1,721,087 research outputs found
Black holes as antimatter factories
No full textWe consider accretion of matter onto a low mass black hole surrounded by ionized medium. We show that, because of the higher mobility of protons than electrons, the black hole would acquire positive electric charge. If the black hole's mass is about or below 10 20 g, the electric field at the horizon can reach the critical value which leads to vacuum instability and electron-positron pair production by the Schwinger mechanism. Since the positrons are ejected by the emergent electric field, while electrons are back-captured, the black hole operates as an antimatter factory which effectively converts protons into positrons. © 2009 IOP Publishing Ltd and SISSA
A black hole conjecture and rare decays in theories with low scale gravity
No full textAbstract: In models with large extra dimensions, where the fundamental gravity scale can be in the electroweak range, gravitational effects in particle physics may be noticeable even at relatively low energies. In this paper, we perform simple estimates of the decays of elementary particles with a black hole intermediate state. Since black holes are believed to violate global symmetries, particle decays can violate lepton and baryon numbers. Whereas previous literature has claimed incompatibility between these rates (e.g. -decay) and existing experimental bounds, we find suppressed baryon and lepton-violating rates due to a new conjecture about the nature of the virtual black holes. We assume here that black holes lighter than the (effective) Planck mass must have zero electric and color charge and zero angular momentum -- this statement is true in classical general relativity and we make the conjecture that it holds in quantum gravity as well. If true, the rates for proton-decay, neutron-antineutron oscillations, and lepton-violating rare decays are suppressed to below experimental bounds even for large extra dimensions with TeV-scale gravity. Neutron-antineutron oscillations and anomalous decays of muons, -leptons, and and -mesons open a promising possibility to observe TeV gravity effects with a minor increase of existing experimental accuracy
Non-singular quantum-inspired gravitational collapse
Full text linkWe consider general relativistic homogeneous gravitational collapses for dust and radiation. We show that replacing the density profile with an effective density justified by some quantum gravity framework leads to the avoidance of the final singularity. The effective density acts on the collapsing cloud by introducing an isotropic pressure, which is negligible at the beginning of the collapse and becomes negative and dominant in the strong field regime. Event horizons never form and therefore the outcome of the collapse is not a black hole, in the sense that there are no regions causally disconnected from future null infinity. Apparent horizons form when the mass of the object exceeds a critical value, disappear when the matter density approaches an upper bound and gravity becomes very weak (asymptotic freedom regime), form again after the bounce as a consequence of the decrease in the matter density, and eventually disappear when the density becomes too low and the matter is radiated away. The possibility of detecting radiation coming from the high density region of a collapsing
astrophysical object in which classically there would be the creation of a singularity could open a new window to experimentally test theories of quantum gravity
Testing conformal gravity with astrophysical black holes
No full textWeyl conformal symmetry can solve the problem the spacetime singularities present in Einstein's gravity. In a recent paper, two of us have found a singularity-free rotating black hole solution in conformal gravity. In addition to the mass M and the spin angular momentum J of the black hole, the new solution has a new parameter, L, which here we consider to be proportional to the black hole mass. Since the solution is conformally equivalent to the Kerr metric, photon trajectories are unchanged, while the structure of an accretion disk around a black hole is affected by the value of the parameter L. In this paper, we show that x-ray data of astrophysical black holes require L/M < 1.2
Primordial black holes and the observed Galactic 511 keV line
No full textThe observed 511 keV line from the Galactic Bulge is a real challenge for theoretical astrophysics: despite a lot of suggested mechanisms, there is still no convincing explanation and the origin of the annihilated positrons remains unknown. Here we discuss the possibility that a population of slowly evaporating primordial black holes with the mass around g ejects (among other particles) low--energy positrons into the Galaxy. In addition to positrons, we have also calculated the spectrum and number density of photons and neutrinos produced by such black holes and found that the photons are potentially observable in the near future, while the neutrino flux is too weak and below the terrestrial and extra--terrestrial backgrounds. Depending on their mass distribution, such black holes could make a small fraction or the whole cosmological dark matter
Spacetime completeness of non-singular black holes in conformal gravity
