3,134 research outputs found

### Stars creating a gravitational repulsion

In the framework of the Theory of General Relativity, models of stars with an
unusual equation of state $\rho c^20$ where $\rho$ is the mass density
and $P$ is the pressure, are constructed. These objects create outside
themselves the forces of gravitational repulsion. The equilibrium of such stars
is ensured by a non-standard balance of forces. Negative mass density, acting
gravitationally on itself, creates an acceleration of the negative mass,
directed from the center. Therefore in the absence of pressure such an object
tends to expand. At the same time, the positive pressure, which falls just like
in ordinary stars from the center to the surface, creates a force directed from
the center. This force acts on the negative mass density, which causes
acceleration directed the opposite of the acting force, that is to the center
of the star. This acceleration balances the gravitational repulsion produced by
the negative mass. Thus, in our models gravity and pressure change roles: the
negative mass tends to create a gravitational repulsion, while the gradient of
the pressure acting on the negative mass tends to compress the star. In this
paper, we construct several models of such a star with various equations of
state.Comment: 6 pages, 4 figure

### Time machines and the Principle of Self-Consistency as a consequence of the Principle of Stationary Action (II): the Cauchy problem for a self-interacting relativistic particle

We consider the action principle to derive the classical, relativistic motion
of a self-interacting particle in a 4-D Lorentzian spacetime containing a
wormhole and which allows the existence of closed time-like curves. In
particular, we study the case of a pointlike particle subject to a
`hard-sphere' self-interaction potential and which can traverse the wormhole an
arbitrary number of times, and show that the only possible trajectories for
which the classical action is stationary are those which are globally
self-consistent. Generically, the multiplicity of these trajectories (defined
as the number of self-consistent solutions to the equations of motion beginning
with given Cauchy data) is finite, and it becomes infinite if certain
constraints on the same initial data are satisfied. This confirms the previous
conclusions (for a non-relativistic model) by Echeverria, Klinkhammer and
Thorne that the Cauchy initial value problem in the presence of a wormhole
`time machine' is classically `ill-posed' (far too many solutions). Our results
further extend the recent claim by Novikov et al. that the `Principle of
self-consistency' is a natural consequence of the `Principle of minimal
action.'Comment: 39 pages, latex fil

### Attempt to find a correlation between the spin of stellar-mass black hole candidates and the power of steady jets: relaxing the Kerr black hole hypothesis

The rotational energy of a black hole can be extracted via the
Blandford-Znajek mechanism and numerical simulations suggest a strong
dependence of the power of the produced jet on the black hole spin. A recent
study has found no evidence for a correlation between the spin and the power of
steady jets. If the measurements of the spin and of the jet power are correct,
it leads one to conclude that steady jets are not powered by the black hole
spin. In this paper, I explore another possibility: I assume that steady jets
are powered by the spin and I check if current observations can be explained if
astrophysical black hole candidates are not the Kerr black hole predicted by
General Relativity. It turns out that this scenario might indeed be possible.
While such a possibility is surely quite speculative, it is definitively
intriguing and can be seriously tested when future more accurate measurements
will be available.Comment: 9 pages, 5 figures. v2: refereed version significantly longer. v3:
"Harvard group" replaced by "Harvard-Smithsonian CfA group" in the tex

### Slim accretion discs: a model for ADAF-SLE transitions

We numerically construct slim, global, vertically integrated models of
optically thin, transonic accretion discs around black holes, assuming a
regularity condition at the sonic radius and boundary conditions at the outer
radius of the disc and near the black hole. In agreement with several previous
studies, we find two branches of shock-free solutions, in which the cooling is
dominated either by advection, or by local radiation. We also confirm that the
part of the accretion flow where advection dominates is in some circumstances
limited in size: it does not extend beyond a certain outer limiting radius. New
results found in our paper concern the location of the limiting radius and
properties of the flow near to it. In particular, we find that beyond the
limiting radius, the advective dominated solutions match on to Shapiro,
Lightman & Eardley (SLE) discs through a smooth transition region. Therefore,
the full global solutions are shock-free and unlimited in size. There is no
need for postulating an extra physical effect (e.g. evaporation) for triggering
the ADAF-SLE transition. It occurs due to standard accretion processes
described by the classic slim disc equations.Comment: 12 pages, 7 figures, MNRAS accepte

### Spectrum for the electric dipole which nonradially falling into a black hole

The electromagnetic bremsstrahlung spectrum for the dipole which falling by a
spiral orbit into the Schwarzschild black hole was found. The characteristic
features in this electromagnetic spectrum can be used for determine of the
black hole mass by the new way. This new way (if implemented) provides higher
accuracy in determining of the black hole mass. Also these features in the
spectrum can be used for determine of the certain characteristics in the black
hole magnetosphere or in the accretion disk characteristics around the black
hole. It is also shown that the asymptotic behavior of this spectrum (at high
frequencies) is practically independent from the impact parameter of the
falling dipole.Comment: 15 pages, 3 figures. To appear in IJMPD, 201

### Disintegration and expansion of wormholes

We consider the process of catastrophic expansion of a spacelike wormhole
after a violation of its equilibrium state. The dynamics of deformation of the
comoving reference frame is investigated. We show that the deformation has a
very specific anisotropic feature. The statement made earlier by other authors,
that in the process of expanding the wormhole connecting two universes these
universes ultimately unite into one universe, is not correct. We show that the
transverse size of the wormhole (its throat) increases and the length of the
corridor decreases which does not correspond to the de Sitter model

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