2,846 research outputs found
How (Not) to Palatini
We revisit the problem of defining non-minimal gravity in the first order
formalism. Specializing to scalar-tensor theories, which may be disguised as
`higher-derivative' models with the gravitational Lagrangians that depend only
on the Ricci scalar, we show how to recast these theories as Palatini-like
gravities. The correct formulation utilizes the Lagrange multiplier method,
which preserves the canonical structure of the theory, and yields the
conventional metric scalar-tensor gravity. We explain the discrepancies between
the na\"ive Palatini and the Lagrange multiplier approach, showing that the
na\"ive Palatini approach really swaps the theory for another. The differences
disappear only in the limit of ordinary General Relativity, where an accidental
redundancy ensures that the na\"ive Palatini works there. We outline the
correct decoupling limits and the strong coupling regimes. As a corollary we
find that the so-called `Modified Source Gravity' models suffer from strong
coupling problems at very low scales, and hence cannot be a realistic
approximation of our universe. We also comment on a method to decouple the
extra scalar using the chameleon mechanism.Comment: 18 pages, LaTeX; added references and minor improvements in sec
Data report for the Siple Coast (Antarctica) project
This report presents data collected during three field seasons of glaciological studies in the Antarctica and describes the methods employed. The region investigated covers the mouths of Ice Streams B and C (the Siple Coast) and Crary Ice Rise on the Ross Ice Shelf. Measurements included in the report are as follows: surface velocity and deformation from repeated satellite geoceiver positions; surface topography from optical levelling; radar sounding of ice thickness; accumulation rates; near-surface densities and temperature profiles; and mapping from aerial photography
Quantization of the First-Order Two-Dimensional Einstein-Hilbert Action
A canonical analysis of the first-order two-dimensional Einstein-Hilbert
action has shown it to have no physical degrees of freedom and to possess an
unusual gauge symmetry with a symmetric field acting as a gauge
function. Some consequences of this symmetry are explored. The action is
quantized and it is shown that all loop diagrams beyond one-loop order vanish.
Furthermore, explicit calculation of the one-loop two-point function shows that
it too vanishes, with the contribution of the ghost loop cancelling that of the
``graviton'' loop
Relating Green-Schwarz and Extended Pure Spinor Formalisms by Similarity Transformation
In order to gain deeper understanding of pure-spinor-based formalisms of
superstring, an explicit similarity transformation is constructed which
provides operator mapping between the light-cone Green-Schwarz (LCGS) formalism
and the extended pure spinor (EPS) formalism, a recently proposed
generalization of the Berkovits' formalism in an enlarged space. By applying a
systematic procedure developed in our previous work, we first construct an
analogous mapping in the bosonic string relating the BRST and the light-cone
formulations. This provides sufficient insights and allows us to construct the
desired mapping in the more intricate case of superstring as well. The success
of the construction owes much to the enlarged field space where pure spinor
constraints are removed and to the existence of the ``B-ghost'' in the EPS
formalism.Comment: 37pages, no figur
Fundamental Strings in Open String Theory at the Tachyonic Vacuum
We show that the world-volume theory on a D-p-brane at the tachyonic vacuum
has solitonic string solutions whose dynamics is governed by the Nambu-Goto
action of a string moving in (25+1) dimensional space-time. This provides
strong evidence for the conjecture that at this vacuum the full (25+1)
dimensional Poincare invariance is restored. We also use this result to argue
that the open string field theory at the tachyonic vacuum must contain closed
string excitations.Comment: LaTeX file, 16 pages, references and clarification adde
Twistor theory of hyper-K{\"a}hler metrics with hidden symmetries
We briefly review the hierarchy for the hyper-K\"ahler equations and define a
notion of symmetry for solutions of this hierarchy. A four-dimensional
hyper-K\"ahler metric admits a hidden symmetry if it embeds into a hierarchy
with a symmetry. It is shown that a hyper-K\"ahler metric admits a hidden
symmetry if it admits a certain Killing spinor. We show that if the hidden
symmetry is tri-holomorphic, then this is equivalent to requiring symmetry
along a higher time and the hidden symmetry determines a `twistor group' action
as introduced by Bielawski \cite{B00}. This leads to a construction for the
solution to the hierarchy in terms of linear equations and variants of the
generalised Legendre transform for the hyper-K\"ahler metric itself given by
Ivanov & Rocek \cite{IR96}. We show that the ALE spaces are examples of
hyper-K\"ahler metrics admitting three tri-holomorphic Killing spinors. These
metrics are in this sense analogous to the 'finite gap' solutions in soliton
theory. Finally we extend the concept of a hierarchy from that of \cite{DM00}
for the four-dimensional hyper-K\"ahler equations to a generalisation of the
conformal anti-self-duality equations and briefly discuss hidden symmetries for
these equations.Comment: Final version. To appear in the August 2003 special issue of JMP on
`Integrability, Topological Solitons, and Beyond
- âŠ