726 research outputs found
Gravity thaws the frozen moduli of the CP^1 lump
The slow motion of a self-gravitating CP^1 lump is investigated in the
approximation of geodesic flow on the moduli space of unit degree static
solutions M_1. It is found that moduli which are frozen in the absence of
gravity, parametrizing the lump's width and internal orientation, may vary once
gravitational effects are included. If gravitational coupling is sufficiently
strong, the presence of the lump shrinks physical space to finite volume, and
the moduli determining the boundary value of the CP^1 field thaw also. Explicit
formulae for the metric on M_1 are found in both the weak and strong coupling
regimes. The geodesic problem for weak coupling is studied in detail, and it is
shown that M_1 is geodesically incomplete. This leads to the prediction that
self-gravitating lumps are unstable.Comment: 6 pages, minor error corrected (conclusions unchanged
Wrapped M2/M5 Duality
A microscopic accounting of the entropy of a generic 5D supersymmetric
rotating black hole, arising from wrapped M2-branes in Calabi-Yau compactified
M-theory, is an outstanding unsolved problem. In this paper we consider an
expansion around the zero-entropy, zero-temperature, maximally rotating ground
state for which the angular momentum J_L and graviphoton charge Q are related
by J_L^2=Q^3. At J_L=0 the near horizon geometry is AdS_2 x S^3. As J_L^2 goes
to Q^3 it becomes a singular quotient of AdS_3 x S^2: more precisely, a
quotient of the near horizon geometry of an M5 wrapped on a 4-cycle whose
self-intersection is the 2-cycle associated to the wrapped-M2 black hole. The
singularity of the AdS_3 quotient is identified as the usual one associated to
the zero-temperature limit, suggesting that the (0,4) wrapped-M5 CFT is dual
near maximality to the wrapped-M2 black hole. As evidence for this, the
microscopic (0,4) CFT entropy and the macroscopic rotating black hole entropy
are found to agree to leading order away from maximality.Comment: 10 pages, no figure
Hidden supersymmetries in supersymmetric quantum mechanics
We discuss the appearance of additional, hidden supersymmetries for simple
0+1 -invariant supersymmetric models and analyse some geometrical
mechanisms that lead to them. It is shown that their existence depends
crucially on the availability of odd order invariant skewsymmetric tensors on
the (generic) compact Lie algebra , and hence on the cohomology
properties of the Lie algebra considered.Comment: Misprints corrected, two refs. added. To appear in NP
M5-brane Effective Action as an On-shell Action in Supergravity
We show that the covariant effective action for M5-brane is a solution to the
Hamilton-Jacobi (H-J) equations of 11-dimensional supergravity. The solution to
the H-J equations reproduces the supergravity solution that represents the
M2-M5 bound states.Comment: 20 pages, references added, typos correcte
Brane Dynamics From the Born-Infeld Action
We use the abelian Born-Infeld action for the worldvolume gauge field and
transverse displacement scalars to explore new aspects of D-brane structure and
dynamics. We study several classic gauge field configurations, including point
charges in any worldvolume dimension and vortices in two worldvolume
dimensions, and show that, with an appropriate excitation of the transverse
coordinate field, they are BPS-saturated solutions. The Coulomb point charge
solutions turn out to represent, with considerable fidelity, fundamental
strings attached to the brane (their magnetic counterparts describe D1-branes
attached to D3-branes). We also show that S-matrix for small excitations
propagating on the point charge solution is consistent with (and gives further
illuminating information about) Polchinski's effective open string boundary
condition.Comment: 16 pages, 3 figures. Minor typos fixe
Could biodiversity loss have increased Australiaâs bushfire threat
Ecosystem engineers directly or indirectly affect the availability of resources
through changing the physical state of biotic and/or abiotic materials. Fossorial
ecosystem engineers have been hypothesized as affecting fire behaviour through
altering litter accumulation and breakdown, however, little evidence of this has
been shown to date. Fire is one of the major ecological processes affecting biodiversity
globally. Australia has seen the extinction of 29 of 315 terrestrial mammal
species in the last 200 years and several of these species were ecosystem engineers
whose fossorial actions may increase the rate of leaf litter breakdown. Thus, their
extinction may have altered the rate of litter accumulation and therefore fire ignition
potential and rate of spread. We tested whether a reduction in leaf litter was
associated with sites where mammalian ecosystem engineers had been reintroduced
using a pair-wise, cross-fence comparison at sites spanning the Australian continent.
