Light Loop Echoes and Blinking Black Holes. (arXiv:1110.2789v1 [astro-ph.HE])

Radiation emitted near a black hole reaches the observer by multiple paths;
and when this radiation varies in time, the time-delays between the various
paths generate a “blinking” effect in the observed light curve L(t) or its
auto-correlation function xi(T)= <L(t)L(t-T)>. For the particularly important
“face-on” configuration (in which the hole is viewed roughly along its spin
axis, while the emission comes roughly from its equatorial plane — e.g. from
the inner edge of its accretion disk, or from the violent flash of a
nearby/infalling star) we calculate the blinking in detail by computing the
time delay Delta t_{j}(r,a) and magnification mu_{j}(r,a) of the jth path
(j=1,2,3,…), relative to the primary path (j=0), as a function of the
emission radius r and black hole spin 0<a/M<1. The particular geometry and
symmetry of the nearly-face-on configuration enhances and “protects” the
blinking signal, making it more detectable and more independent of certain
astrophysical and observational details. The effect can be surprisingly strong:
e.g. for radiation from the innermost stable circular orbit (“ISCO”) of a black
hole of critical spin (a_{crit}/M = 0.853), the j=1,2,3 fluxes are,
respectively, 27%, 2% and 0.1% of the j=0 flux.

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Supersymmetric Solutions in Six Dimensions: A Linear Structure. (arXiv:1110.2781v1 [hep-th])

The equations underlying all supersymmetric solutions of six-dimensional
minimal ungauged supergravity coupled to an anti-self-dual tensor multiplet
have been known for quite a while, and their complicated non-linear form has
hindered all attempts to systematically understand and construct BPS solutions.
In this paper we show that, by suitably re-parameterizing these equations, one
can find a structure that allows one to construct supersymmetric solutions by
solving a sequence of linear equations. We then illustrate this method by
constructing a new class of geometries describing several parallel spirals
carrying D1, D5 and P charge and parameterized by four arbitrary functions of
one variable. A similar linear structure is known to exist in five dimensions,
where it underlies the black hole, black ring and corresponding microstate
geometries. The unexpected generalization of this to six dimensions will have
important applications to the construction of new, more general such
geometries.

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(An)Isotropic models in scalar and scalar-tensor cosmologies. (arXiv:1110.2775v1 [gr-qc])

We study how the constants $ G$ and $ \Lambda$ may vary in different
theoretical models (general relativity with a perfect fluid, scalar
cosmological models (\textquotedblleft quintessence\textquotedblright) with and
without interacting scalar and matter fields and a scalar-tensor model with a
dynamical $ \Lambda$ ) in order to explain some observational results. We apply
the program outlined in section II to study three different geometries which
generalize the FRW ones, which are Bianchi \textrm{V}, \textrm{VII}$ _{0}$ and
\textrm{IX}, under the self-similarity hypothesis. We put special emphasis on
calculating exact power-law solutions which allow us to compare the different
models. In all the studied cases we arrive to the conclusion that the solutions
are isotropic and noninflationary while the cosmological constant behaves as a
positive decreasing time function (in agreement with the current observations)
and the gravitational constant behaves as a growing time function.

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Observational constraints in scalar tensor theory with tachyonic potential. (arXiv:1109.6317v1 [gr-qc])

We study the dynamics of the scalar tensor cosmological model in the presence
of tachyon field. In an alternative approach, in two exponential and power law
form of the scalar field functions in the model, field equations are solved by
simultaneously best fitting the model parameters with the most recent
observational data. This approach gives us an observationally verified
interpretation of the dynamics of the universe. We then discuss the best fitted
of equation of state parameter, the statefinder parameters and the
reconstructed scalar field in the model.

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Generalized Quantization Principle in Canonical Quantum Gravity and Application to Quantum Cosmology. (arXiv:1109.4629v1 [gr-qc])

In this paper is considered a generalized quantization principle for the
gravitational field in canonical quantum gravity, especially with respect to
quantum geometrodynamics. This assumption can be interpreted as a transfer from
the generalized uncertainty principle in quantum mechanics, which is postulated
as generalization of the Heisenberg algebra to introduce a minimal length, to a
corresponding quantization principle concerning the quantities of quantum
gravity. According to this presupposition there have to be given generalized
representations of the operators referring to the observables in the canonical
approach of a quantum description of general relativity. This also leads to
generalized constraints for the states and thus to a generalized Wheeler DeWitt
equation determining a new dynamical behaviour. As a special manifestation of
this modified canonical theory of quantum gravity there is explored quantum
cosmology. The generalized cosmological Wheeler DeWitt equation corresponding
to the application of the generalized quantization principle to the
cosmological degree of freedom is solved by using Sommerfelds polynomial
method.

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Inflation with stable anisotropic hair: is it cosmologically viable?. (arXiv:1109.3456v1 [gr-qc])

Recently an inflationary model with a vector field coupled to the inflaton
was proposed and the phenomenology studied for the Bianchi type I spacetime. It
was found that the model demonstrates a counter-example to the cosmic no-hair
theorem since there exists a stable anisotropically inflationary fix-point. One
of the great triumphs of inflation, however, is that it explains the observed
flatness and isotropy of the universe today without requiring special initial
conditions. Any acceptable model for inflation should thus explain these
observations in a satisfactory way. To check whether the model meets this
requirement, we introduce curvature to the background geometry and consider
axisymmetric spacetimes of Bianchi type II,III and the Kantowski-Sachs metric.
We show that the anisotropic Bianchi type I fix-point is an attractor for the
entire family of such spacetimes. The model is predictive in the sense that the
universe gets close to this fix-point after a few e-folds for a wide range of
initial conditions. If inflation lasts for N e-folds, the curvature at the end
of inflation is typically of order exp(-2N). The anisotropy in the expansion
rate at the end of inflation, on the other hand, while being small on the
one-percent level, is highly significant. We show that after the end of
inflation there will be a period of isotropization lasting for about 2N/3
e-folds. After that the shear scales as the curvature and becomes dominant
around N e-folds after the end of inflation. For plausible bounds on the reheat
temperature the minimum number of e-folds during inflation, required for
consistency with the isotropy of the supernova Ia data, lays in the interval
(21,48). Thus the results obtained for our restricted class of spacetimes
indicates that inflation with anisotropic hair is cosmologically viable.

