38,415 research outputs found
Axiomatic approach to the cosmological constant
A theory of the cosmological constant Lambda is currently out of reach.
Still, one can start from a set of axioms that describe the most desirable
properties a cosmological constant should have. This can be seen in certain
analogy to the Khinchin axioms in information theory, which fix the most
desirable properties an information measure should have and that ultimately
lead to the Shannon entropy as the fundamental information measure on which
statistical mechanics is based. Here we formulate a set of axioms for the
cosmological constant in close analogy to the Khinchin axioms, formally
replacing the dependency of the information measure on probabilities of events
by a dependency of the cosmological constant on the fundamental constants of
nature. Evaluating this set of axioms one finally arrives at a formula for the
cosmological constant that is given by Lambda = (G^2/hbar^4) (m_e/alpha_el)^6,
where G is the gravitational constant, m_e is the electron mass, and alpha_el
is the low energy limit of the fine structure constant. This formula is in
perfect agreement with current WMAP data. Our approach gives physical meaning
to the Eddington-Dirac large number hypothesis and suggests that the observed
value of the cosmological constant is not at all unnatural.Comment: 7 pages, no figures. Some further references adde
Circumstellar environment of the M-type AGB star R Dor. APEX spectral scan at GHz
Our current insights into the circumstellar chemistry of asymptotic giant
branch (AGB) stars are largely based on studies of carbon-rich stars and stars
with high mass-loss rates. In order to expand the current molecular inventory
of evolved stars we present a spectral scan of the nearby, oxygen-rich star R
Dor, a star with a low mass-loss rate (/yr). We
carried out a spectral scan in the frequency ranges 159.0-321.5GHz and
338.5-368.5 GHz (wavelength range 0.8-1.9mm) using the SEPIA/Band-5 and SHeFI
instruments on the APEX telescope and we compare it to previous surveys,
including one of the oxygen-rich AGB star IK Tau, which has a high mass-loss
rate (/yr). The spectrum of R Dor is dominated by
emission lines of SO and the different isotopologues of SiO. We also detect
CO, HO, HCN, CN, PO, PN, SO, and tentatively TiO, AlO, and NaCl.
Sixteen out of approximately 320 spectral features remain unidentified. Among
these is a strong but previously unknown maser at 354.2 GHz, which we suggest
could pertain to HSiO, silanone. With the exception of one, none of these
unidentified lines are found in a similarly sensitive survey of IK Tau
performed with the IRAM 30m telescope. We present radiative transfer models for
five isotopologues of SiO (SiO, SiO, SiO, SiO,
SiO), providing constraints on their fractional abundance and radial
extent. We derive isotopic ratios for C, O, Si, and S and estimate that R Dor
likely had an initial mass in the range 1.3-1.6, in agreement with
earlier findings based on models of HO line emission. From the presence of
spectral features recurring in many of the measured thermal and maser emission
lines we tentatively identify up to five kinematical components in the outflow
of R Dor, indicating deviations from a smooth, spherical wind.Comment: 66 pages, 25 figures, Accepted for publication in Astronomy &
Astrophysics. Fully reduced FITS spectrum made available through CD
Possible Measurable Effects of Dark Energy in Rotating Superconductors
We discuss recent laboratory experiments with rotating superconductors and
show that three so far unexplained experimentally observed effects (anomalous
acceleration signals, anomalous gyroscope signals, Cooper pair mass excess) can
be physically explained in terms of a possible interaction of dark energy with
Cooper pairs. Our approach is based on a Ginzburg-Landau-like model of
electromagnetic dark energy, where gravitationally active photons obtain mass
in the superconductor. We show that this model can account simultaneously for
the anomalous acceleration and anomalous gravitomagnetic fields around rotating
superconductors measured by Tajmar et al. and for the anomalous Cooper pair
mass in superconductive Niobium, measured by Cabrera and Tate. It is argued
that these three different physical effects are ultimately different
experimental manifestations of the simultaneous spontaneous breaking of gauge
invariance, and of the principle of general covariance in superconductive
materials.Comment: 22 page
Macroscopic Features of Light Heavy-Ion Fission Reactions
Global macroscopic features observed in the fully-damped binary processes in
light di-nuclear systems, such as limiting angular momenta, mean total kinetic
energies and energy thresholds for fusion-fission processes (''fission
thresholds") are presented. Their deduced systematics are consistent with that
obtained for heavier systems and follow a fusion-fission picture which can be
described by a realistic rotating liquid drop model considering diffuse-surface
and finite-nuclear-range effects.Comment: 8 pages(REVTeX), 3 figures available upon request, to appear in Phys.
