2,451 research outputs found
Dark Energy and Dark Matter
It is a puzzle why the densities of dark matter and dark energy are nearly
equal today when they scale so differently during the expansion of the
universe. This conundrum may be solved if there is a coupling between the two
dark sectors. In this paper we assume that dark matter is made of cold relics
with masses depending exponentially on the scalar field associated to dark
energy. Since the dynamics of the system is dominated by an attractor solution,
the dark matter particle mass is forced to change with time as to ensure that
the ratio between the energy densities of dark matter and dark energy become a
constant at late times and one readily realizes that the present-day dark
matter abundance is not very sensitive to its value when dark matter particles
decouple from the thermal bath. We show that the dependence of the present
abundance of cold dark matter on the parameters of the model differs
drastically from the familiar results where no connection between dark energy
and dark matter is present. In particular, we analyze the case in which the
cold dark matter particle is the lightest supersymmetric particle.Comment: 4 pages latex, 2 figure
Grading And Assessments: Correlations Of Variables Affecting Teaching And Course Assessments
Faculty assessments are a hot topic in academia. Mostly considered an unproven statistic holding the professor hostage for a good grade, the benefits are still in discussion. At course end, we expect students to have the ability to analyze what we think they should have learned. In reality, most students do not have the maturity to realize correlation for what you put into your education is what you get out. Consensus is that it does not happen the way we plan. Adding to the assessment issue is the clear fact that today most universities are adjunct professor-driven. Given these facts, the authors have statistically studied teaching assessments and associated grade point averages (GPA) representing academic rigor at a major non-profit university over the past three years. The authors’ hypothesis is that there is no correlation between the two statistics. This paper reviews the literature, provides the study methodology, and presents the findings
Spatial and observational homogeneities of the galaxy distribution in standard cosmologies
This work discusses the possible empirical verification of the geometrical
concept of homogeneity of the standard relativistic cosmology considering its
various definitions of distance. We study the physical consequences of the
distinction between the usual concept of spatial homogeneity (SH), as defined
by the Cosmological Principle, and the concept of observational homogeneity
(OH), arguing that OH is in principle falsifiable by means of astronomical
observations, whereas verifying SH is only possible indirectly. Simulated
counts of cosmological sources are produced by means of a generalized
number-distance expression that can be specialized to produce either the counts
of the Einstein-de Sitter (EdS) cosmology, which has SH by construction, or
other types of counts, which do, or do not, have OH by construction.
Expressions for observational volumes and differential densities are derived
with the various cosmological distance definitions in the EdS model. Simulated
counts that have OH by construction do not always exhibit SH features. The
reverse situation is also true. Besides, simulated counts with no OH features
at low redshift start showing OH characteristics at high redshift. The comoving
distance seems to be the only distance definition where both SH and OH appear
simultaneously. The results show that observations indicating possible lack of
OH do not necessarily falsify the standard Friedmannian cosmology, meaning that
this cosmology will not necessarily always produce observable homogeneous
densities. The general conclusion is that the use of different cosmological
distances in the characterization of the galaxy distribution lead to
significant ambiguities in reaching conclusions about the behavior of the
large-scale galaxy distribution in the Universe.Comment: 12 pages, 12 figures, LaTeX. Matches the final version sent to the
journal. Accepted for publication in "Astronomy and Astrophysics
Kination Dominated Reheating and Cold Dark Matter Abundance
We consider the decay of a massive particle under the complete or partial
domination of the kinetic energy density generated by a quintessential
exponential model and we impose a number of observational constraints
originating from nucleosynthesis, the present acceleration of the universe and
the dark-energy-density parameter. We show that the presence of kination causes
a prolonged period during which the temperature is frozen to a plateau value,
much lower than the maximal temperature achieved during the process of
reheating in the absence of kination. The decoupling of a cold dark matter
particle during this period is analyzed, its relic density is calculated both
numerically and semi-analytically and the results are compared with each other.
