1,176 research outputs found
Relatively computably enumerable reals
A real X is defined to be relatively c.e. if there is a real Y such that X is
c.e.(Y) and Y does not compute X. A real X is relatively simple and above if
there is a real Y <_T X such that X is c.e.(Y) and there is no infinite subset
Z of the complement of X such that Z is c.e.(Y). We prove that every nonempty
Pi^0_1 class contains a member which is not relatively c.e. and that every
1-generic real is relatively simple and above.Comment: 5 pages. Significant changes from earlier versio
Classification of Generalized Symmetries for the Vacuum Einstein Equations
A generalized symmetry of a system of differential equations is an
infinitesimal transformation depending locally upon the fields and their
derivatives which carries solutions to solutions. We classify all generalized
symmetries of the vacuum Einstein equations in four spacetime dimensions. To
begin, we analyze symmetries that can be built from the metric, curvature, and
covariant derivatives of the curvature to any order; these are called natural
symmetries and are globally defined on any spacetime manifold. We next classify
first-order generalized symmetries, that is, symmetries that depend on the
metric and its first derivatives. Finally, using results from the
classification of natural symmetries, we reduce the classification of all
higher-order generalized symmetries to the first-order case. In each case we
find that the generalized symmetries are infinitesimal generalized
diffeomorphisms and constant metric scalings. There are no non-trivial
conservation laws associated with these symmetries. A novel feature of our
analysis is the use of a fundamental set of spinorial coordinates on the
infinite jet space of Ricci-flat metrics, which are derived from Penrose's
``exact set of fields'' for the vacuum equations.Comment: 57 pages, plain Te
X-ray Sources and their Optical Counterparts in the Globular Cluster M4
We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the
Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the
core and 19 more within the cluster half-mass radius. The limiting luminosity
of this observation is Lx~10e29 erg/sec for sources associated with the
cluster, the deepest X-ray observation of a globular cluster to date. We
identify 6 X-ray sources with known objects and use ROSAT observations to show
that the brightest X-ray source is variable. Archival data from the Hubble
Space Telescope allow us to identify optical counterparts to 16 X-ray sources.
Based on the X-ray and optical properties of the identifications and the
information from the literature, we classify two (possibly three) sources as
cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources
as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397
suggests a scaling of the number of active binaries in these clusters with the
cluster (core) mass.Comment: 11 pages, 6 figures, accepted for publication in ApJ. Figure 1 and 5
are of reduced qualit
Thermodynamics and Kinetic Theory of Relativistic Gases in 2-D Cosmological Models
A kinetic theory of relativistic gases in a two-dimensional space is
developed in order to obtain the equilibrium distribution function and the
expressions for the fields of energy per particle, pressure, entropy per
particle and heat capacities in equilibrium. Furthermore, by using the method
of Chapman and Enskog for a kinetic model of the Boltzmann equation the
non-equilibrium energy-momentum tensor and the entropy production rate are
determined for a universe described by a two-dimensional Robertson-Walker
metric. The solutions of the gravitational field equations that consider the
non-equilibrium energy-momentum tensor - associated with the coefficient of
bulk viscosity - show that opposed to the four-dimensional case, the cosmic
scale factor attains a maximum value at a finite time decreasing to a "big
crunch" and that there exists a solution of the gravitational field equations
corresponding to a "false vacuum". The evolution of the fields of pressure,
energy density and entropy production rate with the time is also discussed.Comment: 23 pages, accepted in PR
What do young athletes implicitly understand about psychological skills?