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New Black Holes Solutions in a Modified Gravity. (arXiv:1109.2928v1 [gr-qc])

We present a theory of modified gravity, inspired by the gauge theories,
where the commutator algebra of covariant derivative gives us an added term
with respect to the General Relativity, which represents the interaction of
gravity with a substratum. New spherically symmetric solutions of this theory
are obtained and can be view as solutions that reproduce, the mass, the charge,
the cosmological constant and the Rindler acceleration, without coupling with
the matter content, i.e., in the vacuum.

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The CMB bispectrum in the squeezed limit. (arXiv:1109.1822v1 [astro-ph.CO])

The CMB bispectrum generated by second-order effects at recombination can be
calculated analytically when one of the three modes has a wavelength much
longer than the other two and is outside the horizon at recombination. This was
pointed out in \cite{Creminelli:2004pv} and here we correct their results. We
derive a simple formula for the bispectrum, $ f_{\rm NL}^{\rm loc} = – (1/6+
\cos 2 \theta) \cdot (1- 1/2 \cdot d \ln (l_S^2 C_{S})/d \ln l_S)$ , where $ C_S$
is the short scale spectrum and $ \theta$ the relative orientation between the
long and the short modes. This formula is exact and takes into account all
effects at recombination, including recombination-lensing, but neglects all
late-time effects such as ISW-lensing. The induced bispectrum in the squeezed
limit is small and will negligibly contaminate the Planck search for a local
primordial signal: this will be biased only by $ f_{\rm NL}^{\rm
loc}\approx-0.4$ . The above analytic formula includes the primordial
non-Gaussianity of any single-field model. It also represents a consistency
check for second-order Boltzmann codes: we find substantial agreement with the
CMBquick code.

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Constraints on $\Lambda(t)$CDM models as holographic and agegraphic dark energy with the observational Hubble parameter data. (arXiv:1109.1661v1 [astro-ph.CO])

The newly released observational $ H(z)$ data (OHD) is used to constrain
$ \Lambda(t)$ CDM models as holographic and agegraphic dark energy. By the use of
the length scale and time scale as the IR cut-off including Hubble horizon
(HH), future event horizon (FEH), age of the universe (AU), and conformal time
(CT), we achieve four different $ \Lambda(t)$ CDM models which can describe the
present cosmological acceleration respectively. In order to get a comparison
between such $ \Lambda(t)$ CDM models and standard $ \Lambda$ CDM model, we use the
information criteria (IC), $ Om(z)$ diagnostic, and statefinder diagnostic to
measure the deviations. Furthermore, by simulating a larger Hubble parameter
data sample in the redshift range of $ 0.1<z<2.0$ , we get the improved
constraints and more sufficient comparison. We show that OHD is not only able
to play almost the same role in constraining cosmological parameters as SNe Ia
does but also provides the effective measurement of the deviation of the DE
models from standard $ \Lambda$ CDM model. In the holographic and agegraphic
scenarios, the results indicate that the FEH is more preferable than HH
scenario. However, both two time scenarios show better approximations to
$ \Lambda$ CDM model than the length scenarios.

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A relativistic model of the topological acceleration effect. (arXiv:1109.1596v1 [astro-ph.CO])

It has previously been shown heuristically that the topology of the Universe
affects gravity, in the sense that a test particle near a massive object in a
multiply connected universe is subject to a topologically induced acceleration
that opposes the local attraction to the massive object. This effect
distinguishes different comoving 3-manifolds, potentially providing a
theoretical justification for the Poincar\’e dodecahedral space observational
hypothesis and a dynamical test for cosmic topology. It is necessary to check
if this effect occurs in a fully relativistic solution of the Einstein
equations that has a multiply connected spatial section. A Schwarzschild-like
exact solution that is multiply connected in one spatial direction is checked
for analytical and numerical consistency with the heuristic result. The T$ ^1$
(slab space) heuristic result is found to be relativistically correct. For a
fundamental domain size of $ L$ , a slow-moving, negligible-mass test particle
lying at distance $ x$ along the axis from the object of mass $ M$ to its nearest
multiple image, where $ GM/c^2 \ll x \ll L/2$ , has a residual acceleration away
from the massive object of $ 4\zeta(3) G(M/L^3)\,x$ , where $ \zeta(3)$ is
Ap\’ery’s constant. For $ M \sim 10^14 M_\odot$ and $ L \sim 10$ to $ 20\hGpc$ ,
this linear expression is accurate to $ \pm10%$ over $ 3\hMpc \ltapprox x
\ltapprox 2\hGpc$ . Thus, at least in a simple example of a multiply connected
universe, the topological acceleration effect is not an artefact of
Newtonian-like reasoning, and its linear derivation is accurate over about
three orders of magnitude in $ x$ .

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