4,089 research outputs found
How far is it to a sudden future singularity of pressure?
We discuss the constraints coming from current observations of type Ia
supernovae on cosmological models which allow sudden future singularities of
pressure (with the scale factor and the energy density regular). We show that
such a sudden singularity may happen in the very near future (e.g. within ten
million years) and its prediction at the present moment of cosmic evolution
cannot be distinguished, with current observational data, from the prediction
given by the standard quintessence scenario of future evolution. Fortunately,
sudden future singularities are characterized by a momentary peak of infinite
tidal forces only; there is no geodesic incompletness which means that the
evolution of the universe may eventually be continued throughout until another
``more serious'' singularity such as Big-Crunch or Big-Rip.Comment: REVTEX4, 4 pages, 2 figures, references change
A robust method for measuring the Hubble parameter
We obtain a robust, non-parametric, estimate of the Hubble constant from
galaxy linear diameters calibrated using HST Cepheid distances. Our method is
independent of the parametric form of the diameter function and the spatial
distribution of galaxies and is insensitive to Malmquist bias. We include
information on the galaxy rotation velocities; unlike Tully-Fisher, however, we
retain a fully non-parametric treatment. We find km/s/Mpc,
somewhat larger than previous results using galaxy diameters.Comment: 4 pages, 1 figure, Cosmic Flows Workshop, Victoria B.C. Canada, July
1999, ed. S. Courteau, M. Strauss & J. Willick, ASP conf. serie
Strong nonlinear optical response of graphene flakes measured by four-wave mixing
We present the first experimental investigation of nonlinear optical
properties of graphene flakes. We find that at near infrared frequencies a
graphene monolayer exhibits a remarkably high third-order optical nonlinearity
which is practically independent of the wavelengths of incident light. The
nonlinear optical response can be utilized for imaging purposes, with image
contrasts of graphene which are orders of magnitude higher than those obtained
using linear microscopy.Comment: 4 pages, 5 figure
Non-invasive, near-field terahertz imaging of hidden objects using a single pixel detector
Terahertz (THz) imaging has the ability to see through otherwise opaque
materials. However, due to the long wavelengths of THz radiation
({\lambda}=300{\mu}m at 1THz), far-field THz imaging techniques are heavily
outperformed by optical imaging in regards to the obtained resolution. In this
work we demonstrate near-field THz imaging with a single-pixel detector. We
project a time-varying optical mask onto a silicon wafer which is used to
spatially modulate a pulse of THz radiation. The far-field transmission
corresponding to each mask is recorded by a single element detector and this
data is used to reconstruct the image of an object placed on the far side of
the silicon wafer. We demonstrate a proof of principal application where we
image a printed circuit board on the underside of a 115{\mu}m thick silicon
wafer with ~100{\mu}m ({\lambda}/4) resolution. With subwavelength resolution
and the inherent sensitivity to local conductivity provided by the THz probe
frequencies, we show that it is possible to detect fissures in the circuitry
wiring of a few microns in size. Imaging systems of this type could have other
uses where non-invasive measurement or imaging of concealed structures with
high resolution is necessary, such as in semiconductor manufacturing or in
bio-imaging
A physical distance indicator for spiral galaxies
In this paper we derive a Tully Fisher relation from measured I band
photometry and H rotation curves of a large survey of southern sky
spiral galaxies, obtained in Persic \& Salucci (1995) by deprojecting and
folding the raw H data of Mathewson, Ford \& Buchhorn (1992). We
calibrate the relation by combining several of the largest clusters in the
survey, using an iterative maximum likelihood procedure to account for
observational selection effects and Malmquist bias. We also incorporate a
simple model for the line of sight depth of each cluster. Our results indicate
a Tully Fisher relation of intrinsic dispersion mag, corresponding to
a distance error dispersion of . Application of this relation to mapping
the large scale velocity field is underway.Comment: Plain TeX Version 3.0, 4 pages, to appear in `Astrophysical Letters
and Communications' - proceedings of the international workshop on
observational cosmology: `From Galaxies to Galaxy Systems', Sesto, July 199
Cosmological tests of sudden future singularities
We discuss combined constraints, coming from the cosmic microwave background
shift parameter , baryon acoustic oscillations (BAO) distance
parameter , and from the latest type Ia supernovae data, imposed
on cosmological models which allow sudden future singularities of pressure. We
show that due to their weakness such sudden singularities may happen in the
very near future and that at present they can mimic standard dark energy
models.Comment: 8 pages, 2 references adde
Pengaruh Waktu Hidrolisis Dan Konsentrasi Katalisator Asam Sulfat Terhadap Sintesis Furfural Dari Jerami Padi
-Rice straws are one of the hemicellulose-contained wastes, which one of its main components is pentosan. Pentosan can be reacted into furfural with acid catalytic hydrolysis reaction. This research is intended to find out the effect of hydrolysis durations and sulfuric acid (H2SO4) catalyst concentrations towards yield of furfural from rice straws. The synthesis of furfural from rice straws is conducted by refined the rice straws, then hydrolyze it with different concentrations of sulfuric acid solution (1%, 3%, and 5% v/v) and with different durations of hydrolysis reaction (1 hour, 2 hours, 3 hours, 4 hours, dan 5 hours). This hydrolisis reaction produced liquid, which are distilled to separate furfural from sulfuric acid solution. The results of this research show the yield of furfural from rice straws hydrolysis is inversely proportional to sulfuric acid catalyst concentration and hydrolysis duration. The best yield of furfural was 5,441%, which obtained with 1% sulfuric acid catalyst concentration and 1 hour hydrolysis duration
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