593 research outputs found
Black Holes in Cascading Theories: Confinement/Deconfinement Transition and other Thermal Properties
We present numerical evidence for a transition between the Klebanov-Strassler
background and a solution describing a black hole in the class of cascading
solutions in the chirally restored phase. We also present a number of
properties of this solution, including the running of the coupling constant,
the viscosity to entropy ratio and the drag force on a quark moving in this
background.Comment: 34 pages, 7 figures. Version to be published by JHE
More on Penrose limits and non-local theories
We obtain the Penrose limit of six dimensional Non-Commutative Open String
(NCOS) theory and show that in the neighborhood of a particular null
geodesic it leads to an exactly solvable string theory (unlike their
counterparts in four or in other dimensions). We describe the phase structure
of this theory and discuss the Penrose limit in different phases including Open
D-string (OD1) theory. We compute the string spectrum and discuss their
relations with the states of various theories at different phases. We also
consider the case of general null geodesic for which the Penrose limit leads to
string theory in the time dependent pp-wave background and comment on the
renormalization group flow in the dual theory.Comment: latex, 22 pages, minor corrections, references added, published
versio
Spin-gravity coupling and gravity-induced quantum phases
External gravitational fields induce phase factors in the wave functions of
particles. The phases are exact to first order in the background gravitational
field, are manifestly covariant and gauge invariant and provide a useful tool
for the study of spin-gravity coupling and of the optics of particles in
gravitational or inertial fields. We discuss the role that spin-gravity
coupling plays in particular problems.Comment: 18 pages, 1 figur
Self-trapping transition for nonlinear impurities embedded in a Cayley tree
The self-trapping transition due to a single and a dimer nonlinear impurity
embedded in a Cayley tree is studied. In particular, the effect of a perfectly
nonlinear Cayley tree is considered. A sharp self-trapping transition is
observed in each case. It is also observed that the transition is much sharper
compared to the case of one-dimensional lattices. For each system, the critical
values of for the self-trapping transitions are found to obey a
power-law behavior as a function of the connectivity of the Cayley tree.Comment: 6 pages, 7 fig
Holographic Algorithms Beyond Matchgates
Holographic algorithms based on matchgates were introduced by Valiant. These algorithms run in polynomial-time and are intrinsically for planar problems. We introduce two new families of holographic algorithms, which work over general, i.e., not necessarily planar, graphs. The two underlying families of constraint functions are of the affine and product types. These play the role of Kasteleyn’s algorithm for counting planar perfect matchings. The new algorithms are obtained by transforming a problem to one of these two families by holographic reductions. We present a polynomial-time algorithm to decide if a given counting problem has a holographic algorithm using these constraint families. When the constraints are symmetric, we give a polynomial-time decision procedure in the size of the succinct presentation of symmetric constraint functions. This procedure shows that the recent dichotomy theorem for Holant problems with symmetric constraints is polynomial-time decidable
Interacting New Agegraphic Dark Energy in a Cyclic Universe
The main goal of this work is investigation of NADE in the cyclic universe
scenario. Since, cyclic universe is explained by a phantom phase (),
it is shown when there is no interaction between matter and dark energy, ADE
and NADE do not produce a phantom phase, then can not describe cyclic universe.
Therefore, we study interacting models of ADE and NADE in the modified
Friedmann equation. We find out that, in the high energy regime, which it is a
necessary part of cyclic universe evolution, only NADE can describe this
phantom phase era for cyclic universe. Considering deceleration parameter tells
us that the universe has a deceleration phase after an acceleration phase, and
NADE is able to produce a cyclic universe. Also it is found valuable to study
generalized second law of thermodynamics. Since the loop quantum correction is
taken account in high energy regime, it may not be suitable to use standard
treatment of thermodynamics, so we turn our attention to the result of
\citep{29}, which the authors have studied thermodynamics in loop quantum
gravity, and we show that which condition can satisfy generalized second law of
thermodynamics.Comment: 8 pages, 3 figure
Does accelerating universe indicates Brans-Dicke theory
The evolution of universe in Brans-Dicke (BD) theory is discussed in this
paper.
Considering a parameterized scenario for BD scalar field
which plays the role of gravitational "constant" ,
we apply the Markov Chain Monte Carlo method to investigate a global
constraints on BD theory with a self-interacting potential according to the
current observational data: Union2 dataset of type supernovae Ia (SNIa),
high-redshift Gamma-Ray Bursts (GRBs) data, observational Hubble data (OHD),
the cluster X-ray gas mass fraction, the baryon acoustic oscillation (BAO), and
the cosmic microwave background (CMB) data. It is shown that an expanded
universe from deceleration to acceleration is given in this theory, and the
constraint results of dimensionless matter density and parameter
are, and
which is consistent with the
result of current experiment exploration, . In
addition, we use the geometrical diagnostic method, jerk parameter , to
distinguish the BD theory and cosmological constant model in Einstein's theory
of general relativity.Comment: 16 pages, 3 figure
Stability of the Einstein static universe in IR modified Ho\v{r}ava gravity
Recently, Horava proposed a power counting renormalizable theory for
(3+1)-dimensional quantum gravity, which reduces to Einstein gravity with a
non-vanishing cosmological constant in IR, but possesses improved UV behaviors.
In this work, we analyze the stability of the Einstein static universe by
considering linear homogeneous perturbations in the context of an IR
modification of Horava gravity, which implies a `soft' breaking of the
`detailed balance' condition. The stability regions of the Einstein static
universe are parameterized by the linear equation of state parameter w=p/\rho
and the parameters appearing in the Horava theory, and it is shown that a large
class of stable solutions exists in the respective parameter space.Comment: 9 pages, 5 figures; references adde
Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory
Data from the Pierre Auger Observatory are analyzed to search for
anisotropies near the direction of the Galactic Centre at EeV energies. The
exposure of the surface array in this part of the sky is already significantly
larger than that of the fore-runner experiments. Our results do not support
previous findings of localized excesses in the AGASA and SUGAR data. We set an
upper bound on a point-like flux of cosmic rays arriving from the Galactic
Centre which excludes several scenarios predicting sources of EeV neutrons from
Sagittarius . Also the events detected simultaneously by the surface and
fluorescence detectors (the `hybrid' data set), which have better pointing
accuracy but are less numerous than those of the surface array alone, do not
show any significant localized excess from this direction.Comment: Matches published versio
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