439 research outputs found
Non-adiabatic-like accelerated expansion of the late universe in entropic cosmology
In `entropic cosmology', instead of a cosmological constant , an
extra driving term is added to the Friedmann equation and the acceleration
equation, taking into account the entropy and the temperature on the horizon of
the universe. By means of the modified Friedmann and acceleration equations, we
examine a non-adiabatic-like accelerated expansion of the universe in entropic
cosmology. In this study, we consider a homogeneous, isotropic, and spatially
flat universe, focusing on the single-fluid (single-component) dominated
universe at late-times. To examine the properties of the late universe, we
solve the modified Friedmann and acceleration equations, neglecting high-order
corrections for the early universe. We derive the continuity (conservation)
equation from the first law of thermodynamics, assuming non-adiabatic expansion
caused by the entropy and temperature on the horizon. Using the continuity
equation, we formulate the generalized Friedmann and acceleration equations,
and propose a simple model. Through the luminosity distance, it is demonstrated
that the simple model agrees well with both the observed accelerated expansion
of the universe and a fine-tuned standard CDM (lambda cold dark
matter) model. However, we find that the increase of the entropy for the simple
model is likely uniform, while the increase of the entropy for the standard
CDM model tends to be gradually slow especially after the present
time. In other words, the simple model predicts that the present time is not a
special time, unlike for the prediction of the standard CDM model.Comment: 16 pages, 6 figures, revised. Appendices and References were added
and revise
A Novel Pyrochlore Ruthenate: Ca2Ru2O7
Single crystals of a novel ruthenate, Ca2Ru2O7, were obtained. An X-ray
diffraction study on a single crystal revealed that this material crystallizes
in a pyrochlore structure with a lattice parameter, a = 10.197 Angstroms. The
magnetic susceptibility above 30 K is the summation of a Curie-Weiss
contribution and a constant term independent of temperature. The effective
moment per Ru atom is only 0.36 Bohr magnetons, one order of magnitude smaller
than that expected from a localized spin model with S=3/2 for Ru5+. Below 23 K,
the localized spins freeze in a spin-glass state. The resistivity at room
temperature is 2E-3 Ohm cm, comparable to that in metallic, highly correlated
oxides.}Comment: 11 pages, 3 figures, J. Phys. Soc. Jan., in pres
Studies of the motion and decay of axion walls bounded by strings
We discuss the appearance at the QCD phase transition, and the subsequent
decay, of axion walls bounded by strings in N=1 axion models. We argue on
intuitive grounds that the main decay mechanism is into barely relativistic
axions. We present numerical simulations of the decay process. In these
simulations, the decay happens immediately, in a time scale of order the light
travel time, and the average energy of the radiated axions is for . is found to increase
approximately linearly with . Extrapolation of this behaviour
yields in axion models of interest. We find that the
contribution to the cosmological energy density of axions from wall decay is of
the same order of magnitude as that from vacuum realignment, with however large
uncertainties. The velocity dispersion of axions from wall decay is found to be
larger, by a factor or so, than that of axions from vacuum realignment
and string decay. We discuss the implications of this for the formation and
evolution of axion miniclusters and for the direct detection of axion dark
matter on Earth. Finally we discuss the cosmology of axion models with in
which the domain wall problem is solved by introducing a small U(1)
breaking interaction. We find that in this case the walls decay into
gravitational waves.Comment: 37 pages, 10 figures, a minor mistake was corrected, several
references and comments were adde
The Apparent and Intrinsic Shape of the APM Galaxy Clusters
We estimate the distribution of intrinsic shapes of APM galaxy clusters from
the distribution of their apparent shapes. We measure the projected cluster
ellipticities using two alternative methods. The first method is based on
moments of the discrete galaxy distribution while the second is based on
moments of the smoothed galaxy distribution. We study the performance of both
methods using Monte Carlo cluster simulations covering the range of APM cluster
distances and including a random distribution of background galaxies. We find