At Scotia (New South Wales), Karakamia (Western Australia) and Yookamurra
(South Australia) sanctuaries, leaf litter mass ( 24%) and percentage cover
of leaf litter ( 3%) were significantly lower where reintroduced ecosystem engineers
occurred compared to where they were absent, and fire behaviour modelling
illustrated this has substantial impacts on flame height and rate of spread. This
result has major implications for fire behaviour and management globally wherever
ecosystem engineers are now absent as the reduced leaf litter volumes where they
occur will lead to decreased flame height and rate of fire spread. This illustrates
the need to restore the full suite of biodiversity globally.http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1469-17952017-12-31hb2016Centre for Wildlife Managemen
Mirage Cosmology of U(1) Gauge Field on Unstable D3 Brane Universe
An unstable -brane universe governed by the DBI action of the tachyon
field minimally coupled to a U(1) gauge boson is examined. The cosmological
evolution of this coupled system, is further analyzed, in terms of the
expansion rate of the inflating brane, which is highly affected by the presence
of the tachyonic and gauge field charges. We show, that the minimal coupling
makes the effective brane density less divergent. However, for some sectors of
the theory the tachyon is not able to regulate it in an efficient fashion.
Also, a detailed analysis of the dependance of the effective brane density on
the scale factor of the universe is performed, which leads to various
cosmological models.Comment: ReVTeX format 20 pages; v2 1 figure added, one additional paragraph
with extra comments added, enlarged list of references, version to appear in
JHE
Quantum lump dynamics on the two-sphere
It is well known that the low-energy classical dynamics of solitons of
Bogomol'nyi type is well approximated by geodesic motion in M_n, the moduli
space of static n-solitons. There is an obvious quantization of this dynamics
wherein the wavefunction evolves according to the Hamiltonian H_0 equal to
(half) the Laplacian on M_n. Born-Oppenheimer reduction of analogous mechanical
systems suggests, however, that this simple Hamiltonian should receive
corrections including k, the scalar curvature of M_n, and C, the n-soliton
Casimir energy, which are usually difficult to compute, and whose effect on the
energy spectrum is unknown. This paper analyzes the spectra of H_0 and two
corrections to it suggested by work of Moss and Shiiki, namely H_1=H_0+k/4 and
H_2=H_1+C, in the simple but nontrivial case of a single CP^1 lump moving on
the two-sphere. Here M_1=TSO(3), a noncompact kaehler 6-manifold invariant
under an SO(3)xSO(3) action, whose geometry is well understood. The symmetry
gives rise to two conserved angular momenta, spin and isospin. A hidden
isometry of M_1 is found which implies that all three energy spectra are
symmetric under spin-isospin interchange. The Casimir energy is found exactly
on the zero section of TSO(3), and approximated numerically on the rest of M_1.
The lowest 19 eigenvalues of H_i are found for i=0,1,2, and their spin-isospin
and parity compared. The curvature corrections in H_1 lead to a qualitatively
unchanged low-level spectrum while the Casimir energy in H_2 leads to
significant changes. The scaling behaviour of the spectra under changes in the
radii of the domain and target spheres is analyzed, and it is found that the
disparity between the spectra of H_1 and H_2 is reduced when the target sphere
is made smaller.Comment: 35 pages, 3 figure
The kink Casimir energy in a lattice sine-Gordon model
The Casimir energy of quantum fluctuations about the classical kink
configuration is computed numerically for a recently proposed lattice
sine-Gordon model. This energy depends periodically on the kink position and is
found to be approximately sinusoidal.Comment: 10 pages, 4 postscript figure
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