4,445 research outputs found
Echoes of ECOs: gravitational-wave signatures of exotic compact objects and of quantum corrections at the horizon scale
Gravitational waves from binary coalescences provide one of the cleanest
signatures of the nature of compact objects. It has been recently argued that
the post-merger ringdown waveform of exotic ultracompact objects is initially
identical to that of a black-hole, and that putative corrections at the horizon
scale will appear as secondary pulses after the main burst of radiation. Here
we extend this analysis in three important directions: (i) we show that this
result applies to a large class of exotic compact objects with a photon sphere
for generic orbits in the test-particle limit; (ii) we investigate the
late-time ringdown in more detail, showing that it is universally characterized
by a modulated and distorted train of "echoes" of the modes of vibration
associated with the photon sphere; (iii) we study for the first time
equal-mass, head-on collisions of two ultracompact boson stars and compare
their gravitational-wave signal to that produced by a pair of black-holes. If
the initial objects are compact enough as to mimic a binary black-hole
collision up to the merger, the final object exceeds the maximum mass for boson
stars and collapses to a black-hole. This suggests that - in some
configurations - the coalescence of compact boson stars might be almost
indistinguishable from that of black-holes. On the other hand, generic
configurations display peculiar signatures that can be searched for in
gravitational-wave data as smoking guns of exotic compact objects.Comment: 13 pages, RevTex4. v2: typo in equation 7 corrected, references
added, to appear in PR
Astrophysical signatures of boson stars: quasinormal modes and inspiral resonances
Compact bosonic field configurations, or boson stars, are promising dark
matter candidates which have been invoked as an alternative description for the
supermassive compact objects in active galactic nuclei. Boson stars can be
comparable in size and mass to supermassive black holes and they are hard to
distinguish by electromagnetic observations. However, boson stars do not
possess an event horizon and their global spacetime structure is different from
that of a black hole. This leaves a characteristic imprint in the
gravitational-wave emission, which can be used as a discriminant between black
holes and other horizonless compact objects. Here we perform a detailed study
of boson stars and their gravitational-wave signatures in a fully relativistic
setting, a study which was lacking in the existing literature in many respects.
We construct several fully relativistic boson star configurations, and we
analyze their geodesic structure and free oscillation spectra, or quasinormal
modes. We explore the gravitational and scalar response of boson star
spacetimes to an inspiralling stellar-mass object and compare it to its black
hole counterpart. We find that a generic signature of compact boson stars is
the resonant-mode excitation by a small compact object on stable quasi-circular
geodesic motion.Comment: 20 pages, 8 figures. v2: minor corrections, version to be published
in Phys. Rev. D. v3: final versio
Light rings as observational evidence for event horizons: long-lived modes, ergoregions and nonlinear instabilities of ultracompact objects
Ultracompact objects are self-gravitating systems with a light ring. It was
recently suggested that fluctuations in the background of these objects are
extremely long-lived and might turn unstable at the nonlinear level, if the
object is not endowed with a horizon. If correct, this result has important
consequences: objects with a light ring are black holes. In other words, the
nonlinear instability of ultracompact stars would provide a strong argument in
favor of the "black hole hypothesis," once electromagnetic or
gravitational-wave observations confirm the existence of light rings. Here we
explore in some depth the mode structure of ultracompact stars, in particular
constant-density stars and gravastars. We show that the existence of very
long-lived modes -- localized near a second, stable null geodesic -- is a
generic feature of gravitational perturbations of such configurations. Already
at the linear level, such modes become unstable if the object rotates
sufficiently fast to develop an ergoregion. Finally, we conjecture that the
long-lived modes become unstable under fragmentation via a
Dyson-Chandrasekhar-Fermi mechanism at the nonlinear level. Depending on the
structure of the star, it is also possible that nonlinearities lead to the
formation of small black holes close to the stable light ring. Our results
suggest that the mere observation of a light ring is a strong evidence for the
existence of black holes.Comment: 10 pages, RevTeX
A multi-layer edge-on single photon counting silicon microstrip detector for innovative techniques in diagnostic radiology
A three-layer detector prototype, obtained by stacking three edge-on single photon counting silicon microstrip detectors, has been developed and widely tested. This was done in the framework of the Synchrotron Radiation for Medical Physics/Frontier Radiology (SYRMEP/FRONTRAD) collaboration activities, whose aim is to improve the quality of mammographic examinations operating both on the source and on the detector side. The active surface of the device has been fully characterized making use of an edge-scanning technique and of a well-collimated laminar synchrotron radiation beam. The obtained data (interlayer distances, channel correspondence, etc.) have then been used to combine information coming from each detector layer, without causing any loss in spatial and contrast resolution of the device. Contrast and spatial resolution have also been separately evaluated for each detector layer. Moreover, imaging techniques (phase contrast, refraction, and scatter imaging), resulting in an increased visibility of low absorbing details, have been implemented, and their effectiveness has been tested on a biological sample. Finally, the possibility of simultaneously acquiring different kind of images with the different detector layers is discussed. This would result in maximizing the information extracted from the sample, while at the same time the high absorption efficiency of the detector device would allow a low dose delivery
