1,721,222 research outputs found
Acoustic horizons for axially and spherically symmetric fluid flow
No full textWe investigate the formation of acoustic horizons for an inviscid fluid moving in a pipe in the case of stationary, axi-symmetric, one-dimensional (rotations around the symmetry axis are absent) flow. We show that, differently from what is generally believed, the acoustic horizon forms in correspondence to either a local minimum or maximum of the flux tube cross-section. Similarly, the external potential is required to have either a maximum or a minimum at the horizon, so that the external force has to vanish there. Choosing a power-law equation of state for the fluid, P proportional to p(n), we solve the equations of the fluid dynamics and show that the two possibilities are realized, respectively, for n > -I and n < - 1. The Chaplygin gas, characterized by n = - 1, corresponds to the crossover between the normal and unusual behaviour. These results are also extended to the case of spherically symmetric flow. RI Pani, Paolo/G-7412-2012 OI Pani, Paolo/0000-0003-4443-176
I-Love-Q relations for gravastars and the approach to the black-hole limit
No full textThe multipole moments and the tidal Love numbers of neutron stars and quark stars satisfy certain relations which are almost insensitive to the star’s internal structure. A natural question is whether the same relations hold for different compact objects and how they possibly approach the black-hole limit. Here we consider “gravastars,” which are hypothetical compact objects sustained by their internal vacuum energy. Such solutions have been proposed as exotic alternatives to the black-hole paradigm because they can be as compact as black holes and exist in any mass range. By constructing slowly rotating, thin-shell gravastars to quadratic order in the spin, we compute the moment of inertia I, the mass quadrupole moment Q, and the tidal Love number λ in exact form. The I-λ-Q relations of a gravastar are dramatically different from those of an ordinary compact star, but the black-hole limit is continuous; i.e., these quantities approach their Kerr counterparts when the compactness is maximum. Therefore, such relations can be used to discern a gravastar from an ordinary compact star but not to break the degeneracy with the black-hole case. Based on these results, we conjecture that the full multipolar structure and the tidal deformability of a spinning, ultracompact gravastar are identical to those of a Kerr black hole. The approach to the black-hole limit is nonpolynomial, thus differing from the critical behavior recently found for strongly anisotropic neutron stars
Binary pulsars as dark-matter probes
No full textDuring the motion of a binary pulsar around the Galactic center, the pulsar and its companion experience a wind of dark-matter particles that can affect the orbital motion through dynamical friction. We show that this effect produces a characteristic seasonal modulation of the orbit and causes a secular change of the orbital period whose magnitude can be well within the astonishing precision of various binary-pulsar observations. Our analysis is valid for binary systems with orbital period longer than a day. By comparing this effect with pulsar-timing measurements, it is possible to derive model-independent upper bounds on the dark-matter density at different distances D from the Galactic center. For example, the precision timing of J1713+0747 imposes ρDM≲105 GeV/cm3 at D≈7 kpc. The detection of a binary pulsar at D≲10 pc could provide stringent constraints on dark-matter halo profiles and on growth models of the central black hole. The Square Kilometer Array can improve current bounds by 2 orders of magnitude, potentially constraining the local density of dark matter to unprecedented levels
Marco Antonio Pani, Paolo Carboni, Capo e croce: Le ragioni dei pastori, Bianco e nero, sonoro, 104’, Italia, 2013
Full text linkFilm review of Marco Antonio Pani, Paolo Carboni, Capo e croce: Le ragioni dei pastori, Bianco e nero, sonoro, 104’, Italia, 2013.Recensione del film-documentario di Marco Antonio Pani, Paolo Carboni, Capo e Croce. Le ragioni dei pastori, Bianco e nero, sonoro, 104’, Italia, 2013
PERTURBATIONS OF SPINNING BLACK HOLES: SLOW-ROTATION FRAMEWORK
No full textWe discuss a method to study linear perturbations of generic rotating spacetimes in the slow-rotation limit. The framework is valid for any perturbation field and it is particularly advantageous when the field equations are not separable. Using this approach, we show that massive vector perturbations in the Kerr metric exibit strong superradiant instabilities, which put competitive constraints on the mass of the photon: mγ ≲ 10−20 eV
Love in extrema ratio
Full text linkThe tidal deformability of a self-gravitating object leaves an imprint on the gravitational-wave signal of an inspiral which is paramount to measure the internal structure of the binary components. We unveil here a surprisingly unnoticed effect: in the extreme mass-ratio limit the tidal Love number of the central object (i.e. the quadrupole moment induced by the tidal field of its companion) affects the gravitational waveform at the leading order in the mass ratio. This effect acts as a magnifying glass for the tidal deformability of supermassive objects but was so far neglected, probably because the tidal Love numbers of a black hole (the most natural candidate for a compact supermassive object) are identically zero. We argue that extreme mass-ratio inspirals detectable by the future laser interferometric space antenna (LISA) mission might place constraints on the tidal Love numbers of the central object which are roughly eight orders of magnitude more stringent than current ones on neutron stars, potentially probing all models of black hole mimickers proposed so far
Stochastic gravitational-wave background at 3G detectors as a smoking gun for microscopic dark matter relics
Full text linkMicroscopic horizonless relics could form in the early universe either directly through gravitational collapse or as stable remnants of the Hawking evaporation of primordial black holes. In both cases they completely or partially evade cosmological constraints arising from Hawking evaporation and in certain mass ranges can explain the entirety of the dark matter. We systematically explore the stochastic gravitational-wave background associated with the formation of microscopic dark-matter relics in various scenarios, adopting an agnostic approach and discussing the limitations introduced by existing constraints, possible ways to circumvent the latter, and expected astrophysical foregrounds. Interestingly, this signal is at most marginally detectable with current interferometers but could be detectable by third-generations instruments such as the Einstein Telescope, strengthening their potential as discovery machines
Linear stability of nonbidiagonal black holes in massive gravity
No full textWe consider generic linear perturbations of a nonbidiagonal class of static black-hole solutions in massive (bi-)gravity. We show that the quasinormal spectrum of these solutions coincides with that of a Schwarzschild black hole in general relativity, thus proving that these solutions are mode stable. This is in contrast to the case of bidiagonal black-hole solutions which are affected by a radial instability. On the other hand, the full set of perturbation equations is generically richer than that of a Schwarzschild black hole in general relativity, and this affects the linear response of the black hole to external perturbations. Finally, we argue that the generalization of these solutions to the spinning case does not suffer from the superradiant instability, despite the fact that the theory describes a massive graviton
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