138 research outputs found
Atiyah-Hitchin M-Branes
We present new M2 and M5 brane solutions in M-theory based on transverse
Atiyah-Hitchin space and other self-dual geometries. One novel feature of these
solutions is that they have bolt-like fixed points yet still preserve 1/4 of
the supersymmetry. All the solutions can be reduced down to ten dimensional
intersecting brane configurations.Comment: 18 pages, 5 figures, one paragraph added in the conclusions, typos
correcte
New Reducible Five-brane Solutions in M-theory
We construct new M-theory solutions of M5 branes that are a realization of
the fully localized ten dimensional NS5/D6 and NS5/D5 brane intersections.
These solutions are obtained by embedding self-dual geometries lifted to
M-theory. We reduce these solutions down to ten dimensions, obtaining new
D-brane systems in type IIA/IIB supergravity. The worldvolume theories of the
NS5-branes are new non-local, non-gravitational, six dimensional, T-dual little
string theories with eight supersymmetries.Comment: 19 pages, 4 figures, two paragraphs added in conclusions, typos
correcte
Gibbons-Hawking M-branes
We present new M2 and M5-brane solutions in M-theory based on transverse
Gibbons-Hawking spaces. These solutions provide realizations of fully localized
type IIA D2/D6 and NS5/D6 brane intersections. One novel feature of these
solutions is that the metric functions depend on more than two transverse
coordinates (unlike all the other previous known solutions). All the solutions
have eight preserved supersymmetries and the world-volume theories of the
NS5-branes are new non-local, non-gravitational, six dimensional, T-dual little
string theories with eight supersymmetries. We discuss the limits in which the
dynamics of the D2 and NS5-branes decouple from the bulk for these solutions.Comment: 32 pages, 7 figures, few sentences added, a couple of typos
corrected, to appear in JHE
Symmetries of a class of nonlinear fourth order partial differential equations
In this paper we study symmetry reductions of a class of nonlinear fourth
order partial differential equations \be u_{tt} = \left(\kappa u + \gamma
u^2\right)_{xx} + u u_{xxxx} +\mu u_{xxtt}+\alpha u_x u_{xxx} + \beta u_{xx}^2,
\ee where , , , and are constants. This
equation may be thought of as a fourth order analogue of a generalization of
the Camassa-Holm equation, about which there has been considerable recent
interest. Further equation (1) is a ``Boussinesq-type'' equation which arises
as a model of vibrations of an anharmonic mass-spring chain and admits both
``compacton'' and conventional solitons. A catalogue of symmetry reductions for
equation (1) is obtained using the classical Lie method and the nonclassical
method due to Bluman and Cole. In particular we obtain several reductions using
the nonclassical method which are no} obtainable through the classical method
Qualitative Properties of Magnetic Fields in Scalar Field Cosmology
We study the qualitative properties of the class of spatially homogeneous
Bianchi VI_o cosmological models containing a perfect fluid with a linear
equation of state, a scalar field with an exponential potential and a uniform
cosmic magnetic field, using dynamical systems techniques. We find that all
models evolve away from an expanding massless scalar field model in which the
matter and the magnetic field are negligible dynamically. We also find that for
a particular range of parameter values the models evolve towards the usual
power-law inflationary model (with no magnetic field) and, furthermore, we
conclude that inflation is not fundamentally affected by the presence of a
uniform primordial magnetic field. We investigate the physical properties of
the Bianchi I magnetic field models in some detail.Comment: 12 pages, 2 figures in REVTeX format. to appear in Phys. Rev.
Testing homogeneity with the fossil record of galaxies
The standard Friedmann model of cosmology is based on the Copernican
Principle, i.e. the assumption of a homogeneous background on which structure
forms via perturbations. Homogeneity underpins both general relativistic and
modified gravity models and is central to the way in which we interpret
observations of the CMB and the galaxy distribution. It is therefore important
to probe homogeneity via observations. We describe a test based on the fossil
record of distant galaxies: if we can reconstruct key intrinsic properties of
galaxies as functions of proper time along their worldlines, we can compare
such properties at the same proper time for our galaxy and others. We achieve
this by computing the lookback time using radial Baryon Acoustic Oscillations,
and the time along galaxy world line using stellar physics, allowing us to
probe homogeneity, in principle anywhere inside the past light cone. Agreement
in the results would be an important consistency test -- although it would not
in itself prove homogeneity. Any significant deviation in the results however
would signal a breakdown of homogeneity.Comment: Accepted for publication in JCAP. Matches published version. Minor
changes: ref. added and longer discussion on performing the test
observationally. Results unchange
A note on the minimum effort control problem
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/33486/1/0000891.pd
Do primordial Lithium abundances imply there's no Dark Energy?
Explaining the well established observation that the expansion rate of the
universe is apparently accelerating is one of the defining scientific problems
of our age. Within the standard model of cosmology, the repulsive 'dark energy'
supposedly responsible has no explanation at a fundamental level, despite many
varied attempts. A further important dilemma in the standard model is the
Lithium problem, which is the substantial mismatch between the theoretical
prediction for 7-Li from Big Bang Nucleosynthesis and the value that we observe
today. This observation is one of the very few we have from along our past
worldline as opposed to our past lightcone. By releasing the untested
assumption that the universe is homogeneous on very large scales, both apparent
acceleration and the Lithium problem can be easily accounted for as different
aspects of cosmic inhomogeneity, without causing problems for other
cosmological phenomena such as the cosmic microwave background. We illustrate
this in the context of a void model.Comment: 14 pages, 4 figures. v2: minor rearrangements in the text, comments
and references expanded, results unchange
The Planetary Nebula Luminosity Function at the Dawn of Gaia
The [O III] 5007 Planetary Nebula Luminosity Function (PNLF) is an excellent
extragalactic standard candle. In theory, the PNLF method should not work at
all, since the luminosities of the brightest planetary nebulae (PNe) should be
highly sensitive to the age of their host stellar population. Yet the method
appears robust, as it consistently produces < 10% distances to galaxies of all
Hubble types, from the earliest ellipticals to the latest-type spirals and
irregulars. It is therefore uniquely suited for cross-checking the results of
other techniques and finding small offsets between the Population I and
Population II distance ladders. We review the calibration of the method and
show that the zero points provided by Cepheids and the Tip of the Red Giant
Branch are in excellent agreement. We then compare the results of the PNLF with
those from Surface Brightness Fluctuation measurements, and show that, although
both techniques agree in a relative sense, the latter method yields distances
that are ~15% larger than those from the PNLF. We trace this discrepancy back
to the calibration galaxies and argue that, due to a small systematic error
associated with internal reddening, the true distance scale likely falls
between the extremes of the two methods. We also demonstrate how PNLF
measurements in the early-type galaxies that have hosted Type Ia supernovae can
help calibrate the SN Ia maximum magnitude-rate of decline relation. Finally,
we discuss how the results from space missions such as Kepler and Gaia can help
our understanding of the PNLF phenomenon and improve our knowledge of the
physics of local planetary nebulae.Comment: 12 pages, invited review at the conference "The Fundamental Cosmic
Distance Scale: State of the Art and Gaia Perspective", to appear in
Astrophysics and Space Scienc
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