253 research outputs found
Orientifolds and Slumps in G_2 and Spin(7) Metrics
We discuss some new metrics of special holonomy, and their roles in string
theory and M-theory. First we consider Spin(7) metrics denoted by C_8, which
are complete on a complex line bundle over CP^3. The principal orbits are S^7,
described as a triaxially squashed S^3 bundle over S^4. The behaviour in the
S^3 directions is similar to that in the Atiyah-Hitchin metric, and we show how
this leads to an M-theory interpretation with orientifold D6-branes wrapped
over S^4. We then consider new G_2 metrics which we denote by C_7, which are
complete on an R^2 bundle over T^{1,1}, with principal orbits that are
S^3\times S^3. We study the C_7 metrics using numerical methods, and we find
that they have the remarkable property of admitting a U(1) Killing vector whose
length is nowhere zero or infinite. This allows one to make an everywhere
non-singular reduction of an M-theory solution to give a solution of the type
IIA theory. The solution has two non-trivial S^2 cycles, and both carry
magnetic charge with respect to the R-R vector field. We also discuss some
four-dimensional hyper-Kahler metrics described recently by Cherkis and
Kapustin, following earlier work by Kronheimer. We show that in certain cases
these metrics, whose explicit form is known only asymptotically, can be related
to metrics characterised by solutions of the su(\infty) Toda equation, which
can provide a way of studying their interior structure.Comment: Latex, 45 pages; minor correction
Supersymmetric Non-singular Fractional D2-branes and NS-NS 2-branes
We obtain regular deformed D2-brane solutions with fractional D2-branes
arising as wrapped D4-branes. The space transverse to the D2-brane is a
complete Ricci-flat 7-manifold of G_2 holonomy, which is asymptotically conical
with principal orbits that are topologically CP^3 or the flag manifold
SU(3)/(U(1) x U(1)). We obtain the solution by first constructing an L^2
normalisable harmonic 3-form. We also review a previously-obtained regular
deformed D2-brane whose transverse space is a different 7-manifold of G_2
holonomy, with principal orbits that are topologically S^3 x S^3. This
describes D2-branes with fractional NS-NS 2-branes coming from the wrapping of
5-branes, which is supported by a non-normalisable harmonic 3-form on the
7-manifold. We prove that both types of solutions are supersymmetric,
preserving 1/16 of the maximal supersymmetry and hence that they are dual to
{\cal N}=1 three-dimensional gauge theories. In each case, the spectrum for
minimally-coupled scalars is discrete, indicating confinement in the infrared
region of the dual gauge theories. We examine resolutions of other branes, and
obtain necessary conditions for their regularity. The resolution of many of
these seems to lie beyond supergravity. In the process of studying these
questions, we construct new explicit examples of complete Ricci-flat metrics.Comment: Latex, 30 page
BPS Branes in Cosmology
The possibility to study the M/string theory cosmology via 5d bulk & brane
action is investigated. The role of the 4-form field in the theory of BPS
branes in 5d is clarified. We describe arguments suggesting that the effective
4d description of the universe in the ekpyrotic scenario (hep-th/0103239)
should lead to contraction rather than expansion of the universe. To verify
these arguments, we study the full 5d action prior to its integration over the
5th dimension. We show that if one adds the potential V(Y) to the action of the
bulk brane, then the metric ansatz used in the ekpyrotic scenario does not
solve the dilaton and gravitational equations. To find a consistent
cosmological solution one must use a more general metric ansatz and a complete
5d description of the brane interaction instead of simply adding an effective
4d bulk brane potential V(Y).Comment: 18 pages, revte
Closed string tachyons, flips and conifolds
Following the analysis of tachyons and orbifold flips described in
hep-th/0412337, we study nonsupersymmetric analogs of the supersymmetric
conifold singularity and show using their toric geometry description that they
are nonsupersymmetric orbifolds of the latter. Using linear sigma models, we
see that these are unstable to localized closed string tachyon condensation and
exhibit flip transitions between their two small resolutions (involving
2-cycles), in the process mediating mild dynamical topology change. Our
analysis shows that the structure of these nonsupersymmetric conifolds as
quotients of the supersymmetric conifold obstructs the 3-cycle deformation of
such singularities, suggesting that these nonsupersymmetric conifolds decay by
evolving towards their stable small resolutions.Comment: Latex, 22 pgs, 2 figs. v4: matches JHEP version, 29 pgs, 3 figures,
more elaborate Introduction, various clarifications adde
The Centurion 18 telescope of the Wise Observatory
We describe the second telescope of the Wise Observatory, a 0.46-m Centurion
18 (C18) installed in 2005, which enhances significantly the observing
possibilities. The telescope operates from a small dome and is equipped with a
large-format CCD camera. In the last two years this telescope was intensively
used in a variety of monitoring projects.