One reason sport psychologists teach psychological skills is to enhance performance in sport; but the value of psychological skills for young athletes is questionable because of the qualitative and quantitative differences between children and adults in their understanding of abstract concepts such as mental skills. To teach these skills effectively to young athletes, sport psychologists need to appreciate what young athletes implicitly understand about such skills because maturational (e.g., cognitive, social) and environmental (e.g., coaches) factors can influence the progressive development of children and youth. In the present qualitative study, we explored young athletes’ (aged 10–15 years) understanding of four basic psychological skills: goal setting, mental imagery, self-talk, and relaxation. Young athletes (n = 118: 75 males and 43 females) completed an open-ended questionnaire to report their understanding of these four basic psychological skills. Compared with the older youth athletes, the younger youth athletes were less able to explain the meaning of each psychological skill. Goal setting and mental imagery were better understood than self-talk and relaxation. Based on these findings, sport psychologists should consider adapting interventions and psychoeducational programs to match young athletes’ age and developmental level
Evolution of the pairing pseudogap in the spectral function with interplane anisotropy
We study the pairing pseudogap in the spectral function as a function of
interplane coupling. The analytical expressions for the self-energy in the
critical regime are obtained for any degree of anisotropy. The frequency
dependence of the self-energy is found to be qualitatively different in two and
three dimensions, and the crossover from two to three dimensional behavior is
discussed. In particular, by considering the anisotropy of the Fermi velocity
and gap along the Fermi surface, we can qualitatively explain recent
photoemission experiments on high temperature superconductors concerning the
temperature dependent Fermi arcs seen in the pseudogap phase.Comment: 20 pages, revtex, 5 encapsulated postscript figures include
Magneto-optical trap for metastable helium at 389 nm
We have constructed a magneto-optical trap (MOT) for metastable triplet
helium atoms utilizing the 2 3S1 -> 3 3P2 line at 389 nm as the trapping and
cooling transition. The far-red-detuned MOT (detuning Delta = -41 MHz)
typically contains few times 10^7 atoms at a relatively high (~10^9 cm^-3)
density, which is a consequence of the large momentum transfer per photon at
389 nm and a small two-body loss rate coefficient (2 * 10^-10 cm^3/s < beta <
1.0 * 10^-9 cm^3/s). The two-body loss rate is more than five times smaller
than in a MOT on the commonly used 2 3S1 -> 2 3P2 line at 1083 nm. Furthermore,
we measure a temperature of 0.46(1) mK, a factor 2.5 lower as compared to the
1083 nm case. Decreasing the detuning to Delta= -9 MHz results in a cloud
temperature as low as 0.25(1) mK, at small number of trapped atoms. The 389 nm
MOT exhibits small losses due to two-photon ionization, which have been
investigated as well.Comment: 11 page
An adaptive inelastic magnetic mirror for Bose-Einstein condensates
We report the reflection and focussing of a Bose-Einstein condensate by a new
pulsed magnetic mirror. The mirror is adaptive, inelastic, and of extremely
high optical quality. The deviations from specularity are less than 0.5 mrad
rms, making this the best atomic mirror demonstrated to date. We have also used
the mirror to realize the analog of a beam-expander, producing an ultra-cold
collimated fountain of matter wavesComment: 4 pages, 4 figure
A strongly first order electroweak phase transition from strong symmetry-breaking interactions
We argue that a strongly first order electroweak phase transition is natural
in the presence of strong symmetry-breaking interactions, such as technicolor.
We demonstrate this using an effective linear scalar theory of the
symmetry-breaking sector.Comment: LaTex, 15 pages, 3 figures in EPS format. Phys. Rev. D approved
Typographically Correct version, minor grammatical change
Late Holocene isotope hydrology of Lake Qinghai, NE Tibetan Plateau: effective moisture variability and atmospheric circulation changes
A sub-centennial-resolution record of lacustrine carbonate oxygen isotopes (δ<sup>18</sup>O<sub>C</sub>) from the closed-basin Lake Qinghai on the NE Tibetan Plateau shows pronounced variability over the past 1500 years. Changes in δ<sup>18</sup>O<sub>C</sub> in hydrologically closed lakes are often interpreted in terms of changing effective moisture. Under this interpretation our record would imply increasing effective moisture during the Little Ice Age (LIA) compared to the Medieval Warm Period (MWP). However, independent evidence from other archives strongly suggests the Asian summer monsoon was stronger during the MWP and weakened during the LIA. Controls other than effective moisture (the balance of water inputs over evaporative loss) must therefore have contributed to the δ<sup>18</sup>O<sub>C</sub> values. We propose the LIA signal in Lake Qinghai resulted from a reduction in evaporation caused by colder air temperatures, coupled with a decrease in oxygen isotope composition of input waters as a result of an increase in the relative importance of westerly-derived precipitation. Our results caution against simplistic interpretations of carbonate oxygen isotope records from hydrologically closed lakes and suggest all possible controlling factors must be taken into account in order to avoid misleading palaeoclimatic reconstructions
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