219 research outputs found
Nutrient Enrichment Increases Mortality of Mangroves
Nutrient enrichment of the coastal zone places intense pressure on marine communities. Previous studies have shown that growth of intertidal mangrove forests is accelerated with enhanced nutrient availability. However, nutrient enrichment favours growth of shoots relative to roots, thus enhancing growth rates but increasing vulnerability to environmental stresses that adversely affect plant water relations. Two such stresses are high salinity and low humidity, both of which require greater investment in roots to meet the demands for water by the shoots. Here we present data from a global network of sites that documents enhanced mortality of mangroves with experimental nutrient enrichment at sites where high sediment salinity was coincident with low rainfall and low humidity. Thus the benefits of increased mangrove growth in response to coastal eutrophication is offset by the costs of decreased resilience due to mortality during drought, with mortality increasing with soil water salinity along climatic gradients
Causality in AdS/CFT and Lovelock theory
We explore the constraints imposed on higher curvature corrections of the
Lovelock type due to causality restrictions in the boundary of asymptotically
AdS space-time. In the framework of AdS/CFT, this is related to positivity of
the energy constraints that arise in conformal collider physics. We present
explicit analytic results that fully address these issues for cubic Lovelock
gravity in arbitrary dimensions and give the formal analytic results that
comprehend general Lovelock theory. The computations can be performed in two
ways, both by considering a thermal setup in a black hole background and by
studying the scattering of gravitons with a shock wave in AdS. We show that
both computations coincide in Lovelock theory. The different helicities, as
expected, provide the boundaries defining the region of allowed couplings. We
generalize these results to arbitrary higher dimensions and discuss their
consequences on the shear viscosity to energy density ratio of CFT plasmas, the
possible existence of Boulware-Deser instabilities in Lovelock theory and the
extent to which the AdS/CFT correspondence might be valid for arbitrary
dimensions.Comment: 35 pages, 20 figures; v2: minor amendments and clarifications
include
AdS_7/CFT_6, Gauss-Bonnet Gravity, and Viscosity Bound
We study the relation between the causality and the positivity of energy
bounds in Gauss-Bonnet gravity in AdS_7 background and find a precise
agreement. Requiring the group velocity of metastable states to be bounded by
the speed of light places a bound on the value of Gauss-Bonnet coupling. To
find the positivity of energy constraints we compute the parameters which
determine the angular distribution of the energy flux in terms of three
independent coefficients specifying the three-point function of the
stress-energy tensor. We then relate the latter to the Weyl anomaly of the
six-dimensional CFT and compute the anomaly holographically. The resulting
upper bound on the Gauss-Bonnet coupling coincides with that from causality and
results in a new bound on viscosity/entropy ratio.Comment: 21 page, harvmac; v2: reference adde
Causality constraints in AdS/CFT from conformal collider physics and Gauss-Bonnet gravity
We explore the relation between positivity of the energy constraints in
conformal field theories and causality in their dual gravity description. Our
discussion involves CFTs with different central charges whose description, in
the gravity side, requires the inclusion of quadratic curvature corrections. It
is enough, indeed, to consider the Gauss-Bonnet term. We find that both sides
of the AdS/CFT correspondence impose a restriction on the Gauss-Bonnet
coupling. In the case of 6d supersymmetric CFTs, we show the full matching of
these restrictions. We perform this computation in two ways. First by
considering a thermal setup in a black hole background. Second by scrutinizing
the scattering of gravitons with a shock wave in AdS. The different helicities
provide the corresponding lower and upper bounds. We generalize these results
to arbitrary higher dimensions and comment on some hints and puzzles they
prompt regarding the possible existence of higher dimensional CFTs and the
extent to which the AdS/CFT correspondence would be valid for them.Comment: 31 pages, 5 figures; v2: typos fixed, cosmetic amendments and
references adde
Entanglement Entropy for Singular Surfaces
We study entanglement entropy for regions with a singular boundary in higher
dimensions using the AdS/CFT correspondence and find that various singularities
make new universal contributions. When the boundary CFT has an even spacetime
dimension, we find that the entanglement entropy of a conical surface contains
a term quadratic in the logarithm of the UV cut-off. In four dimensions, the
coefficient of this contribution is proportional to the central charge 'c'. A
conical singularity in an odd number of spacetime dimensions contributes a term
proportional to the logarithm of the UV cut-off. We also study the entanglement
entropy for various boundary surfaces with extended singularities. In these
cases, similar universal terms may appear depending on the dimension and
curvature of the singular locus.Comment: 66 pages,4 figures. Some typos are removed and a reference is adde
Holographic Lovelock Gravities and Black Holes
We study holographic implications of Lovelock gravities in AdS spacetimes.
