5,021 research outputs found
Dark matter vs. modifications of the gravitational inverse-square law. Results from planetary motion in the solar system
Dark matter or modifications of the Newtonian inverse-square law in the
solar-system are studied with accurate planetary astrometric data. From
extra-perihelion precession and possible changes in the third Kepler's law, we
get an upper limit on the local dark matter density, rho_{DM} < 3*10^{-16}
kg/m^3 at the 2-sigma confidence level. Variations in the 1/r^2 behavior are
considered in the form of either a possible Yukawa-like interaction or a
modification of gravity of MOND type. Up to scales of 10^{11} m,
scale-dependent deviations in the gravitational acceleration are really small.
We examined the MOND interpolating function mu in the regime of strong gravity.
Gradually varying mu suggested by fits of rotation curves are excluded, whereas
the standard form mu(x)= x/(1+x^2)^{1/2} is still compatible with data. In
combination with constraints from galactic rotation curves and theoretical
considerations on the external field effect, the absence of any significant
deviation from inverse square attraction in the solar system makes the range of
acceptable interpolating functions significantly narrow. Future radio ranging
observations of outer planets with an accuracy of few tenths of a meter could
either give positive evidence of dark matter or disprove modifications of
gravity.Comment: 7 pages, 4 figures, accepted for publication in MNRA
Signals of primordial phase transitions on CMB maps
The analysis of the CMB anisotropies is a rich source of cosmological
informations. In our study, we simulated the signals produced by the relics of
a first order phase transition occured during an inflationary epoch in the
early Universe. These relics are bubbles of true vacuum that leave a
characteristic non-Gaussian imprint on the CMB. We use different statistical
estimators in order to evaluate this non-Gaussianity. We obtain some limits on
the allowed values of the bubble parameters comparing our results with the
experimental data.
We also predict the possibility to detect this signal with the next high
resolution experiments.Comment: 2 pages, submitted to Proceedings of 9th Marcel Grossmann meetin
Present limits to cosmic bubbles from the COBE-DMR three point correlation function
The existence of large scale voids in several galaxy surveys suggests the
occurence of an inflationary first order phase transition. This process
generates primordial bubbles that, before evolving into the present voids,
leave at decoupling a non-Gaussian imprint on the CMB. I this paper we evaluate
an analytical expression of the collapsed three point correlation function from
the bubble temperature fluctuations. Comparing the results with COBE-DMR
measures, we obtain upper limits on the allowed non-Gaussianity and hence on
the bubble parameters.Comment: 4 pages, 3 figures; submitted to MNRA
Detecting stable massive neutral particles through particle lensing
Stable massive neutral particles emitted by astrophysical sources undergo
deflection under the gravitational potential of our own galaxy. The deflection
angle depends on the particle velocity and therefore non-relativistic particles
will be deflected more than relativistic ones. If these particles can be
detected through neutrino telescopes, cosmic ray detectors or directional dark
matter detectors, their arrival directions would appear aligned on the sky
along the source-lens direction. On top of this deflection, the arrival
direction of non-relativistic particles is displaced with respect to the
relativistic counterpart also due to the relative motion of the source with
respect to the observer; this induces an alignment of detections along the sky
projection of the source trajectory. The final alignment will be given by a
combination of the directions induced by lensing and source proper motion. We
derive the deflection-velocity relation for the Milky Way halo and suggest that
searching for alignments on detection maps of particle telescopes could be a
way to find new particles or new astrophysical phenomena.Comment: 17 pages, 7 figures. Accepted by PR
Observational Constraints on Silent Quartessence
We derive new constraints set by SNIa experiments (`gold' data sample of
Riess et al.), X-ray galaxy cluster data (Allen et al. Chandra measurements of
the X-ray gas mass fraction in 26 clusters), large scale structure (Sloan
Digital Sky Survey spectrum) and cosmic microwave background (WMAP) on the
quartessence Chaplygin model. We consider both adiabatic perturbations and
intrinsic non-adiabatic perturbations such that the effective sound speed
vanishes (Silent Chaplygin). We show that for the adiabatic case, only models
with equation of state parameter are allowed: this
means that the allowed models are very close to \LambdaCDM. In the Silent case,
however, the results are consistent with observations in a much broader range,
-0.3<\alpha<0.7.Comment: 7 pages, 12 figures, to be submitted to JCA
General CMB and Primordial Trispectrum Estimation
We present trispectrum estimation methods which can be applied to general
non-separable primordial and CMB trispectra. We present a general optimal
estimator for the connected part of the trispectrum, for which we derive a
quadratic term to incorporate the effects of inhomogeneous noise and masking.
