14 research outputs found
Accretion with back reaction
We calculate analytically a back reaction of the stationary spherical
accretion flow near the event horizon and near the inner Cauchy horizon of the
charged black hole. It is shown that corresponding back-reaction corrections to
the black hole metric depend only on the fluid accretion rate and diverge in
the case of an extremely charged black hole. In result, the test fluid
approximation for stationary accretion is violated for extreme black holes.
This behavior of the accreting black hole is in accordance with the third law
of black hole thermodynamics, forbidding the practical attainability of the
extreme state.Comment: 5 pages, 2 figures; new figure and references adde
Clusters of primordial black holes
The Primordial Black Holes (PBHs) are gradually involved into consideration
as the phenomenon having reliable basis. We discuss here the possibility of
their agglomeration into clusters that may have several prominent observable
features. The clusters can form due to closed domain walls appearance in the
natural and the hybrid inflation with subsequent evolution and gravitational
collapse. Early dustlike stages of dominance of heavy metastable dissipative
particles, at which star-like objects are formed, can also naturally lead to
formation of black hole clusters, remaining in the Universe after decay of
particles, from which they have originated. The dynamical evolution of such
clusters discussed here is of the crucial importance. Such a model inherits all
the advantages of the single PBHs like possible explanation of existence of
supermassive black holes (origin of the early quasars), binary BH merges
registered by LIGO/Virgo through gravitational waves, contribution to
reionization of the Universe, but also has additional benefits. The cluster
could alleviate or completely avoid existing constraints on the single PBH
abundance making PBHs a real dark matter candidate. The most of existing
constraints on (single) PBH density should be re-considered as applied to the
clusters. Also unidentified cosmic gamma-ray point-like sources could be
(partially) accounted for by them. One can conclude, that it seems really to be
much more viable model with respect to the single PBHs.Comment: v2: both the text and bibliography are essentially extended,
coincides with EPJC versio
Small-scale clumps in the galactic halo and dark matter annihilation
Production of small-scale DM clumps is studied in the standard cosmological
scenario with an inflation-produced primeval fluctuation spectrum. Special
attention is given to three following problems: (i) The mass spectrum of
small-scale clumps with is calculated with tidal
destruction of the clumps taken into account within the hierarchical model of
clump structure. Only 0.1 - 0.5% of small clumps survive the stage of tidal
destruction in each logarithmic mass interval . (ii) The mass
distribution of clumps has a cutoff at due to diffusion of DM
particles out of a fluctuation and free streaming at later stage.
is a model dependent quantity. In the case the neutralino, considered as a pure
bino, is a DM particle, . (iii) The
evolution of density profile in a DM clump does not result in the singularity
because of formation of the core under influence of tidal interaction. The
radius of the core is , where is radius of the clump. The
applications for annihilation of DM particles in the Galactic halo are studied.
The number density of clumps as a function of their mass, radius and distance
to the Galactic center is presented. The enhancement of annihilation signal due
to clumpiness, valid for arbitrary DM particles, is calculated. In spite of
small survival probability, the annihilation signal in most cases is dominated
by clumps. For observationally preferable value of index or primeval
fluctuation spectrum , the enhancement of annihilation signal is
described by factor 2 - 5 for different density profiles in a clump.Comment: inor changes in text and 2 references adde
Physical origin of the dark spot at the image of supermassive black hole SgrA* revealed by the EHT collaboration
In this comment we elucidate the physical origin of the dark spot at the
image of supermassive black hole SgrA* presented very recently by the EHT
collaboration. It is argued that this dark spot, which is noticeably smaller of
the classical black hole shadow, is the northern hemisphere of the event
horizon globe. At the same time, the outer boundary of this dark spot is an
equator on the event horizon globe.Comment: 6 pages, 4 figure
Supervisor of the Universe
In this paper, conformal invariant gravitation, based on Weyl geometry, is considered. In addition to the gravitational and matter action integrals, the interaction between the Weyl vector (entered in Weyl geometry) and the vector, representing the world line of the independent observer, are introduced. It is shown that the very existence of such an interaction selects the exponentially growing scale factor solutions among the cosmological vacua