124 research outputs found
The holographic bound in the scalar-tensor and gravities
The holographic bound has been extended to the different theory of gravities
such as scalar-tensor gravity and gravity according to the Noether
charge definition of the entropy for a black hole surface. We have introduced
some popular examples of the flat FRW cosmology in order to investigate
holographic bound in scalar-tensor and gravity. Using the holographic
bound, we put an additional constraint on the scalar-tensor gravity and
gravity parameters. We also discuss about the transformation from Jordan frame
to Einstein frame.Comment: accepted in European Physical Journal C (epjc), a section added to
the pape
Black hole shadow with a cosmological constant for cosmological observers
We investigate the effect of the cosmological constant on the angular size of
a black hole shadow. It is known that the accelerated expansion which is
created by the cosmological constant changes the angular size of the black hole
shadow for static observers. However, the shadow size must be calculated for
the appropriate cosmological observes. We calculate the angular size of the
shadow measured by cosmological comoving observers by projecting the shadow
angle to this observer rest frame. We show that the shadow size tends to zero
as the observer approaches the cosmological horizon. We estimate the angular
size of the shadow for a typical supermassive black hole, e.g M87. It is found
that the angular size of the shadow for cosmological observers and static
observers is approximately the same at these scales of mass and distance. We
present a catalog of supermassive black holes and calculate the effect of the
cosmological constant on their shadow size and find that the effect could be
for distant known sources like the Phoenix Cluster supermassive
black hole.Comment: 8 pages, 2 figures, a few typos in table numbers and text correcte
Radiation from the LTB black hole
Does a dynamical black hole embedded in a cosmological FRW background emit
Hawking radiation where a globally defined event horizon does not exist? What
are the differences to the Schwarzschild black hole? What about the first law
of black hole mechanics? We face these questions using the LTB cosmological
black hole model recently published. Using the Hamilton-Jacobi and radial null
geodesic-methods suitable for dynamical cases, we show that it is the apparent
horizon which contributes to the Hawking radiation and not the event horizon.
The Hawking temperature is calculated using the two different methods giving
the same result. The first law of LTB black hole dynamics and the thermal
character of the radiation is also dealt with.Comment: 9 pages, revised version, Europhysics Letter 2012 97 2900
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