10,181 research outputs found
Impact of the tidal p-g instability on the gravitational wave signal from coalescing binary neutron stars
Recent studies suggest that coalescing neutron stars are subject to a fluid
instability involving the nonlinear coupling of the tide to -modes and
-modes. Its influence on the inspiral dynamics and thus the gravitational
wave signal is, however, uncertain because we do not know precisely how the
instability saturates. Here we construct a simple, physically motivated model
of the saturation that allows us to explore the instability's impact as a
function of the model parameters. We find that for plausible assumptions about
the saturation, current gravitational wave detectors might miss of
events if only point particle waveforms are used. Parameters such as the chirp
mass, component masses, and luminosity distance might also be significantly
biased. On the other hand, we find that relatively simple modifications to the
point particle waveform can alleviate these problems and enhance the science
that emerges from the detection of binary neutron stars.Comment: 15 pages, 12 figures, 1 tabl
Gauss-Bonnet dark energy by Lagrange multipliers
A string-inspired effective theory of gravity, containing Gauss-Bonnet
invariant interacting with a scalar field, is considered in view of obtaining
cosmological dark energy solutions. A Lagrange multiplier is inserted into the
action in order to achieve the cosmological reconstruction by selecting
suitable forms of couplings and potentials. Several cosmological exact
solutions (including dark energy of quintessence, phantom or Little Rip type)
are derived in presence and in absence of the Lagrange multiplier showing the
difference in the two dynamical approaches. In the models that we consider, the
Lagrange multiplier behaves as a sort of dust fluid that realizes the
transitions between matter dominated and dark energy epochs. The relation
between Lagrange multipliers and Noether symmetries is discussed.Comment: 14 pages, expanded version to appear in PR
Positive or negative voting premium: what happened to private benefits in Italy?
A large body of research deals with voting premium as a proxy of private benefit of control. Almost all of them find positive voting premium, in particular in Italy. Therefore appears interesting to ask what is the current status of private benefits of control in Italy in the last decade (2007-2017). Surprisingly, we show three major findings: i) reduction of non-voting share in the Italian scenario; ii) prevalence of negative voting rights premium more than positive ones, thus conflicting with the assumption and the observations by other researchers; iii) limits of the voting premium method. Our aim is that this study, despite its limitations, may encourage further researches focused on the analysis of the improvement and the change in the Italian corporate governance. The article points out that interesting evidence already exists, although still much remains to do in the future
Generalized energy conditions in Extended Theories of Gravity
Theories of physics can be considered viable if the initial value problem and
the energy conditions are formulated self-consistently. The former allow a
uniquely determined dynamical evolution of the system, and the latter guarantee
that causality is preserved and that "plausible" physical sources have been
considered. In this work, we consider the further degrees of freedom related to
curvature invariants and scalar fields in Extended Theories of Gravity (ETG).
These new degrees of freedom can be recast as effective perfect fluids that
carry different meanings with respect to the standard matter fluids generally
adopted as sources of the field equations. It is thus somewhat misleading to
apply the standard general relativistic energy conditions to this effective
energy-momentum, as the latter contains the matter content and a geometrical
quantity, which arises from the ETG considered. Here, we explore this subtlety,
extending on previous work, in particular, to cases with the contracted Bianchi
identities with diffeomorphism invariance and to cases with generalized
explicit curvature-matter couplings, which imply the non-conservation of the
energy-momentum tensor. Furthermore, we apply the analysis to specific ETGs,
such as scalar-tensor gravity, gravity and modified Gauss-Bonnet
gravity. Interesting results appear such as matter that may exhibit unusual
thermodynamical features, for instance, and gravity that retains its attractive
character in the presence of negative pressures; or alternatively, we verify
that repulsive gravity may occur for standard matter.Comment: 12 pages, version accepted for publication in Phys.Rev.
Energy conditions in modified gravity
We consider generalized energy conditions in modified theories of gravity by
taking into account the further degrees of freedom related to scalar fields and
curvature invariants. The latter are usually recast as generalized {\it
geometrical fluids} that have different meanings with respect to the standard
matter fluids generally adopted as sources of the field equations. More
specifically, in modified gravity the curvature terms are grouped in a tensor
and a coupling that can be reorganized in effective
Einstein field equations, as corrections to the energy-momentum tensor of
matter. The formal validity of such inequalities does not assure some basic
requirements such as the attractive nature of gravity, so that the energy
conditions have to be considered in a wider sense.Comment: 4 pages. V2: 5 pages; version to appear in Physics Letters B. V3:
typo in Eq. (4) correcte
Exact charged black-hole solutions in D-dimensional f(T) gravity: torsion vs curvature analysis
We extract exact charged black-hole solutions with flat transverse sections
in the framework of D-dimensional Maxwell-f(T) gravity, and we analyze the
singularities and horizons based on both torsion and curvature invariants.
Interestingly enough, we find that in some particular solution subclasses there
appear more singularities in the curvature scalars than in the torsion ones.
This difference disappears in the uncharged case, or in the case where f(T)
gravity becomes the usual linear-in-T teleparallel gravity, that is General
Relativity. Curvature and torsion invariants behave very differently when
matter fields are present, and thus f(R) gravity and f(T) gravity exhibit
different features and cannot be directly re-casted each other.Comment: 24 pages, 3 figures. arXiv admin note: text overlap with
arXiv:1110.402
Patterns of trading profiles at the Nordic Stock Exchange. A correlation-based approach
We investigate the trading behavior of Finnish individual investors trading
the stocks selected to compute the OMXH25 index in 2003 by tracking the
individual daily investment decisions. We verify that the set of investors is a
highly heterogeneous system under many aspects. We introduce a correlation
based method that is able to detect a hierarchical structure of the trading
profiles of heterogeneous individual investors. We verify that the detected
hierarchical structure is highly overlapping with the cluster structure
obtained with the approach of statistically validated networks when an
appropriate threshold of the hierarchical trees is used. We also show that the
combination of the correlation based method and of the statistically validated
method provides a way to expand the information about the clusters of investors
with similar trading profiles in a robust and reliable way.Comment: 25 pages, 8 figure
Hybrid modified gravity unifying local tests, galactic dynamics and late-time cosmic acceleration
The non-equivalence between the metric and Palatini formalisms of
gravity is an intriguing feature of these theories. However, in the recently
proposed hybrid metric-Palatini gravity, consisting of the superposition of the
metric Einstein-Hilbert Lagrangian with an term constructed \`{a}
la Palatini, the "true" gravitational field is described by the interpolation
of these two non-equivalent approaches. The theory predicts the existence of a
light long-range scalar field, which passes the local constraints and affects
the galactic and cosmological dynamics. Thus, the theory opens new
possibilities for a unified approach, in the same theoretical framework, to the
problems of dark energy and dark matter, without distinguishing a priori matter
and geometric sources, but taking their dynamics into account under the same
standard.Comment: 8 pages. Received an Honorable Mention in the Gravity Research
Foundation Essay Contest 2013. V2: references added; version to appear in the
International Journal of Modern Physics
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