10,204 research outputs found
Pursuing Parameters for Critical Density Dark Matter Models
We present an extensive comparison of models of structure formation with
observations, based on linear and quasi-linear theory. We assume a critical
matter density, and study both cold dark matter models and cold plus hot dark
matter models. We explore a wide range of parameters, by varying the fraction
of hot dark matter , the Hubble parameter and the spectral
index of density perturbations , and allowing for the possibility of
gravitational waves from inflation influencing large-angle microwave background
anisotropies. New calculations are made of the transfer functions describing
the linear power spectrum, with special emphasis on improving the accuracy on
short scales where there are strong constraints. For assessing early object
formation, the transfer functions are explicitly evaluated at the appropriate
redshift. The observations considered are the four-year {\it COBE} observations
of microwave background anisotropies, peculiar velocity flows, the galaxy
correlation function, and the abundances of galaxy clusters, quasars and damped
Lyman alpha systems. Each observation is interpreted in terms of the power
spectrum filtered by a top-hat window function. We find that there remains a
viable region of parameter space for critical-density models when all the dark
matter is cold, though must be less than 0.5 before any fit is found and
significantly below unity is preferred. Once a hot dark matter component is
invoked, a wide parameter space is acceptable, including . The
allowed region is characterized by \Omega_\nu \la 0.35 and 0.60 \la n \la
1.25, at 95 per cent confidence on at least one piece of data. There is no
useful lower bound on , and for curious combinations of the other parameters
it is possible to fit the data with as high as 0.65.Comment: 19 pages LaTeX file (uses mn.sty). Figures *not* included due to
length. We strongly recommend obtaining the full paper, either by WWW at
http://star-www.maps.susx.ac.uk/papers/lsstru_papers.html (UK) or
http://www.bartol.udel.edu/~bob/papers (US), or by e-mailing ARL. Final
version, to appear MNRAS. Main revision is update to four-year COBE data.
Miscellaneous other changes and reference updates. No significant changes to
principal conclusion
Study of the Correlations Between the Highest Energy Cosmic Ray Showers and Gamma Ray Bursts
We examine the correlation between the arrival direction of ultra high energy
cosmic ray showers and gamma ray bursts in the third BATSE catalog. We find no
correlation between the two data sets. We also find no correlations between a
pre-BATSE burst catalog and the Haverah Park Ultra High Energy shower set that
cover approximately the same period of time.Comment: 1 uuencoded g-zipped postscript file containing text and figure
Self-Nulling Lock-in Detection Electronics for Capacitance Probe Electrometer
A multi-channel electrometer voltmeter that employs self-nulling lock-in detection electronics in conjunction with a mechanical resonator with noncontact voltage sensing electrodes has been developed for space-based measurement of an Internal Electrostatic Discharge Monitor (IESDM). The IESDM is new sensor technology targeted for integration into a Space Environmental Monitor (SEM) subsystem used for the characterization and monitoring of deep dielectric charging on spacecraft. Use of an AC-coupled lock-in amplifier with closed-loop sense-signal nulling via generation of an active guard-driving feedback voltage provides the resolution, accuracy, linearity and stability needed for long-term space-based measurement of the IESDM. This implementation relies on adjusting the feedback voltage to drive the sense current received from the resonator s variable-capacitance-probe voltage transducer to approximately zero, as limited by the signal-to-noise performance of the loop electronics. The magnitude of the sense current is proportional to the difference between the input voltage being measured and the feedback voltage, which matches the input voltage when the sense current is zero. High signal-to-noise-ratio (SNR) is achieved by synchronous detection of the sense signal using the correlated reference signal derived from the oscillator circuit that drives the mechanical resonator. The magnitude of the feedback voltage, while the loop is in a settled state with essentially zero sense current, is an accurate estimate of the input voltage being measured. This technique has many beneficial attributes including immunity to drift, high linearity, high SNR from synchronous detection of a single-frequency carrier selected to avoid potentially noisy 1/f low-frequency spectrum of the signal-chain electronics, and high accuracy provided through the benefits of a driven shield encasing the capacitance- probe transducer and guarded input triaxial lead-in. Measurements obtained from a 2- channel prototype electrometer have demonstrated good accuracy (|error| < 0.2 V) and high stability. Twenty-four-hour tests have been performed with virtually no drift. Additionally, 5,500 repeated one-second measurements of 100 V input were shown to be approximately normally distributed with a standard deviation of 140 mV
On the Applicability of Weak-Coupling Results in High Density QCD
Quark matter at asymptotically high baryon chemical potential is in a color
superconducting state characterized by a gap Delta. We demonstrate that
although present weak-coupling calculations of Delta are formally correct for
mu -> Infinity, the contributions which have to this point been neglected are
large enough that present results can only be trusted for mu >> mu_c ~ 10^8
MeV. We make this argument by using the gauge dependence of the present
calculation as a diagnostic tool. It is known that the present calculation
yields a gauge invariant result for mu -> Infinity; we show, however, that the
gauge dependence of this result only begins to decrease for mu > mu_c, and
conclude that the result can certainly not be trusted for mu < mu_c. In an
appendix, we set up the calculation of the influence of the Meissner effect on
the magnitude of the gap. This contribution to Delta is, however, much smaller
than the neglected contributions whose absence we detect via the resulting
gauge dependence.Comment: 21 pages, 3 figures, uses LaTeX2e and ReVTeX, updated figures, made
minor text change
Amplitude-Stabilized Oscillator for a Capacitance-Probe Electrometer
A multichannel electrometer voltmeter that employs a mechanical resonator maintained in sustained amplitude-stabilized oscillation has been developed for the space-based measurement of an Internal Electrostatic Discharge Monitor (IESDM) sensor. The IESDM is new sensor technology targeted for integration into a Space Environmental Monitor (SEM) subsystem used for the characterization and monitoring of deep dielectric charging on spacecraft. Creating a stable oscillator from the mechanical resonator was achieved by employing magnetic induction for sensing the resonator s velocity, and forcing a current through a coil embedded in the resonator to produce a Lorentz actuation force that overcomes the resonator s dissipative losses. Control electronics employing an AGC loop provide conditions for stabilized, constant amplitude harmonic oscillation. The prototype resonator was composed of insulating FR4 printed-wireboard (PWB) material containing a flat, embedded, rectangular coil connected through flexure springs to a base PWB, and immersed in a magnetic field having two regions of opposite field direction generated by four neodymium block magnets. In addition to maintaining the mechanical movement needed for the electrometer s capacitor-probe transducer, this oscillator provides a reference signal for synchronous detection of the capacitor probe s output signal current so drift of oscillation frequency due to environmental effects is inconsequential
- …