988 research outputs found
Torsion, an alternative to dark matter?
We confront Einstein-Cartan's theory with the Hubble diagram. An affirmative
answer to the question in the title is compatible with today's supernovae data.Comment: 14 pp, 3 figures. Version 2 matches the version published in Gen.
Rel. Grav., references added. Version 3 corrects a factor 3 in Cartan's
equations to become
Beyond Einstein-Cartan gravity: Quadratic torsion and curvature invariants with even and odd parity including all boundary terms
Recently, gravitational gauge theories with torsion have been discussed by an
increasing number of authors from a classical as well as from a quantum field
theoretical point of view. The Einstein-Cartan(-Sciama-Kibble) Lagrangian has
been enriched by the parity odd pseudoscalar curvature (Hojman, Mukku, and
Sayed) and by torsion square and curvature square pieces, likewise of even and
odd parity. (i) We show that the inverse of the so-called Barbero-Immirzi
parameter multiplying the pseudoscalar curvature, because of the topological
Nieh-Yan form, can only be appropriately discussed if torsion square pieces are
included. (ii) The quadratic gauge Lagrangian with both parities, proposed by
Obukhov et al. and Baekler et al., emerges also in the framework of Diakonov et
al.(2011). We establish the exact relations between both approaches by applying
the topological Euler and Pontryagin forms in a Riemann-Cartan space expressed
for the first time in terms of irreducible pieces of the curvature tensor.
(iii) Only in a Riemann-Cartan spacetime, that is, in a spacetime with torsion,
parity violating terms can be brought into the gravitational Lagrangian in a
straightforward and natural way. Accordingly, Riemann-Cartan spacetime is a
natural habitat for chiral fermionic matter fields.Comment: 12 page latex, as version 2 an old file was submitted by mistake,
this is now the real corrected fil
Constraining Type Ia supernova models: SN 2011fe as a test case
The nearby supernova SN 2011fe can be observed in unprecedented detail.
Therefore, it is an important test case for Type Ia supernova (SN Ia) models,
which may bring us closer to understanding the physical nature of these
objects. Here, we explore how available and expected future observations of SN
2011fe can be used to constrain SN Ia explosion scenarios. We base our
discussion on three-dimensional simulations of a delayed detonation in a
Chandrasekhar-mass white dwarf and of a violent merger of two white
dwarfs-realizations of explosion models appropriate for two of the most
widely-discussed progenitor channels that may give rise to SNe Ia. Although
both models have their shortcomings in reproducing details of the early and
near-maximum spectra of SN 2011fe obtained by the Nearby Supernova Factory
(SNfactory), the overall match with the observations is reasonable. The level
of agreement is slightly better for the merger, in particular around maximum,
but a clear preference for one model over the other is still not justified.
Observations at late epochs, however, hold promise for discriminating the
explosion scenarios in a straightforward way, as a nucleosynthesis effect leads
to differences in the 55Co production. SN 2011fe is close enough to be followed
sufficiently long to study this effect.Comment: Accepted for publication in The Astrophysical Journal Letter
Weak Lensing from Space I: Instrumentation and Survey Strategy
A wide field space-based imaging telescope is necessary to fully exploit the
technique of observing dark matter via weak gravitational lensing. This first
paper in a three part series outlines the survey strategies and relevant
instrumental parameters for such a mission. As a concrete example of hardware
design, we consider the proposed Supernova/Acceleration Probe (SNAP). Using
SNAP engineering models, we quantify the major contributions to this
telescope's Point Spread Function (PSF). These PSF contributions are relevant
to any similar wide field space telescope. We further show that the PSF of SNAP
or a similar telescope will be smaller than current ground-based PSFs, and more
isotropic and stable over time than the PSF of the Hubble Space Telescope. We
outline survey strategies for two different regimes - a ``wide'' 300 square
degree survey and a ``deep'' 15 square degree survey that will accomplish
various weak lensing goals including statistical studies and dark matter
mapping.Comment: 25 pages, 8 figures, 1 table, replaced with Published Versio
Supernova / Acceleration Probe: A Satellite Experiment to Study the Nature of the Dark Energy
The Supernova / Acceleration Probe (SNAP) is a proposed space-based
experiment designed to study the dark energy and alternative explanations of
the acceleration of the Universe's expansion by performing a series of
complementary systematics-controlled measurements. We describe a
self-consistent reference mission design for building a Type Ia supernova
Hubble diagram and for performing a wide-area weak gravitational lensing study.
A 2-m wide-field telescope feeds a focal plane consisting of a 0.7
square-degree imager tiled with equal areas of optical CCDs and near infrared
sensors, and a high-efficiency low-resolution integral field spectrograph. The
SNAP mission will obtain high-signal-to-noise calibrated light-curves and
spectra for several thousand supernovae at redshifts between z=0.1 and 1.7. A
wide-field survey covering one thousand square degrees resolves ~100 galaxies
per square arcminute. If we assume we live in a cosmological-constant-dominated
Universe, the matter density, dark energy density, and flatness of space can
all be measured with SNAP supernova and weak-lensing measurements to a
systematics-limited accuracy of 1%. For a flat universe, the
density-to-pressure ratio of dark energy can be similarly measured to 5% for
the present value w0 and ~0.1 for the time variation w'. The large survey area,
depth, spatial resolution, time-sampling, and nine-band optical to NIR
photometry will support additional independent and/or complementary dark-energy
measurement approaches as well as a broad range of auxiliary science programs.
(Abridged)Comment: 40 pages, 18 figures, submitted to PASP, http://snap.lbl.go
Two Loop Scalar Self-Mass during Inflation
We work in the locally de Sitter background of an inflating universe and
consider a massless, minimally coupled scalar with a quartic self-interaction.
We use dimensional regularization to compute the fully renormalized scalar
self-mass-squared at one and two loop order for a state which is released in
Bunch-Davies vacuum at t=0. Although the field strength and coupling constant
renormalizations are identical to those of lfat space, the geometry induces a
non-zero mass renormalization. The finite part also shows a sort of growing
mass that competes with the classical force in eventually turning off this
system's super-acceleration.Comment: 31 pages, 5 figures, revtex4, revised for publication with extended
list of reference
The detection of neutron clusters
A new approach to the production and detection of bound neutron clusters is
presented. The technique is based on the breakup of beams of very neutron-rich
nuclei and the subsequent detection of the recoiling proton in a liquid
scintillator. The method has been tested in the breakup of 11Li, 14Be and 15B
beams by a C target. Some 6 events were observed that exhibit the
characteristics of a multineutron cluster liberated in the breakup of 14Be,
most probably in the channel 10Be+4n. The various backgrounds that may mimic
such a signal are discussed in detail.Comment: 11 pages, 12 figures, LPCC 01-1
Search for supersymmetry with a dominant R-parity violating LQDbar couplings in e+e- collisions at centre-of-mass energies of 130GeV to 172 GeV
A search for pair-production of supersymmetric particles under the assumption
that R-parity is violated via a dominant LQDbar coupling has been performed
using the data collected by ALEPH at centre-of-mass energies of 130-172 GeV.
The observed candidate events in the data are in agreement with the Standard
Model expectation. This result is translated into lower limits on the masses of
charginos, neutralinos, sleptons, sneutrinos and squarks. For instance, for
m_0=500 GeV/c^2 and tan(beta)=sqrt(2) charginos with masses smaller than 81
GeV/c^2 and neutralinos with masses smaller than 29 GeV/c^2 are excluded at the
95% confidence level for any generation structure of the LQDbar coupling.Comment: 32 pages, 30 figure
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