Using plausible values (from the viewpoint of particle models) for the mass and
the thermal averaged cross section times the velocity of the cold relic, we
investigate scenaria of equilibrium or non-equilibrium production. In both
cases, acceptable results for the cold dark matter abundance can be obtained,
by constraining the initial energy density of the decaying particle, its decay
width, its mass and the averaged number of the produced cold relics. The
required plateau value of the temperature is, in most cases, lower than about
40 GeVComment: Final versio
Topological approach to phase transitions and inequivalence of statistical ensembles
The relation between thermodynamic phase transitions in classical systems and
topology changes in their state space is discussed for systems in which
equivalence of statistical ensembles does not hold. As an example, the
spherical model with mean field-type interactions is considered. Exact results
for microcanonical and canonical quantities are compared with topological
properties of a certain family of submanifolds of the state space. Due to the
observed ensemble inequivalence, a close relation is expected to exist only
between the topological approach and one of the statistical ensembles. It is
found that the observed topology changes can be interpreted meaningfully when
compared to microcanonical quantities.Comment: 9 pages, 1 figur
Transactional Criticism and Aesthetic Literary Experiences: Examining Complex Responses in Light of the Teacher\u27s Purpose
In classroom literature discussions, teachers orchestrate situations in which readers and texts come together. Approaches teachers use may differ in terms of the stance or purpose for reading encouraged. Rosenblatt (1978, 1985) describes two stances readers can take while reading literary works. An efferent stance indicates a reader\u27s attention is focused on information to be retained after reading and can result in a study of the text. An aesthetic stance, on the other hand, occurs when the reader\u27s attention is on the livedthrough experience of the story and the experiences, thoughts, feelings, images, and associations which are evoked. Rosenblatt (1978, 1983, 1986) contends that although the appropriate stance when reading literature is the aesthetic stance, most literature in schools is taught from an efferent approach. Research describing teaching approaches used in schools seems to support this contention (Sacks, 1987; Walmsley and Walp, 1989; Zarillo and Cox, 1992)
RadICAL stack: A localisation method for dynamic gamma/neutron fields
A variation of the RadICAL (Radiation Imaging Cylinder Activity Locator) system capable of operating in a dynamic environment, such as that created by active interrogation techniques, has been developed. RadICAL is a novel method for locating a radiological source using a rotating detector element. The detector geometry is that of a thin sheet and is rotated to present a constantly changing surface area to the source; it therefore generates a characteristic temporal response which can be used to determine the source direction. The time required to determine the direction of a source make it unsuitable for dynamic environments and so an alternative method is presented that uses a stack of identical scintillator slabs positioned at fixed horizontal angles around a central axis. By comparing count rates from each slab to a standard response curve, using a specially developed algorithm, the direction of a source can be determined without the need to rotate the detector. EJ-299-33 plastic scintillator was used to allow detection of separate neutron and gamma events in a mixed field through pulse shape discrimination. A four element detector was built and shown to achieve a positional accuracy of approximately 4.4 degrees when exposed to a 1.44MBq 137 Cs source at distances of up to 2m. The same detector was used to discriminate separate neutron and gamma events in a mixed field, which allows for the possibility of locating a neutron source within a gamma rich environment
Neutron/gamma pulse shape discrimination in EJ-299-34 at high flux
The effect of scintillator geometry on the quality of neutron/γ pulse shape discrimination (PSD) in EJ-299 plastic scintillator, using a digital charge integration PSD algorithm has been studied. It is shown that the PSD Figure of Merit (FOM) reduces as the geometry of the scintillator moves from a cube-like shape towards a flat panel shape. The PSD performance in this material at high flux irradiation is investigated with performance deteriorating at rates of ∼107 photons/s. The use of EJ-299 for security applications, with a focus on active interrogation environments is explored in conjunction with a system capable of neutron/γ separation and localisation
Electroweak Baryogenesis: Concrete in a SUSY Model with a Gauge Singlet
SUSY models with a gauge singlet easily allow for a strong first order
electroweak phase transition (EWPT) if the vevs of the singlet and Higgs fields
are of comparable size. We discuss the profile of the stationary expanding
bubble wall and CP-violation in the effective potential, in particular
transitional CP-violation inside the bubble wall during the EWPT. The
dispersion relations for charginos contain CP-violating terms in the WKB
approximation. These enter as source terms in the Boltzmann equations for the
(particle--antiparticle) chemical potentials and fuel the creation of a baryon
asymmetry through the weak sphaleron in the hot phase. This is worked out for
concrete parameters.Comment: 46 pages, LaTeX, 11 figures, discussion of source terms and transport
equations modified, version to appear in Nucl. Phys.
On the mean-field spherical model
Exact solutions are obtained for the mean-field spherical model, with or
without an external magnetic field, for any finite or infinite number N of
degrees of freedom, both in the microcanonical and in the canonical ensemble.
The canonical result allows for an exact discussion of the loci of the Fisher
zeros of the canonical partition function. The microcanonical entropy is found
to be nonanalytic for arbitrary finite N. The mean-field spherical model of
finite size N is shown to be equivalent to a mixed isovector/isotensor
sigma-model on a lattice of two sites. Partial equivalence of statistical
ensembles is observed for the mean-field spherical model in the thermodynamic
limit. A discussion of the topology of certain state space submanifolds yields
insights into the relation of these topological quantities to the thermodynamic
behavior of the system in the presence of ensemble nonequivalence.Comment: 21 pages, 5 figure
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