Testing General Relativity on Horizon Scales and the Primordial non-Gaussianity. (arXiv:1109.0998v1 [astro-ph.CO])

The proper general relativistic description of the observed galaxy power
spectrum is substantially different from the standard Newtonian description on
large scales, providing a unique opportunity to test general relativity on
horizon scales. Using the Einstein equations, the general relativistic effects
can be classified as two new terms that represent the velocity and the
gravitational potential, coupling to the time evolution of galaxy number
density and Hubble parameter. Compared to the dominant density and velocity
redshift-space distortion terms, the former scales as H/k and correlates the
real and imaginary parts of the Fourier modes, while the latter scales as
(H/k)^2, where k is the comoving wave number and H is the conformal Hubble
parameter. We use the recently developed methods to reduce the sampling
variance and shot noise to show that in an all sky galaxy redshift survey at
low redshift the velocity term can be measured at 10-sigma confidence level, if
one can utilize halos of mass M>10^{10} Msun, while the gravitational potential
term itself can only be marginally detected. We also demonstrate that the
general relativistic effect is not degenerate with the primordial non-Gaussian
signature in galaxy bias, and the ability to detect the primordial
non-Gaussianity is little compromised.

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Implications of a viscosity bound on black hole accretion. (arXiv:1108.5177v1 [astro-ph.HE])

Motivated by the viscosity bound in gauge/gravity duality, we consider the
ratio of shear viscosity (eta) to entropy density (s) in black hole accretion
flows. We use both an ideal gas equation of state and the QCD equation of state
obtained from lattice for the fluid accreting onto a Kerr black hole. The QCD
equation of state is considered since the temperature of accreting matter is
expected to approach 10^{12}K in certain hot flows. We find that in both the
cases eta/s is small only for primordial black holes and several orders of
magnitude larger than any known fluid for stellar and supermassive black holes.
We show that a lower bound on the mass of primordial black holes leads to a
lower bound on eta/s and vice versa. Finally we speculate that the
Shakura-Sunyaev viscosity parameter should decrease with increasing density
and/or temperatures.

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Towards a formal description of the collapse approach to the inflationary origin of the seeds of cosmic structure. (arXiv:1108.4928v1 [gr-qc])

Inflation plays a central role in our current understanding of the universe.
According to the standard viewpoint the homogeneous and isotropic mode of the
inflaton field drove an early phase of nearly exponential expansion of the
universe, while the quantum fluctuations (uncertainties) of the other modes
gave rise to the seeds of cosmic structure. However, if we accept that the
accelerated expansion lead the universe into an essentially homogeneous and
isotropic space-time with the state of all the matter fields in their vacuum
except for the zero mode of the inflaton field, we can not escape the
conclusion that the state of the universe as a whole would remain always
homogeneous and isotropic. It was recently proposed in [A. Perez, H. Sahlmann
and D. Sudarsky, "On the quantum origin of the seeds of cosmic structure,"
Class. Quant. Grav. \textbf{23}, 2317-2354 (2006)] that a collapse
(representing physics beyond the established paradigm, and presumably
associated with a quantum-gravity effect \`a la Penrose) of the state function
of the inflaton field might be the missing element, and thus would be
responsible for the emergence of the primordial inhomogeneities. Here we will
discuss in detail a formalism that relies strongly on quantum field theory on
curved space-times, and within which we can implement a detailed description of
such a process. We will see that the picture that emerges clarifies many
aspects of the problem, and is conceptually quite transparent. We will find
nonetheless that the results lead us to argue that it is not fully compatible
with a purely geometric description of space-time.

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Logarithmic Corrections to N=2 Black Hole Entropy: An Infrared Window into the Microstates. (arXiv:1108.3842v1 [hep-th])

Logarithmic corrections to the extremal black hole entropy can be computed
purely in terms of the low energy data — the spectrum of massless fields and
their interaction. The demand of reproducing these corrections provides a
strong constraint on any microscopic theory of quantum gravity that attempts to
explain the black hole entropy. Using quantum entropy function formalism we
compute logarithmic corrections to the entropy of half BPS black holes in N=2
supersymmetric string theories. Our results allow us to test various proposals
for the measure in the OSV formula, and we find agreement with the measure
proposed by Denef and Moore if we assume their result to be valid at weak
topological string coupling. Our analysis also gives the logarithmic
corrections to the entropy of extremal Reissner-Nordstrom black holes in
ordinary Einstein-Maxwell theory.

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Einstein Gravity as a Conformal Field Theory on Horizons of Stationary and Axisymmetric Black Holes. (arXiv:1108.3841v1 [hep-th])

We carry out a Kaluza-Klein reduction of the Einstein-Hilbert action along
the ignorable coordinates of stationary and axisymmetric black holes. Rigid
diffeomorphism invariance of the $ m$ -ignorable coordinates then become a global
$ SL(m,R)$ gauge symmetry of the reduced theory. Mass and angular momentum of
the black holes are related to generators of an $ SL(2,R)$ subgroup of the full
symmetry. Related to each angular momentum there is also an $ SL(2,R)$ subgroup.
On the horizon, this $ SL(2,R)$ can be extended to the full Witt algebra, which
is an exact symmetry of the reduced action when the black hole temperature is
zero. So on the horizon, the Kaluza-Klein reduced action is a conformal field
theory. The infinite dimensional symmetries are explicitly broken when the
system is at a finite temperature, i.e. when the black hole temperature is
nonzero.