For a generic Lovelock gravity in arbitrary spacetime dimensions we formulate
the existence condition for asymptotically AdS black holes. We consider small
fluctuations around these black holes and determine the constraint on Lovelock
parameters by demanding causality of the boundary theory. For the case of cubic
Lovelock gravity in seven spacetime dimensions we compute the holographic Weyl
anomaly and determine the three point functions of the stress energy tensor in
the boundary CFT. Remarkably, these correlators happen to satisfy the same
relation as the one imposed by supersymmetry. We then compute the energy flux;
requiring it to be positive is shown to be completely equivalent to requiring
causality of the finite temperature CFT dual to the black hole. These
constraints are not stringent enough to place any positive lower bound on the
value of viscosity. Finally, we conjecture an expression for the energy flux
valid for any Lovelock theory in arbitrary dimensions.Comment: 31 pages, 1 figure, harvmac, references added, calculation of
viscosity/entropy ratio include
Black Holes in Quasi-topological Gravity
We construct a new gravitational action which includes cubic curvature
interactions and which provides a useful toy model for the holographic study of
a three parameter family of four- and higher-dimensional CFT's. We also
investigate the black hole solutions of this new gravity theory. Further we
examine the equations of motion of quasi-topological gravity. While the full
equations in a general background are fourth-order in derivatives, we show that
the linearized equations describing gravitons propagating in the AdS vacua
match precisely the second-order equations of Einstein gravity.Comment: 33 pages, 4 figures; two references adde
Holographic studies of quasi-topological gravity
Quasi-topological gravity is a new gravitational theory including
curvature-cubed interactions and for which exact black hole solutions were
constructed. In a holographic framework, classical quasi-topological gravity
can be thought to be dual to the large limit of some non-supersymmetric
but conformal gauge theory. We establish various elements of the AdS/CFT
dictionary for this duality. This allows us to infer physical constraints on
the couplings in the gravitational theory. Further we use holography to
investigate hydrodynamic aspects of the dual gauge theory. In particular, we
find that the minimum value of the shear-viscosity-to-entropy-density ratio for
this model is .Comment: 45 pages, 6 figures. v2: References adde
Atmospheric Evolution
Earth's atmosphere has evolved as volatile species cycle between the
atmosphere, ocean, biomass and the solid Earth. The geochemical, biological and
astrophysical processes that control atmospheric evolution are reviewed from an
"Earth Systems" perspective, with a view not only to understanding the history
of Earth, but also to generalizing to other solar system planets and
exoplanets.Comment: 34 pages, 3 figures, 2 tables. Accepted as a chapter in
"Encyclopaedia of Geochemistry", Editor Bill White, Springer-Nature, 201
Lovelock theories, holography and the fate of the viscosity bound
We consider Lovelock theories of gravity in the context of AdS/CFT. We show
that, for these theories, causality violation on a black hole background can
occur well in the interior of the geometry, thus posing more stringent
constraints than were previously found in the literature. Also, we find that
instabilities of the geometry can appear for certain parameter values at any
point in the geometry, as well in the bulk as close to the horizon. These new
sources of causality violation and instability should be related to CFT
features that do not depend on the UV behavior. They solve a puzzle found
previously concerning unphysical negative values for the shear viscosity that
are not ruled out solely by causality restrictions. We find that, contrary to
previous expectations, causality violation is not always related to positivity
of energy. Furthermore, we compute the bound for the shear viscosity to entropy
density ratio of supersymmetric conformal field theories from d=4 till d=10 -
i.e., up to quartic Lovelock theory -, and find that it behaves smoothly as a
function of d. We propose an approximate formula that nicely fits these values
and has a nice asymptotic behavior when d goes to infinity for any Lovelock
gravity. We discuss in some detail the latter limit. We finally argue that it
is possible to obtain increasingly lower values for the shear viscosity to
entropy density ratio by the inclusion of more Lovelock terms.Comment: 42 pages, 17 figures, JHEP3.cls. v2: reference adde
- …