The operation of the C18 is now automatic, requiring only start-up at the
beginning of a night and close-down at dawn. The observations are mostly
performed remotely from the Tel Aviv campus or even from the observer's home.
The entire facility was erected for a component cost of about 70k$ and a labor
investment of a total of one man-year.
We describe three types of projects undertaken with this new facility: the
measurement of asteroid light variability with the purpose of determining
physical parameters and binarity, the following-up of transiting extrasolar
planets, and the study of AGN variability. The successful implementation of the
C18 demonstrates the viability of small telescopes in an age of huge
light-collectors, provided the operation of such facilities is very efficient.Comment: 16 pages, 13 figures, some figures quality was degraded, accepted for
publication in Astrophysics and Space Scienc
Supersymmetric solutions of PT-/non-PT-symmetric and non-Hermitian Screened Coulomb potential via Hamiltonian hierarchy inspired variational method
The supersymmetric solutions of PT-symmetric and Hermitian/non-Hermitian
forms of quantum systems are obtained by solving the Schrodinger equation for
the Exponential-Cosine Screened Coulomb potential. The Hamiltonian hierarchy
inspired variational method is used to obtain the approximate energy
eigenvalues and corresponding wave functions.Comment: 13 page
de Sitter Vacua in String Theory
We outline the construction of metastable de Sitter vacua of type IIB string
theory. Our starting point is highly warped IIB compactifications with
nontrivial NS and RR three-form fluxes. By incorporating known corrections to
the superpotential from Euclidean D-brane instantons or gaugino condensation,
one can make models with all moduli fixed, yielding a supersymmetric AdS
vacuum. Inclusion of a small number of anti-D3 branes in the resulting warped
geometry allows one to uplift the AdS minimum and make it a metastable de
Sitter ground state. The lifetime of our metastable de Sitter vacua is much
greater than the cosmological timescale of 10^10 years. We also prove, under
certain conditions, that the lifetime of dS space in string theory will always
be shorter than the recurrence time.Comment: 12 pages, 2 figs, added comments on the thin wall approximation to
tunnelin
The fate of Reissner-Nortstr\"{o}m black hole in the Einstein-Yang-Mills-Higgs system
We study about an evaporating process of black holes in SO(3)
Einstein-Yang-Mills-Higgs system. We consider a massless scalar field which
couple neither with the Yang-Mills field nor with the Higgs field surrounding
the black hole. We discuss differences in evaporating rate between a monopole
black hole and a Reissner-Nortstr\"{o}m (RN) black hole.Comment: 9 pages, 8 figure
Is null-point reconnection important for solar flux emergence?
The role of null-point reconnection in a 3D numerical MHD model of solar
emerging flux is investigated. The model consists of a twisted magnetic flux
tube rising through a stratified convection zone and atmosphere to interact and
reconnect with a horizontal overlying magnetic field in the atmosphere. Null
points appear as the reconnection begins and persist throughout the rest of the
emergence, where they can be found mostly in the model photosphere and
transition region, forming two loose clusters on either side of the emerging
flux tube. Up to 26 nulls are present at any one time, and tracking in time
shows that there is a total of 305 overall, despite the initial simplicity of
the magnetic field configuration. We find evidence for the reality of the nulls
in terms of their methods of creation and destruction, their balance of signs,
their long lifetimes, and their geometrical stability. We then show that due to
the low parallel electric fields associated with the nulls, null-point
reconnection is not the main type of magnetic reconnection involved in the
interaction of the newly emerged flux with the overlying field. However, the
large number of nulls implies that the topological structure of the magnetic
field must be very complex and the importance of reconnection along separators
or separatrix surfaces for flux emergence cannot be ruled out.Comment: 26 pages, 12 figures. Added one referenc
Non-minimally Coupled Tachyonic Inflation in Warped String Background
We show that the non-minimal coupling of tachyon field to the scalar
curvature, as proposed by Piao et al, with the chosen coupling parameter does
not produce the effective potential where the tachyon field can roll down from
T=0 to large along the slope of the potential. We find a correct choice of
the parameters which ensures this requirement and support slow-roll inflation.
However, we find that the cosmological parameter found from the analysis of the
theory are not in the range obtained from observations. We then invoke warped
compactification and varying dilaton field over the compact manifold, as
proposed by Raeymaekers, to show that in such a setup the observed parameter
space can be ensured.Comment: minor typos corrected and references adde
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