analysis of a database that includes 160
funding deals signed in Italy, we research for empirical evidence of the
determinants and effects of VC and PE investments. We find that VC and PE
funds are more likely to finance younger and smaller firms. We confirm the
presence of the certification effect under new circumstances applying to SMEs
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