7,681 research outputs found
Family planning in Pakistan: a review of selected service statistics, 1966-67 (Part I and II)
The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c
We present a combined analysis of previously published high-precision radial
velocities and astrometry for the GJ876 planetary system using a
self-consistent model that accounts for the planet-planet interactions.
Assuming the three planets so far identified in the system are coplanar, we
find that including the astrometry in the analysis does not result in a
best-fit inclination significantly different than that found by Rivera and
collaborators from analyzing the radial velocities alone. In this unique case,
the planet-planet interactions are of such significance that the radial
velocity data set is more sensitive to the inclination of the system through
the dependence of the interactions on the true masses of the two gas giant
planets in the system (planets b and c). The astrometry does allow
determination of the absolute orbital inclination (i.e. distinguishing between
i and 180-i) and longitude of the ascending node for planet b, which allows us
to quantify the mutual inclination angle between its orbit and planet c's orbit
when combined with the dynamical considerations. We find that the planets have
a mutual inclination of 5.0 +3.9 -2.3 degrees. This result constitutes the
first determination of the degree of coplanarity in an exoplanetary system
around a normal star. That we find the two planets' orbits are nearly coplanar,
like the orbits of the Solar System planets, indicates that the planets likely
formed in a circumstellar disk, and that their subsequent dynamical evolution
into a 2:1 mean motion resonance only led to excitation of a small mutual
inclination. This investigation demonstrates how the degree of coplanarity for
other exoplanetary systems could also be established using data obtained from
existing facilities.Comment: 9 pages, accepted for publication in A&
Spherical collapse of dark energy with an arbitrary sound speed
We consider a generic type of dark energy fluid, characterised by a constant
equation of state parameter w and sound speed c_s, and investigate the impact
of dark energy clustering on cosmic structure formation using the spherical
collapse model. Along the way, we also discuss in detail the evolution of dark
energy perturbations in the linear regime. We find that the introduction of a
finite sound speed into the picture necessarily induces a scale-dependence in
the dark energy clustering, which in turn affects the dynamics of the spherical
collapse in a scale-dependent way. As with other, more conventional fluids, we
can define a Jeans scale for the dark energy clustering, and hence a Jeans mass
M_J for the dark matter which feels the effect of dark energy clustering via
gravitational interactions. For bound objects (halos) with masses M >> M_J, the
effect of dark energy clustering is maximal. For those with M << M_J, the dark
energy component is effectively homogeneous, and its role in the formation of
these structures is reduced to its effects on the Hubble expansion rate. To
compute quantitatively the virial density and the linearly extrapolated
threshold density, we use a quasi-linear approach which is expected to be valid
up to around the Jeans mass. We find an interesting dependence of these
quantities on the halo mass M, given some w and c_s. The dependence is the
strongest for masses lying in the vicinity of M ~ M_J. Observing this
M-dependence will be a tell-tale sign that dark energy is dynamic, and a great
leap towards pinning down its clustering properties.Comment: 25 pages, 6 figures, matches version published in JCA
Conditions for the discovery of solution horizons
We present necessary and sufficient conditions for discrete infinite horizon optimization problems with unique solutions to be solvable. These problems can be equivalently viewed as the task of finding a shortest path in an infinite directed network. We provide general forward algorithms with stopping rules for their solution. The key condition required is that of weak reachability, which roughly requires that for any sequence of nodes or states, it must be possible from optimal states to reach states close in cost to states along this sequence. Moreover the costs to reach these states must converge to zero. Applications are considered in optimal search, undiscounted Markov decision processes, and deterministic infinite horizon optimization.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47927/1/10107_2005_Article_BF01581244.pd
Constraining Evolution of Quintessence with CMB and SNIa Data
The equation of state of the hypothetical dark energy component, which
constitutes about two thirds of the critical density of the universe, may be
very different from that of a cosmological constant. Employing a
phenomenological model, we investigate semi-analytically the constraints
imposed on the scalar quintessence by supernovae observations, and by the
acoustic scale extracted from recent CMB data. We show that a universe with a
quintessence-dominated phase in the dark age is consistent with the current
observational constraints.Comment: replaced with a revised pape
Condensate cosmology -- dark energy from dark matter
Imagine a scenario in which the dark energy forms via the condensation of
dark matter at some low redshift. The Compton wavelength therefore changes from
small to very large at the transition, unlike quintessence or metamorphosis. We
study CMB, large scale structure, supernova and radio galaxy constraints on
condensation by performing a 4 parameter likelihood analysis over the Hubble
constant and the three parameters associated with Q, the condensate field:
Omega_Q, w_f and z_t (energy density and equation of state today, and redshift
of transition). Condensation roughly interpolates between Lambda CDM (for large
z_t) and sCDM (low z_t) and provides a slightly better fit to the data than
Lambda CDM. We confirm that there is no degeneracy in the CMB between H and z_t
and discuss the implications of late-time transitions for the Lyman-alpha
forest. Finally we discuss the nonlinear phase of both condensation and
metamorphosis, which is much more interesting than in standard quintessence
models.Comment: 13 pages, 13 colour figures. Final version with discussion of TE
cross-correlation spectra for condensation and metamorphosis in light of the
WMAP result
Lower critical field H_c1 and barriers for vortex entry in Bi_2Sr_2CaCu_2O_{8+delta} crystals
The penetration field H_p of Bi_2Sr_2CaCu_2O_{8+delta} crystals is determined
from magnetization curves for different field sweep rates dH/dt and
temperatures. The obtained results are consistent with theoretical reports in
the literature about vortex creep over surface and geometrical barriers. The
frequently observed low-temperature upturn of H_p is shown to be related to
metastable configurations due to barriers for vortex entry. Data of the true
lower critical field H_c1 are presented. The low-temperature dependence of H_c1
is consistent with a superconducting state with nodes in the gap function.
[PACS numbers: 74.25.Bt, 74.60.Ec, 74.60.Ge, 74.72.Hs
High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications
Superconducting technology provides most sensitive field detectors, promising
implementations of qubits and high field magnets for medical imaging and for
most powerful particle accelerators. Thus, with the discovery of new
superconducting materials, such as the iron pnictides, exploring their
potential for applications is one of the foremost tasks. Even if the critical
temperature Tc is high, intrinsic electronic properties might render
applications rather difficult, particularly if extreme electronic anisotropy
prevents effective pinning of vortices and thus severely limits the critical
current density, a problem well known for cuprates. While many questions
concerning microscopic electronic properties of the iron pnictides have been
successfully addressed and estimates point to a very high upper critical field,
their application potential is less clarified. Thus we focus here on the
critical currents, their anisotropy and the onset of electrical dissipation in
high magnetic fields up to 65 T. Our detailed study of the transport properties
of optimally doped SmFeAs(O,F) single crystals reveals a promising combination
of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities
along all crystal directions. This favorable intragrain current transport in
SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a
crucial requirement for possible applications. Essential in these experiments
are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with
sub-\mu\m^2 cross-section, with current along and perpendicular to the
crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed
magnetic fields. The pinning forces have been characterized by scaling the
magnetically measured "peak effect"
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