768 research outputs found
Scalar Field Theory on Non-commutative Snyder Space-Time
We construct a scalar field theory on the Snyder non-commutative space-time.
The symmetry underlying the Snyder geometry is deformed at the co-algebraic
level only, while its Poincar\'e algebra is undeformed. The Lorentz sector is
undeformed at both algebraic and co-algebraic level, but the co-product for
momenta (defining the star-product) is non-co-associative. The Snyder-deformed
Poincar\'e group is described by a non-co-associative Hopf algebra. The
definition of the interacting theory in terms of a non-associative star-product
is thus questionable. We avoid the non-associativity by the use of a space-time
picture based on the concept of realization of a non-commutative geometry. The
two main results we obtain are: (i) the generic (namely for any realization)
construction of the co-algebraic sector underlying the Snyder geometry and (ii)
the definition of a non-ambiguous self interacting scalar field theory on this
space-time. The first order correction terms of the corresponding Lagrangian
are explicitly computed. The possibility to derive Noether charges for the
Snyder space-time is also discussed.Comment: 10 pages; v2: introduction rewritten, co-algebraic analysis improved,
references added; to appear in PR
Wodzicki Residue for Operators on Manifolds with Cylindrical Ends
We define the Wodzicki Residue TR(A) for A in a space of operators with
double order (m_1,m_2). Such operators are globally defined initially on R^n
and then, more generally, on a class of non-compact manifolds, namely, the
manifolds with cylindrical ends. The definition is based on the analysis of the
associate zeta function. Using this approach, under suitable ellipticity
assumptions, we also compute a two terms leading part of the Weyl formula for a
positive selfadjoint operator belonging the mentioned class in the case
m_1=m_2.Comment: 24 pages, picture changed, added references, corrected typo
ENSO in the Mid-Holocene according to CSM and HadCM3
The offline linearized ocean–atmosphere model (LOAM), which was developed to quantify the impact of the climatological mean state on the variability of the El Niño–Southern Oscillation (ENSO), is used to illuminate why ENSO changed between the modern-day and early/mid-Holocene simulations in two climate modeling studies using the NCAR Climate System Model (CSM) and the Hadley Centre Coupled Model, version 3 (HadCM3). LOAM reproduces the spatiotemporal variability simulated by the climate models and shows both the reduction in the variance of ENSO and the changes in the spatial structure of the variance during the early/mid-Holocene. The mean state changes that are important in each model are different and, in both cases, are also different from those hypothesized to be important in the original papers describing these simulations. In the CSM simulations, the ENSO mode is stabilized by the mean cooling of the SST. This reduces atmospheric heating anomalies that in turn give smaller wind stress anomalies, thus weakening the Bjerknes feedback. Within the ocean, a change in the thermocline structure alters the spatial pattern of the variance, shifting the peak variance farther east, but does not reduce the overall amount of ENSO variance. In HadCM3, the ENSO mode is stabilized by a combination of a weaker thermocline and weakened horizontal surface currents. Both of these reduce the Bjerknes feedback by reducing the ocean’s SST response to wind stress forcing. This study demonstrates the importance of considering the combined effect of a mean state change on the coupled ocean–atmosphere system: conflicting and erroneous results are obtained for both models if only one model component is considered in isolation
Many-nodes/many-links spinfoam: the homogeneous and isotropic case
I compute the Lorentzian EPRL/FK/KKL spinfoam vertex amplitude for regular
graphs, with an arbitrary number of links and nodes, and coherent states peaked
on a homogeneous and isotropic geometry. This form of the amplitude can be
applied for example to a dipole with an arbitrary number of links or to the
4-simplex given by the compete graph on 5 nodes. All the resulting amplitudes
have the same support, independently of the graph used, in the large j (large
volume) limit. This implies that they all yield the Friedmann equation: I show
this in the presence of the cosmological constant. This result indicates that
in the semiclassical limit quantum corrections in spinfoam cosmology do not
come from just refining the graph, but rather from relaxing the large j limit.Comment: 8 pages, 4 figure
Physical boundary Hilbert space and volume operator in the Lorentzian new spin-foam theory
A covariant spin-foam formulation of quantum gravity has been recently
developed, characterized by a kinematics which appears to match well the one of
canonical loop quantum gravity. In this paper we reconsider the implementation
of the constraints that defines the model. We define in a simple way the
boundary Hilbert space of the theory, introducing a slight modification of the
embedding of the SU(2) representations into the SL(2,C) ones. We then show
directly that all constraints vanish on this space in a weak sense. The
vanishing is exact (and not just in the large quantum number limit.) We also
generalize the definition of the volume operator in the spinfoam model to the
Lorentzian signature, and show that it matches the one of loop quantum gravity,
as does in the Euclidean case.Comment: 11 page
Climate Engineering Responses to Climate Emergencies
Despite efforts to stabilize CO_2 concentrations, it is possible that the
climate system could respond abruptly with catastrophic consequences.
Intentional intervention in the climate system to avoid or ameliorate such
consequences has been proposed as one possible response, should such a scenario
arise. In a one-week study, the authors of this report conducted a technical
review and evaluation of proposed climate engineering concepts that might serve
as a rapid palliative response to such climate emergency scenarios.
Because of their potential to induce a prompt (less than one year) global
cooling, this study concentrated on Shortwave Climate Engineering (SWCE)
methods for moderately reducing the amount of shortwave solar radiation
reaching the Earth. The study's main objective was to outline a decade-long
agenda of technical research that would maximally reduce the uncertainty
surrounding the benefits and risks associated with SWCE. For rigor of technical
analysis, the study focused the research agenda on one particular SWCE
concept--stratospheric aerosol injection--and in doing so developed several
conceptual frameworks and methods valuable for assessing any SWCE proposal.Comment: 66 pp., 5 figs., published by Novim, Santa Barbara, Cal., revised
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