No full textWe explicitly prove that the Weyl conformal symmetry solves the black hole singularity problem, otherwise unavoidable in a generally covariant local or non-local gravitational theory. Moreover, we yield explicit examples of local and non-local theories enjoying Weyl and diffeomorphism symmetry (in short co-covariant theories). Following the seminal paper by Narlikar and Kembhavi, we provide an explicit construction of singularity-free spherically symmetric and axi-symmetric exact solutions for black hole spacetimes conformally equivalent to the Schwarzschild or the Kerr spacetime. We first check the absence of divergences in the Kretschmann invariant for the rescaled metrics. Afterwords, we show that the new types of black holes are geodesically complete and linked by a Newman-Janis transformation just as in standard general relativity (based on Einstein-Hilbert action). Furthermore, we argue that no massive or massless particles can reach the former Schwarzschild singularity or touch the former Kerr ring singularity in a finite amount of their proper time or of their affine parameter. Finally, we discuss the Raychaudhuri equation in a co-covariant theory and we show that the expansion parameter for congruences of both types of geodesics (for massless and massive particles) never reaches minus infinity. Actually, the null geodesics become parallel at the r=0 point in the Schwarzschild spacetime (the origin) and the focusing of geodesics is avoided. The arguments of regularity of curvature invariants, geodesic completeness, and finiteness of geodesics' expansion parameter ensure us that we are dealing with singularity-free and geodesically-complete black hole spacetimes
A general study of regular and singular black hole solutions in Einstein’s conformal gravity
No full textAbstract We study some general properties of two black hole solutions in Einstein’s conformal gravity. Both solutions can be obtained from the Kerr metric with a suitable conformal rescaling, which leads, respectively, to a regular and a singular spacetime. In addition to the mass M and the spin angular momentum J of the black hole, these solutions are characterized by a new parameter, L, which may be expected to be of the order of the black hole mass. We study the geodesic motion and we calculate the radiative efficiency of a putative accretion disk around these black holes, which allows us to get an estimate of an upper bound on the value of L. Lastly, we explore the possibility of “destroying” the event horizon of these black holes. Within our approach, we are not able to destroy the event horizon of the regular black hole solution, while we can in the case of the singular one
Formation and evaporation of an electrically charged black hole in conformal gravity
No full textAbstract Extending previous work on the formation and the evaporation of black holes in conformal gravity, in the present paper we study the gravitational collapse of a spherically symmetric and electrically charged thin shell of radiation. The process creates a singularity-free black hole. Assuming that in the evaporation process the charge Q is constant, the final product of the evaporation is an extremal remnant with M=Q , which is reached in an infinite amount of time. We also discuss the issue of singularity and thermodynamics of black holes in Weyl’s conformal gravity
How to Detect X-Rays and Gamma-Rays from Space: Optics and Detectors
Full text linkThe measurable quantities of the sky’s light, for any wavelength, are energy, position, arrival time, and polarization. Each of them reveal different information about the science target (e.g. gas dynamics, state and distribution of the matter, temperature, luminosity) and require specific detecting solutions. In the study of X-rays and gamma-rays up to the TeV regime, their absorption by the atmosphere (by 50% at 30 km altitude for 1 MeV photon) requires the development of space applications. The science goals of the mission define which technological benchmark should be maximised (e.g. energy or spatial resolution), but the final design of high energy instruments is the result of a trade-off analysis among the detection specifications, the need for space-borne electronic systems and materials, and the limited resources in mass budget, electrical power, and telemetry rates
The Dawn of Black Holes
Full text linkIn the last decades, luminous accreting super-massive black holes have been
discovered within the first Gyr after the Big Bang, but their origin is still
an unsolved mystery. We discuss our state-of-the-art theoretical knowledge of
their formation physics and early growth, and describe the results of dedicated
observational campaigns in the X-ray band. We also provide an overview of how
these systems can be used to derive cosmological parameters. Finally, we point
out some open issues, in light of future electro-magnetic and
gravitational-wave astronomical facilities.Comment: This Chapter will appear in the Section "Active Galactic Nuclei in X
and Gamma-rays" (Section Editors: A. de Rosa, C. Vignali) of the "Handbook of
X-ray and Gamma-ray Astrophysics" (Editors in chief: C. Bambi and A.
Santangelo
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