that the first method suffers from severe systematic biases, whereas the second
is more reliable. After excluding clusters dominated by substructure and
quantifying the systematic biases in our estimated shape parameters, we recover
a corrected distribution of projected ellipticities. We use the non-parametric
kernel method to estimate the smooth apparent ellipticity distribution, and
numerically invert a set of integral equations to recover the corresponding
distribution of intrinsic ellipticities under the assumption that the clusters
are either oblate or prolate spheroids. The prolate spheroidal model fits the
APM cluster data best.Comment: 8 pages, including 7 figures, accepted for publication in MNRA
Dark matter and structure formation a review
This paper provides a review of the variants of dark matter which are thought
to be fundamental components of the universe and their role in origin and
evolution of structures and some new original results concerning improvements
to the spherical collapse model. In particular, I show how the spherical
collapse model is modified when we take into account dynamical friction and
tidal torques
MAMBO Mapping of Spitzer c2d Small Clouds and Cores
AIMS: To study the structure of nearby (< 500 pc) dense starless and
star-forming cores with the particular goal to identify and understand
evolutionary trends in core properties, and to explore the nature of Very Low
Luminosity Objects (< 0.1 L_sun; VeLLOs). METHODS: Using the MAMBO bolometer
array, we create maps unusually sensitive to faint (few mJy per beam) extended
(approx. 5 arcmin) thermal dust continuum emission at 1.2 mm wavelength.
Complementary information on embedded stars is obtained from Spitzer, IRAS, and
2MASS. RESULTS: Our maps are very rich in structure, and we characterize
extended emission features (``subcores'') and compact intensity peaks in our
data separately to pay attention to this complexity. We derive, e.g., sizes,
masses, and aspect ratios for the subcores, as well as column densities and
related properties for the peaks. Combination with archival infrared data then
enables the derivation of bolometric luminosities and temperatures, as well as
envelope masses, for the young embedded stars. CONCLUSIONS: (abridged) Starless
and star-forming cores occupy the same parameter space in many core properties;
a picture of dense core evolution in which any dense core begins to actively
form stars once it exceeds some fixed limit in, e.g., mass, density, or both,
is inconsistent with our data. Comparison of various evolutionary indicators
for young stellar objects in our sample (e.g., bolometric temperatures) reveals
inconsistencies between some of them, possibly suggesting a revision of some of
these indicators.Comment: Accepted to A&A. In total 46 pages, with 20 pages of tables, figures,
and appendices. High-resolution version of this article at
https://www.xythosondemand.com/home/harvard_iic/Users/jkauffma/Public/mambo_spitzer.pd
On the cosmological mass function theory
This paper provides, from one side, a review of the theory of the
cosmological mass function from a theoretical point of view, starting from the
seminal paper of Press & Shechter (1974) to the last developments (Del Popolo &
Gambera (1998, 1999), Sheth & Tormen 1999 (ST), Sheth, Mo & Tormen 2001 (ST1),
Jenkins et al. 2001 (J01), Shet & Tormen 2002 (ST2), Del Popolo 2002a, Yagi et
al. 2004 (YNY)), and from another side some improvements on the multiplicity
function models in literature. ...Comment: Astronomy Reports, in prin
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Petri Nets Validation of Markovian Models of Emergency Department Arrivals
International audienceModeling of hospital’s Emergency Departments (ED) is vital for optimisation of health services offered to patients that shows up at an ED requiring treatments with different level of emergency. In this paper we present a modeling study whose contribution is twofold: first, based on a dataset relative to the ED of an Italian hospital, we derive different kinds of Markovian models capable to reproduce, at different extents, the statistical character of dataset arrivals; second, we validate the derived arrivals model by interfacing it with a Petri net model of the services an ED patient undergoes. The empirical assessment of a few key performance indicators allowed us to validate some of the derived arrival process model, thus confirming that they can be used for predicting the performance of an ED
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