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Fixing a Parameter of the Galactic Halo: A Mathematical Modelling by Hamiltonian Method. (arXiv:1108.0926v1 [gr-qc])

We illustrate how the mathematical modelling of the equations of motion in
terms of autonomous Hamiltonian dynamical system can definitively fix a sign
for an otherwise indefinite sign of a certain astrophysical parameter. To
illustrate it, we shall consider the Mannheim-Kazanas-de Sitter solution of
Weyl gravity containing the parameter {\gamma}, which is believed to be
significant in the halo gravity. The strategy we adopt is to calculate the
maximum radius up to which the halo supports stable material circular orbits.
The maximum radius for several observed lenses are calculated for both signs of
{\gamma}, and with the observed value of cosmological constant {\Lambda}. These
lenses (all having approximately the Einstein radius R_{E}{\approx}10^23 cm)
consistently yield a maximum radius R_{max}^{stable}({\simeq}4.25{\times}10^27
cm) inside the de Sitter radius of the universe only when {\gamma} is negative,
while a positive {\gamma} yields R_{max}^{stable} always exceeding the de
Sitter radius.

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Impact of a Pioneer/Rindler-type acceleration on the Oort cloud. (arXiv:1108.0409v1 [gr-qc])

According to a recent modified model of gravity at large distances, a radial
constant and uniform extra-acceleration A_Rin of Rindler type acts upon a test
particle p in the static field of a central mass M if certain conditions are
satisfied. Among other things, it was proposed as a potentially viable
explanation of a part of the Pioneer anomaly. We study the impact that an
anomalous Rindler-type term as large as |A_Rin| \sim 10^-10 m s^-2 may have on
the the orbital dynamics of a typical object of the Oort cloud whose
self-energy is quite smaller than its putative Rindler energy. By taking a
typical comet moving along a highly eccentric and inclined orbit throughout the
expected entire extension of the Oort cloud (0.02 pc-1 pc), it turns out that
the addition of an outward Rindler-like acceleration, i.e. for A_Rin > 0, does
not allow bound orbits. Instead, if A_Rin < 0, the resulting numerically
integrated trajectory is limited in space, but it radically differs from the
standard Keplerian ellipse. In particular, the heliocentric distance of the
comet gets markedly reduced and experiences high frequency oscillations, its
speed is increased, and the overall pattern of the trajectory is quite
isotropic. As a consequence, the standard picture of the Oort cloud is
radically altered since its modified orbits are much less sensitive to the
disturbing actions of the Galactic tide and nearby passing stars whose effects,
in the standard scenario, are responsible for the phenomenology on which our
confidence in the existence of the cloud itself is based. The present analysis
may be supplemented in future by further statistical Monte Carlo-type
investigations by randomly varying the initial conditions of the comets.

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Higher Derivative Corrections to Holographic Entanglement Entropy for AdS Solitons. (arXiv:1107.4363v1 [hep-th])

We investigate the behaviors of holographic entanglement entropy for AdS
soliton geometries in the presence of higher derivative corrections. We
calculate the leading higher derivative corrections for the AdS5 setup in type
IIB string and for the AdS4,7 ones in M-theory. We also study the holographic
entanglement entropy in Gauss-Bonnet gravity and study how the
confinement/deconfinement phase transition observed in AdS solitons is affected
by the higher derivative corrections.

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The fate of Cyg X-1: an empirical limit on BH-NS merger rate. (arXiv:1107.4106v1 [astro-ph.GA])

The recent distance determination allowed precise estimation of the orbital
parameters of Cyg X-1, which contains a massive 14.8 Msun BH with a 19.2 Msun O
star companion. This system appears to be the clearest example of a potential
progenitor of a BH-NS system. We follow the future evolution of Cyg X-1, and
show that it will soon encounter a Roche lobe overflow episode, followed
shortly by a Type Ib/c supernova and the formation of a NS. It is demonstrated
that in majority of cases (>70%) the supernova and associated natal kick
disrupts the binary due to the fact that the orbit expanded significantly in
the Roche lobe overflow episode. In the reminder of cases (<30%) the newly
formed BH-NS system is too wide to coalesce in the Hubble time. Only
sporadically (1%) a Cyg X-1 like binary may form a coalescing BH-NS system
given a favorable direction and magnitude of the natal kick. If Cyg X-1 like
channel (X-ray active phase shorter than 10 Myr) is the only or dominant way to
form BH-NS binaries in the Galaxy we can estimate the empirical BH-NS merger
rate in the Galaxy at the level of 0.001 per Myr. This rate is so low that the
detection of BH-NS systems in gravitational radiation is highly unlikely,
generating Advanced LIGO/VIRGO detection rates at the level of only 1 per
century. If BH-NS inspirals are in fact detected, it will indicate that the
formation of these systems proceeds via some alternative and yet unobserved
channels.

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Inflationary non-Gaussianities in the most general second-order scalar-tensor theories. (arXiv:1107.3917v1 [gr-qc])

For very general scalar-field theories in which the equations of motion are
at second-order, we evaluate the three-point correlation function of primordial
scalar perturbations generated during inflation. We show that the shape of
non-Gaussianities is well approximated by the equilateral type. The equilateral
non-linear parameter f_NL^equil is derived on the quasi de Sitter background
where the slow-variation parameters are much smaller than unity. We apply our
formula for f_NL^equil to a number of single-field models of inflation–such as
k-inflation, k-inflation with Galileon terms, potential-driven Galileon
inflation, nonminimal coupling models (including field-derivative coupling
models), and Gauss-Bonnet gravity.

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