Gravitational instabilities of superspinars
Superspinars are ultracompact objects whose mass M and angular momentum J
violate the Kerr bound (cJ/GM^2>1). Recent studies analyzed the observable
consequences of gravitational lensing and accretion around superspinars in
astrophysical scenarios. In this paper we investigate the dynamical stability
of superspinars to gravitational perturbations, considering either purely
reflecting or perfectly absorbing boundary conditions at the "surface" of the
superspinar. We find that these objects are unstable independently of the
boundary conditions, and that the instability is strongest for relatively small
values of the spin. Also, we give a physical interpretation of the various
instabilities that we find. Our results (together with the well-known fact that
accretion tends to spin superspinars down) imply that superspinars are very
unlikely astrophysical alternatives to black holes.Comment: 15 pages, 9 figures, 1 table. v2: Fig. 8 and Section I improved. v3:
minor changes to match the published versio
Gravitational waves from extreme mass-ratio inspirals in Dynamical Chern-Simons gravity
Dynamical Chern-Simons gravity is an interesting extension of General
Relativity, which finds its way in many different contexts, including string
theory, cosmological settings and loop quantum gravity. In this theory, the
gravitational field is coupled to a scalar field by a parity-violating term,
which gives rise to characteristic signatures. Here we investigate how
Chern-Simons gravity would affect the quasi-circular inspiralling of a small,
stellar-mass object into a large non-rotating supermassive black hole, and the
accompanying emission of gravitational and scalar waves. We find the relevant
equations describing the perturbation induced by the small object, and we solve
them through the use of Green's function techniques. Our results show that for
a wide range of coupling parameters, the Chern-Simons coupling gives rise to an
increase in total energy flux, which translates into a fewer number of
gravitational-wave cycles over a certain bandwidth. For space-based
gravitational-wave detectors such as LISA, this effect can be used to constrain
the coupling parameter effectively.Comment: RevTex4, 18 pages, 7 figures, 1 tabl
Bone Remodelling in BioShape
AbstractMany biological phenomena are inherently multiscale, i.e. they are characterised by interactions involving different scales at the same time. This is the case of bone remodelling, where macroscopic behaviour (at organ and tissue scale) and microstructure (at cell scale) strongly influence each other. Consequently, several approaches have been defined to model such a process at different spatial and temporal levels and, in particular, in terms of continuum properties, abstracting in this way from a realistic – and more complex – cellular scenario. While a large amount of information is available to validate such models separately, more work is needed to integrate all levels fully in a faithful multiscale model.In this scenario, we propose the use of BioShape, a 3D particle-based, scale-independent, geometry and space oriented simulator. It is used to define and integrate a cell and tissue scale model for bone remodelling in terms of shapes equipped with perception, interaction and movement capabilities. Their in-silico simulation allows for tuning continuum-based tissutal and cellular models, as well as for better understanding – both in qualitative and in quantitative terms – the blurry synergy between mechanical and metabolic factors triggering bone remodelling
Sustainable Concrete with Recycled Aggregates: experiences and perspective
The recycling of concrete construction and demolition wastes to obtain coarse recycled aggregates for structural concrete production represents an interesting strategy fostering circular economy in the construction sector. In this work, the effects of parent concretes on coarse recycled aggregates and on new structural concretes produced with different replacement percentages of recycled aggregates have been investigated. The quality of parent concrete seems not directly related to the mechanical properties of the concrete prepared with recycled aggregates, while the mix design plays a key role. In addition, tests on concrete specimens (i.e. plinths) have been carried out to demonstrate the feasibility of structural elements with recycled aggregates concrete. In the manuscript we present an overview of these results, highlighting pros and cons of using concrete with recycled aggregates for future developments of the concrete construction market, also stressing the influence of climate change
Effect of parent concrete on the performance of recycled aggregate concrete
Recycling concrete construction waste is a promising way towards sustainable construction. Indeed, replacing natural aggregates with recycled aggregates obtained from concrete waste lowers the environmental impact of concrete constructions and improves natural resource conservation. This paper reports on an experimental study on mechanical and durability properties of concretes casted with recycled aggregates obtained from two different parent concretes, belonging to two structural elements of the old Cagliari stadium. The effects of parent concretes on coarse recycled aggregates and on new structural concretes produced with different replacement percentages of these recycled aggregates are investigated. Mechanical properties (compressive strength, modulus of elasticity, and splitting tensile strength) and durability properties (water absorption, freeze thaw, and chloride penetration resistance) are experimentally evaluated and analyzed as fundamental features to assess structural concrete behavior. The results show that the mechanical performance of recycled concrete is not related to the parent concrete characteristics. Furthermore, the resistance to pressured water penetration is not reduced by the presence of recycled aggregates, and instead, it happens for the chloride penetration resistance. The resistance to frost–thawing seems not related to the recycled aggregates replacement percentage, while an influence of the parent concrete has been assessed
- …