4,271 research outputs found
Dark-matter dynamical friction versus gravitational-wave emission in the evolution of compact-star binaries
The measured orbital period decay of compact-star binaries, with
characteristic orbital periods ~days, is explained with very high
precision by the gravitational wave (GW) emission of an inspiraling binary in
vacuum. However, the binary gravitational binding energy is also affected by an
usually neglected phenomenon, namely the dark matter dynamical friction (DMDF)
produced by the interaction of the binary components with their respective DM
gravitational wakes. The entity of this effect depends on the orbital period
and on the local value of the DM density, hence on the position of the binary
in the Galaxy. We evaluate the DMDF produced by three different DM profiles:
the Navarro-Frenk-White (NFW), the non-singular-isothermal-sphere (NSIS) and
the Ruffini-Arg\"uelles-Rueda (RAR) profile based on self-gravitating keV
fermions. We first show that indeed, due to their Galactic position, the GW
emission dominates over the DMDF in the NS-NS, NS-WD and WD-WD binaries for
which measurements of the orbital decay exist. Then, we evaluate the conditions
under which the effect of DMDF on the binary evolution becomes comparable to,
or overcomes, the one of the GW emission. We find that, for instance for
-- NS-WD, --~ NS-NS, and
--~ WD-WD, located at 0.1~kpc, this occurs at orbital
periods around 20--30 days in a NFW profile while, in a RAR profile, it occurs
at about 100 days. For closer distances to the Galactic center, the DMDF effect
increases and the above critical orbital periods become interestingly shorter.
Finally, we also analyze the system parameters for which DMDF leads to an
orbital widening instead of orbital decay. All the above imply that a
direct/indirect observational verification of this effect in compact-star
binaries might put strong constraints on the nature of DM and its Galactic
distribution.Comment: 15 pages, 12 figures, 2 tables, accepted for publication in Phys.
Rev. D, 201
Rational Hausdorff Divisors: a New approach to the Approximate Parametrization of Curves
In this paper we introduce the notion of rational Hausdorff divisor, we
analyze the dimension and irreducibility of its associated linear system of
curves, and we prove that all irreducible real curves belonging to the linear
system are rational and are at finite Hausdorff distance among them. As a
consequence, we provide a projective linear subspace where all (irreducible)
elements are solutions to the approximate parametrization problem for a given
algebraic plane curve. Furthermore, we identify the linear system with a plane
curve that is shown to be rational and we develop algorithms to parametrize it
analyzing its fields of parametrization. Therefore, we present a generic answer
to the approximate parametrization problem. In addition, we introduce the
notion of Hausdorff curve, and we prove that every irreducible Hausdorff curve
can always be parametrized with a generic rational parametrization having
coefficients depending on as many parameters as the degree of the input curve
Update on an Electromagnetic Basis for Inertia, Gravitation, the Principle of Equivalence, Spin and Particle Mass Ratios
A possible connection between the electromagnetic quantum vacuum and inertia
was first published by Haisch, Rueda and Puthoff (1994). If correct, this would
imply that mass may be an electromagnetic phenomenon and thus in principle
subject to modification, with possible technological implications for
propulsion. A multiyear NASA-funded study at the Lockheed Martin Advanced
Technology Center further developed this concept, resulting in an independent
theoretical validation of the fundamental approach (Rueda and Haisch, 1998ab).
Distortion of the quantum vacuum in accelerated reference frames results in a
force that appears to account for inertia. We have now shown that the same
effect occurs in a region of curved spacetime, thus elucidating the origin of
the principle of equivalence (Rueda, Haisch and Tung, 2001). A further
connection with general relativity has been drawn by Nickisch and Mollere
(2002): zero-point fluctuations give rise to spacetime micro-curvature effects
yielding a complementary perspective on the origin of inertia. Numerical
simulations of this effect demonstrate the manner in which a massless
fundamental particle, e.g. an electron, acquires inertial properties; this also
shows the apparent origin of particle spin along lines originally proposed by
Schroedinger. Finally, we suggest that the heavier leptons (muon and tau) may
be explainable as spatial-harmonic resonances of the (fundamental) electron.
They would carry the same overall charge, but with the charge now having
spatially lobed structure, each lobe of which would respond to higher frequency
components of the electromagnetic quantum vacuum, thereby increasing the
inertia and thus manifesting a heavier mass.Comment: 10 pages, 4 figures, AIP Conf. Proc., Space Technology and
Applications International Forum (STAIF-2003
Query Expansion of Zero-Hit Subject Searches: Using a Thesaurus in Conjunction with NLP Techniques
The focus of our study is zero-hit queries in keyword subject searches and the effort of increasing recall in these cases by reformulating and, then, expanding the initial queries using an external source of knowledge, namely a thesaurus. To this end, the objectives of this study are twofold. First, we perform the mapping of query terms to the thesaurus terms. Second, we use the matched terms to expand the user’s initial query by taking advantage of the thesaurus relations and implementing natural language processing (NLP) techniques. We report on the overall procedure and elaborate on key points and considerations of each step of the process
Strong lensing by fermionic dark matter in galaxies
It has been shown that a self-gravitating system of massive keV fermions in
thermodynamic equilibrium correctly describes the dark matter (DM) distribution
in galactic halos and predicts a denser quantum core towards the center of the
configuration. Such a quantum core, for a fermion mass in the range of keV
keV, can be an alternative interpretation of the
central compact object in Sgr A*. We present in this work the gravitational
lensing properties of this novel DM model in Milky Way-like spiral galaxies. We
describe the lensing effects of the pure DM component both on halo scales,
where we compare them to the effects of the Navarro-Frenk-White and the
Non-Singular Isothermal Sphere DM models, and near the galaxy center, where we
compare them with the effects of a Schwarzschild BH. For the particle mass
leading to the most compact DM core, keV, we draw the
following conclusions. At distances pc from the center of the
lens the effect of the central object on the lensing properties is negligible.
However, we show that measurements of the deflection angle produced by the DM
distribution in the outer region at a few kpc, together with rotation curve
data, could help to discriminate between different DM models. We show that at
distances pc strong lensing effects, such as multiple images and
Einstein rings, may occur. Large differences in the deflection angle produced
by a DM central core and a central BH appear at distances
pc; in this regime the weak-field formalism is no longer applicable and the
exact general-relativistic formula has to be used. We find that quantum DM
cores do not show a photon sphere what implies that they do not cast a shadow.
Similar conclusions apply to the other DM distributions for other fermion
masses in the above specified range and for other galaxy types.Comment: 10 pages, 8 figures. v2: Version published in PR
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