1,236 research outputs found
Saltmarsh resilience to periodic shifts in tidal channels
Resilience of coastal ecosystems to climate change is largely determined by the interaction between plants and the surrounding tidal environment. Research has tended to focus on processes operating at the local scale to explain resilience mechanisms, overlooking potentially important landscape-scale processes and patterns. We show from aerial images spanning 67 years across 3 estuaries that saltmarsh loss was compensated by expansion elsewhere in the estuary when tidal channels shifted position. Compensatory expansion rates were as high as 6 m/yr. This phenomenon of “geomorphic compensation” represents a hitherto overlooked large-scale self-organizing pattern that facilitates the long-term persistence of marshes in estuaries. The geomorphic compensation pattern likely also occurs in other hydrological systems including mangrove forests, and seagrass meadows, and river islands. Compensatory erosion-expansion patterns occurred at the same time as net marsh extent increased by between 120 and 235% across all three estuaries. Marsh expansion mostly occurred in the lower parts of each estuary, where channel migration and compensatory expansion was less evident. Patterns of geomorphic compensation therefore appear to operate at discrete spatio-temporal scales, nested within a hierarchy of coastal morphodynamic processes that govern longer-term patterns of either net marsh gain or loss. Coastal ecosystem resilience can therefore only be fully appreciated when examining erosion and expansion patterns at both local and landscape scales. The intrinsic dynamics of marshes described here have important implications for the long-term delivery of ecosystem services
Comments on Charges and Near-Horizon Data of Black Rings
We study how the charges of the black rings measured at the asymptotic
infinity are encoded in the near-horizon metric and gauge potentials,
independent of the detailed structure of the connecting region. Our analysis
clarifies how different sets of four-dimensional charges can be assigned to a
single five-dimensional object under the Kaluza-Klein reduction. Possible
choices are related by the Witten effect on dyons and by the large gauge
transformation in four and five dimensions, respectively.Comment: 30 pages, 1 figure; v2: additional references; v3: published versio
Brane Universes with Gauss-Bonnet-Induced-Gravity
The DGP brane world model allows us to get the observed late time
acceleration via modified gravity, without the need for a ``dark energy''
field. This can then be generalised by the inclusion of high energy terms, in
the form of a Gauss-Bonnet bulk. This is the basis of the
Gauss-Bonnet-Induced-Gravity (GBIG) model explored here with both early and
late time modifications to the cosmological evolution. Recently the simplest
GBIG models (Minkowski bulk and no brane tension) have been analysed. Two of
the three possible branches in these models start with a finite density
``Big-Bang'' and with late time acceleration. Here we present a comprehensive
analysis of more general models where we include a bulk cosmological constant
and brane tension. We show that by including these factors it is possible to
have late time phantom behaviour.Comment: 12 pages, 19 figures. Minor modifications to text, comments on
phantom behaviour added. References added. As submitted to JCA
Entropy Function for Non-extremal D1D5 and D2D6NS5-branes
We apply the entropy function formalism to non-extremal D1D5 and
D2D6NS5-branes whose throat approximation is given by the Schwarzschild black
hole in AdS_3\times S^3\times T^4 and AdS_3\times S^2\times S^1\times T^4,
respectively. We find the Bekenstein-Hawking entropy and the (alpha')^3R^4
corrections from the value of the entropy function at its saddle point. While
the higher derivative terms have no effect on the temperature, they decrease
the value of the entropy.Comment: 17 Pages, Latex file; Minor additions, version published in JHE
Entropy Functions with 5D Chern-Simons terms
In this note we reconsider Sen's entropy function analysis for 5D
supergravity actions containing Chern-Simons terms. The apparent lack of gauge
invariance is usually tackled via a 4D reduction. Here we motivate how a
systematic 5D procedure also works. In doing so, it becomes important to
identify the correct 5D charges. In particular, we perform explicit
calculations for the black ring and 5D black hole. In the black ring analysis,
we find Chern-Simons induced spectral flow shifts emerging out of Sen's
formalism. We find that the entropy function nevertheless remains gauge
invariant and the resulting electric charges are identified as Page charges.
For the black hole too, 5D gauge invariance is confirmed. Our 5D analysis
enables us to fix a mismatch that arose in the electric charges of Goldstein
and Jena's 4D-reduced calculation. Finally we provide an interpretation for the
e^0 - p^0 exchange in the entropy function as an interpolation between black
hole and black ring geometries in Taub-NUT.Comment: 27 page
Non-supersymmetric Attractors in Born-Infeld Black Holes with a Cosmological Constant
We investigate the attractor mechanism for spherically symmetric extremal
black holes in Einstein-Born-Infeld-dilaton theory of gravity in
four-dimensions, in the presence of a cosmological constant. We look for
solutions analytic near the horizon by using perturbation method. It is shown
that the values of the scalar fields at the horizon are only dependent on the
charges carried by the black hole and are irrelevant in their asymptotic
values. This analysis supports the validity of non-supersymmetric attractors in
the presence of higher derivative interactions in the gauge fields part and in
non-asymptotically flat spacetime.Comment: 18 pages, no figu
On Entropy Function for Supersymmetric Black Rings
The entropy function for five-dimensional supersymmetric black rings, which
are solutions of minimal supergravity, is calculated via both
on-shell and off-shell formalism. We find that at the tree level, the entropy
function obtained from both perspectives can reproduce the Bekenstein-Hawking
entropy. We also compute the higher order corrections to the entropy arising
form five-dimensional Gauss-Bonnet term as well as supersymmetric
completion respectively and compare the results with previous microscopic
calculations.Comment: 17 pages, no figure, JHEP3 style, to appear in JHEP
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