We describe a general algorithm for creating simulated maps with given
arbitrary (and independent) power spectra, bispectra and trispectra. We propose
a universal definition of the trispectrum parameter , so that the
integrated bispectrum on the observational domain can be consistently compared
between theoretical models. We define a shape function for the primordial
trispectrum, together with a shape correlator and a useful parametrisation for
visualizing the trispectrum. We derive separable analytic CMB solutions in the
large-angle limit for constant and local models. We present separable mode
decompositions which can be used to describe any primordial or CMB bispectra on
their respective wavenumber or multipole domains. By extracting coefficients of
these separable basis functions from an observational map, we are able to
present an efficient estimator for any given theoretical model with a
nonseparable trispectrum. The estimator has two manifestations, comparing the
theoretical and observed coefficients at either primordial or late times. These
mode decomposition methods are numerically tractable with order
operations for the CMB estimator and approximately order for the general
primordial estimator (reducing to order in both cases for a special class
of models). We also demonstrate how the trispectrum can be reconstructed from
observational maps using these methods.Comment: 38 pages, 9 figures. In v2 Figures 4-7 are altered slightly and some
extra references are included in the bibliography. v3 matches version
submitted to journal. Includes discussion of special case
Non-local dilaton coupling to dark matter: cosmic acceleration and pressure backreaction
A model of non-local dilaton interactions, motivated by string duality
symmetries, is applied to a scenario of "coupled quintessence" in which the
dilaton dark energy is non-locally coupled to the dark-matter sources. It is
shown that the non-local effects tend to generate a backreaction which -- for
strong enough coupling -- can automatically compensate the acceleration due to
the negative pressure of the dilaton potential, thus asymptotically restoring
the standard (dust-dominated) decelerated regime. This result is illustrated by
analytical computations and numerical examples.Comment: 11 pages, 1 figure ep
Seismic performance of bridges isolated with FPS
The scope of the present study is focused on the evaluation of the seismic response of bridges isolated by single concave sliding pendulum isolators (FPS) for the different structural properties when the presence of the rigid abutment is considered or neglected (i.e., isolated viaducts). In this way, they have been defined two specific multi-degree-of-freedom (mdof) models to simulate the elastic behavior of the reinforced concrete pier in combination to the infinitely rigid presence of the deck and to the presence of the rigid abutment if considered. Both the numerical models also account for the non-linear velocity-dependent behavior of the FPS bearings. Considering the aleatory uncertainty in the seismic input by means of several natural records with different characteristics, a parametric analysis is developed for several structural properties. The relevant results expressed as the statistics in non-dimensional form with respect to the seismic intensity have permitted to study the differences between the two numerical models in relation to the effectiveness of the seismic isolation
Optimal DCFP bearing properties and seismic performance assessment in nondimensional form for isolated bridges
The study analyzes the influence of double concave friction pendulum (DCFP) isolator properties on the seismic performance of isolated multispan continuous deck bridges. The behavior of these systems is analyzed by employing an eight-degree-of-freedom model accounting for the pier flexibility in addition to the rigid presence of both abutment and deck, whereas the DCFP isolator behavior is described combining two single FP devices in series. The uncertainty in the seismic input is taken into account by considering a set of nonfrequent natural records with different characteristics. The variation of the statistics of the response parameters relevant to the seismic performance of the isolated bridges is investigated through the proposal of a nondimensionalization of the motion equations, with respect to the seismic intensity, within an extensive parametric study carried out for different isolator and bridge properties. Moreover, two cases related to different ratios between the sliding friction coefficients of the two surfaces of the DCFP devices are analyzed with the aim also to evaluate the corresponding optimal values able to minimize the seismic demand to the pier. In this way, all the presented nondimensional results are useful for the preliminary design or retrofit of multispan continuous deck bridges, isolated with DCFP devices, located in any site and in relation, especially, to the seismic ultimate limit states
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