3,957 research outputs found
Assessment of early-modern observations of comets and supernovae: Focus on pre-telescopic European astrometric and physical data
The two-century period prior to the publication of Newton's Principia (first edition 1687; third edition 1726) was most important in terms of the radical changes that occurred in the observation, perception, and understanding of celestial objects that in turn spurred Newton to deduce his laws of gravitation and motion. Surprisingly, much of the available observational data embedded in contemporary texts from that two-century period has remained unused by modern astronomers, and this thesis (a) describes large amounts of data that were found and reanalyzed during the course of this Ph.D. research project, (b) places these data and their resulting analyses in context with the astronomy of the early-modern era, and (c) shows how modern astronomers and historians benefit from such information. The emphasis is placed here on west-European observations, as observations made elsewhere (eastern Europe, Asia) were isolated (not communicated for convenient rapid use by contemporary astronomers elsewhere) and did not develop or employ the level of precision that was utilized by western European astronomers through the extensive discussions that developed from correspondence and publication in Europe
Controlling Chaos through Compactification in Cosmological Models with a Collapsing Phase
We consider the effect of compactification of extra dimensions on the onset
of classical chaotic "Mixmaster" behavior during cosmic contraction. Assuming a
universe that is well-approximated as a four-dimensional
Friedmann-Robertson--Walker model (with negligible Kaluza-Klein excitations)
when the contraction phase begins, we identify compactifications that allow a
smooth contraction and delay the onset of chaos until arbitrarily close the big
crunch. These compactifications are defined by the de Rham cohomology (Betti
numbers) and Killing vectors of the compactification manifold. We find
compactifications that control chaos in vacuum Einstein gravity, as well as in
string theories with N = 1 supersymmetry and M-theory. In models where chaos is
controlled in this way, the universe can remain homogeneous and flat until it
enters the quantum gravity regime. At this point, the classical equations
leading to chaotic behavior can no longer be trusted, and quantum effects may
allow a smooth approach to the big crunch and transition into a subsequent
expanding phase. Our results may be useful for constructing cosmological models
with contracting phases, such as the ekpyrotic/cyclic and pre-big bang models.Comment: 1 figure. v2/v3: minor typos correcte
Hypermatrix factors for string and membrane junctions
The adjoint representations of the Lie algebras of the classical groups
SU(n), SO(n), and Sp(n) are, respectively, tensor, antisymmetric, and symmetric
products of two vector spaces, and hence are matrix representations. We
consider the analogous products of three vector spaces and study when they
appear as summands in Lie algebra decompositions. The Z3-grading of the
exceptional Lie algebras provide such summands and provides representations of
classical groups on hypermatrices. The main natural application is a formal
study of three-junctions of strings and membranes. Generalizations are also
considered.Comment: 25 pages, 4 figures, presentation improved, minor correction
Chandra Observations of SDSS J1004+4112: Constraints on the Lensing Cluster and Anomalous X-Ray Flux Ratios of the Quadruply Imaged Quasar
We present results from Chandra observations of SDSS J1004+4112, a strongly
lensed quasar system with a maximum image separation of 15". All four bright
images of the quasar, as well as resolved X-ray emission originating from the
lensing cluster, are clearly detected. The emission from the lensing cluster
extends out to approximately 1.5 arcmin. We measure the bolometric X-ray
luminosity and temperature of the lensing cluster to be 4.7e44 erg s^-1 and 6.4
keV, consistent with the luminosity-temperature relation for distant clusters.
The mass estimated from the X-ray observation shows excellent agreement with
the mass derived from gravitational lensing. The X-ray flux ratios of the
quasar images differ markedly from the optical flux ratios, and the combined
X-ray spectrum of the images possesses an unusually strong Fe Kalpha emission
line, both of which are indicative of microlensing.Comment: 9 pages, 5 figures. Accepted for publication in ApJ. Version with
high-quality color figures at
http://cosmic.riken.jp/ota/publications/index.htm
Probing the Origins of the CIV and Fe Kalpha Baldwin Effect
We use UV/optical and X-ray observations of 272 radio-quiet Type 1 AGNs and
quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra
are drawn from the Hubble Space Telescope, International Ultraviolet Explorer
and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra
and XMM-Newton archives. We apply correlation and partial-correlation analyses
to the equivalent widths, continuum monochromatic luminosities, and alpha_ox,
which characterizes the relative X-ray to UV brightness. The equivalent width
of the CIV 1549 emission line is correlated with both alpha_ox and luminosity.
We find that by regressing l_UV with EW(CIV) and alpha_ox, we can obtain
tighter correlations than by regressing l_UV with only EW(CIV). Both
correlation and regression analyses imply that l_UV is not the only factor
controlling the changes of EW(CIV); alpha_ox (or, equivalently, the soft X-ray
emission) plays a fundamental role in the formation and variation of CIV.
Variability contributes at least 60% of the scatter of the EW(CIV)-l_UV
relation and at least 75% of the scatter of the of the EW(CIV)-alpha_ox
relation.
In our sample, narrow Fe Kalpha 6.4 keV emission lines are detected in 50
objects. Although narrow Fe Kalpha exhibits a BEff similar to that of CIV, its
equivalent width has almost no dependence on either alpha_ox or EW(CIV). This
suggests that the majority of narrow Fe Kalpha emission is unlikely to be
produced in the broad emission-line region. We do find suggestive correlations
between the emission-line luminosities of CIV and Fe Kalpha, which could be
potentially used to estimate the detectability of the Fe Kalpha line of quasars
from rest-frame UV spectroscopic observations
Fermi surface instabilities at finite Temperature
We present a new method to detect Fermi surface instabilities for interacting
systems at finite temperature. We first apply it to a list of cases studied
previously, recovering already known results in a very economic way, and
obtaining most of the information on the phase diagram analytically. As an
example, in the continuum limit we obtain the critical temperature as an
implicit function of the magnetic field and the chemical potential
. By applying the method to a model proposed to describe reentrant
behavior in , we reproduce the phase diagram obtained
experimentally and show the presence of a non-Fermi Liquid region at
temperatures above the nematic phase.Comment: 10 pages, 10 figure
Kaluza-Klein Induced Gravity Inflation
A D-dimensional induced gravity theory is studied carefully in a
dimensional Friedmann-Robertson-Walker space-time. We try to extract
information of the symmetry breaking potential in search of an inflationary
solution with non-expanding internal-space. We find that the induced gravity
model imposes strong constraints on the form of symmetry breaking potential in
order to generate an acceptable inflationary universe. These constraints are
analyzed carefully in this paper.Comment: 10 pages, title changed, corrected some typos, two additional
comments adde
- …