Rev.
Measuring interstellar magnetic fields by radio synchrotron emission
Radio synchrotron emission, its polarization and its Faraday rotation are
powerful tools to study the strength and structure of interstellar magnetic
fields. The total intensity traces the strength and distribution of total
magnetic fields. Total fields in gas-rich spiral arms and bars of nearby
galaxies have strengths of 20-30 Gauss, due to the amplification of
turbulent fields, and are dynamically important. In the Milky Way, the total
field strength is about 6 G near the Sun and several 100 G in
filaments near the Galactic Center. -- The polarized intensity measures ordered
fields with a preferred orientation, which can be regular or anisotropic
fields. Ordered fields with spiral structure exist in grand-design, barred,
flocculent and even in irregular galaxies. The strongest ordered fields are
found in interarm regions, sometimes forming "magnetic spiral arms" between the
optical arms. Halo fields are X-shaped, probably due to outflows. -- The
Faraday rotation of the polarization vectors traces coherent regular fields
which have a preferred direction. In some galaxies Faraday rotation reveals
large-scale patterns which are signatures of dynamo fields. However, in most
galaxies the field has a complicated structure and interacts with local gas
flows. In the Milky Way, diffuse polarized radio emission and Faraday rotation
of the polarized emission from pulsars and background sources show many
small-scale and large-scale magnetic features, but the overall field structure
in our Galaxy is still under debate.Comment: 10 pages, 5 figures. To be published in "Cosmic Magnetic Fields: From
Planets, to Stars and Galaxies", K.G. Strassmeier, A.G. Kosovichev & J.E.
Beckman, eds., Proc. IAU Symp. 259, CU
The basic analytics of access to financial services
Access to financial services, or rather the lack thereof, is often indiscriminately decried as a problem in many developing countries. The authors argue that the"problem of access"should rather be analyzed by identifying different demand and supply constraints. They use the concept of an access possibilities frontier, drawn for a given set of state variables, to distinguish between cases where a financial system settles below the constrained optimum, cases where this constrained optimum is too low, and-in credit services-cases where the observed outcome is excessively high. They distinguish between payment and savings services and fixed intermediation costs, on the one hand, and lending services and different sources of credit risk, on the other hand. The authors include both supply and demand side frictions that can lead to lower access. The analysis helps identify bankable and banked population, the binding constraint to close the gap between the two, and policies to prudently expand the bankable population. This new conceptual framework can inform the debate on adequate policies to expand access to financial services and can serve as the basis for an informed measurement of access.Banks&Banking Reform,Economic Theory&Research,Markets and Market Access,Access to Markets,Financial Intermediation
Surface energetics and structure of the Ge wetting layer on Si(100)
Ge deposited on Si(100) initially forms heteroepitaxial layers, which grow to a critical thickness of ~3 MLs before the appearance of three-dimensional strain relieving structures. Experimental observations reveal that the surface structure of this Ge wetting layer is a dimer vacancy line (DVL) superstructure of the unstrained Ge(100) dimer reconstruction. In the following, the results of first-principles calculations of the thickness dependence of the wetting layer surface excess energy for the c(4×2) and 4×6 DVL surface reconstructions are reported. These results predict a wetting layer critical thickness of ~3 MLs, which is largely unaffected by the presence of dimer vacancy lines. The 4×6 DVL reconstruction is found to be thermodynamically stable with respect to the c(4×2) structure for wetting layers at least 2 ML thick. A strong correlation between the fraction of total surface induced deformation present in the substrate and the thickness dependence of wetting layer surface energy is also shown
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