641 research outputs found
Current-carrying string loops in black-hole spacetimes with a repulsive cosmological constant
Current-carrying string loop dynamics in Schwarzschild-de Sitter spacetimes
characterized by the cosmological parameter {\lambda}=1/3{\Lambda}M^2 is
investigated. With attention concentrated to the axisymmetric motion of string
loops it is shown that the resulting motion is governed by the presence of an
outer tension barrier and an inner angular momentum barrier that are influenced
by the black hole gravitational field given by the mass M and the cosmic
repulsion given by the cosmological constant {\Lambda}. The gravitational
attraction could cause capturing of the string having low energy by the black
hole or trapping in its vicinity; with high enough energy, the string can
escape (scatter) to infinity. The role of the cosmic repulsion becomes
important in vicinity of the so-called static radius where the gravitational
attraction is balanced by the cosmic repulsion-it is demonstrated both in terms
of the effective potential of the string motion and the basin boundary method
reflecting its chaotic character, that a potential barrier exists along the
static radius behind which no trapped oscillations may exist. The trapped
states of the string loops, governed by the interplay of the gravitating mass M
and the cosmic repulsion, are allowed only in Schwarzschild-de Sitter
spacetimes with the cosmological parameter {\lambda}<{\lambda}_trap 0.00497.
The trapped oscillations can extend close to the radius of photon circular
orbit, down to r_mt 3.3M.Comment: 21 pages, 22 figure
Influence of the relict cosmological constant on accretion discs
Surprisingly, the relict cosmological constant has a crucial influence on
properties of accretion discs orbiting black holes in quasars and active
galactic nuclei. We show it by considering basic properties of both the
geometrically thin and thick accretion discs in the Kerr-de Sitter black-hole
(naked-singularity) spacetimes. Both thin and thick discs must have an outer
edge allowing outflow of matter into the outer space, located nearby the so
called static radius, where the gravitational attraction of a black hole is
balanced by the cosmological repulsion. Jets produced by thick discs can be
significantly collimated after crossing the static radius. Extension of discs
in quasars is comparable with extension of the associated galaxies, indicating
a possibility that the relict cosmological constant puts an upper limit on
extension of galaxies.Comment: 15 pages, 4 figures, invited pape
Forces in Kerr spacetimes with a repulsive cosmological constant
Forces defined in the framework of optical reference geometry are introduced
in the case of stationary and axially symmetric Kerr black-hole and
naked-singularity spacetimes with a repulsive cosmological constant. Properties
of the forces acting on test particles moving along circular orbits in the
equatorial plane are discussed, whereas it is shown where the gravitational
force vanishes and changes its orientation and where the centrifugal force
vanishes and changes its orientation independently of the velocity of test
particles related to the optical geometry; the Coriolis force does not vanish
for the velocity being non-zero. The spacetimes are classified according to the
number of circular orbits where the gravitational and centrifugal forces
vanish
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