3D simulations of the accretion process in Kerr space-time with arbitrary value of the spin parameter

We present the results of three-dimensional general relativistic hydrodynamic simulations of adiabatic and spherically symmetric accretion in Kerr space-time. We consider compact objects with spin parameter $|a_*| \le 1$ (black holes) and with $|a_*| > 1$ (super-spinars). Our full three-dimensional simulations confirm the formation of equatorial outflows for high values of $|a_*|$, as found in our previous work in 2.5 dimensions. We show that the critical value of $|a_*|$ determining the onset of powerful outflows depends mainly on the radius of the compact object. The phenomenon of equatorial outflows can hardly occur around a black hole and may thus be used to test the bound $|a_*| \le 1$ for astrophysical black hole candidates.Comment: 13 pages, 9 figures. v2: refereed versio

A revision of the Generalized Uncertainty Principle

The Generalized Uncertainty Principle arises from the Heisenberg Uncertainty Principle when gravity is taken into account, so the leading order correction to the standard formula is expected to be proportional to the gravitational constant $G_N = L_{Pl}^2$. On the other hand, the emerging picture suggests a set of departures from the standard theory which demand a revision of all the arguments used to deduce heuristically the new rule. In particular, one can now argue that the leading order correction to the Heisenberg Uncertainty Principle is proportional to the first power of the Planck length $L_{Pl}$. If so, the departures from ordinary quantum mechanics would be much less suppressed than what is commonly thought.Comment: 6 pages, 1 figur

Dangerous implications of a minimum length in quantum gravity

The existence of a minimum length and a generalization of the Heisenberg uncertainty principle seem to be two fundamental ingredients required in any consistent theory of quantum gravity. In this letter we show that they would predict dangerous processes which are phenomenologically unacceptable. For example, long--lived virtual super--Planck mass black holes may lead to rapid proton decay. Possible solutions of this puzzle are briefly discussed.Comment: 5 pages, no figure. v3: refereed versio

Suppression of thermal conductivity in graphene nanoribbons with rough edges

We analyze numerically the thermal conductivity of carbon nanoribbons with ideal and rough edges. We demonstrate that edge disorder can lead to a suppression of thermal conductivity by several orders of magnitude. This effect is associated with the edge-induced Anderson localization and suppression of the phonon transport, and it becomes more pronounced for longer nanoribbons and low temperatures.Comment: 6 pages, 8 figure

Thick disk accretion in Kerr space-time with arbitrary spin parameters

In this paper we extend our previous works on spherically symmetric accretion onto black holes and super-spinars to the case in which the fluid has a finite angular momentum initially. We run 2.5D and 3D general relativistic hydrodynamic simulations of the accretion of a fat disk. We study how the accretion process changes by changing the values of the parameters of our model. We show that the value of the fluid angular momentum critically determines turn-on and off the production of powerful equatorial outflows around super-spinars. For corotating disks, equatorial outflows are efficiently generated, even for relatively low spin parameters or relatively large super-spinar radii. For counterrotating disks, equatorial outflows are instead significantly suppressed, and they are possible only in limited cases. We also study accretion around a tilted disk.Comment: 11 pages, 10 figure

A note on the black hole information paradox in de Sitter spacetimes

The possibility of stable or quasi--stable Planck mass black hole remnants as solution to the black hole information paradox is commonly believed phenomenologically unacceptable: since we have to expect a black hole remnant for every possible initial state, the number of remnants should be infinite. This would lead to remnant pair production in any physical process with a total available energy roughly exceeding the Planck mass, against trivial evidences. In this note I point out that the number of remnants in our Universe could be finite, at least if the value of the cosmological constant is positive, as present observational data could indicate. Nevertheless, it is not clear if a huge but finite number of states is phenomenologically allowed.Comment: 4 pages, 1 figure. v3: refereed versio

Dark Energy and the mass of galaxy clusters

Up to now, Dark Energy evidences are based on the dynamics of the universe on very large scales, above 1 Gpc. Assuming it continues to behave like a cosmological constant $\Lambda$ on much smaller scales, I discuss its effects on the motion of non-relativistic test-particles in a weak gravitational field and I propose a way to detect evidences of $\Lambda \neq 0$ at the scale of about 1 Mpc: the main ingredient is the measurement of galaxy cluster masses.Comment: 5 pages, no figures, references adde

Natural extension of the Generalised Uncertainty Principle

We discuss a gedanken experiment for the simultaneous measurement of the position and momentum of a particle in de Sitter spacetime. We propose an extension of the so-called generalized uncertainty principle (GUP) which implies the existence of a minimum observable momentum. The new GUP is directly connected to the nonzero cosmological constant, which becomes a necessary ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as published in CQ