1,199 research outputs found
Properties of the Cosmological Density Distribution Function
The properties of the probability distribution function of the cosmological
continuous density field are studied. We present further developments and
compare dynamically motivated methods to derive the PDF. One of them is based
on the Zel'dovich approximation (ZA). We extend this method for arbitrary
initial conditions, regardless of whether they are Gaussian or not. The other
approach is based on perturbation theory with Gaussian initial fluctuations. We
include the smoothing effects in the PDFs. We examine the relationships between
the shapes of the PDFs and the moments. It is found that formally there are no
moments in the ZA, but a way to resolve this issue is proposed, based on the
regularization of integrals. A closed form for the generating function of the
moments in the ZA is also presented, including the smoothing effects. We
suggest the methods to build PDFs out of the whole series of the moments, or
out of a limited number of moments -- the Edgeworth expansion. The last
approach gives us an alternative method to evaluate the skewness and kurtosis
by measuring the PDF around its peak. We note a general connection between the
generating function of moments for small r.m.s and the non-linear
evolution of the overdense spherical fluctuation in the dynamical models. All
these approaches have been applied in 1D case where the ZA is exact, and simple
analytical results are obtained. The 3D case is analyzed in the same manner and
we found a mutual agreement in the PDFs derived by different methods in the the
quasi-linear regime. Numerical CDM simulation was used to validate the accuracy
of considered approximations. We explain the successful log-normal fit of the
PDF from that simulation at moderate as mere fortune, but not as a
universal form of density PDF in general.Comment: 30 pages in Plain Tex, 1 table and 11 figures available as postscript
files by anonymous ftp from ftp.cita.utoronto.ca in directory
/cita/francis/lev, IFA-94-1
Comment on "Melting of Isolated Tin Nanoparticles"
Comment on the paper of T.Bachels, H. J. G\"{u}ntherodt and R.Sch\"{a}fer :
"Melting of Isolated Tin Nanoparticles".Comment: Accepted for publication in Phys. Rev. Let
On Metric Preheating
We consider the generation of super-horizon metric fluctuations during an
epoch of preheating in the presence of a scalar field \chi quadratically
coupled to the inflaton. We find that the requirement of efficient broad
resonance is concomitant with a severe damping of super-horizon \delta\chi
quantum fluctuations during inflation. Employing perturbation theory with
backreaction included as spatial averages to second order in the scalar fields
and in the metric, we argue that the usual inflationary prediction for metric
perturbations on scales relevant for structure formation is not strongly
modified.Comment: 5 latex pages, 1 postscript figure included, uses revtex.sty in two
column format and epsf.sty, some typos corrected and references added. Links
and further material at http://astro.uchicago.edu/home/web/sigl/r4.htm
Exact identification of the radion and its coupling to the observable sector
Braneworld models in extra dimensions can be tested in laboratory by the
coupling of the radion to the Standard Model fields. The identification of the
radion as a canonically normalized field involves a careful General Relativity
treatment: if a bulk scalar is responsible for the stabilization of the system,
its fluctuations are entangled with the perturbations of the metric and they
also have to be taken into account (similarly to the well-developed theory of
scalar metric perturbations in 4D cosmology with a scalar field). Extracting a
proper dynamical variable in a warped geometry/scalar setting is a nontrivial
task, performed so far only in the limit of negligible backreaction of the
scalar field on the background geometry. We perform the general calculation,
diagonalizing the action up to second order in the perturbations and
identifying the physical eigenmodes of the system for any amplitude of the bulk
scalar. This computation allows us to derive a very simple expression for the
exact coupling of the eigenmodes to the Standard Model fields on the brane,
valid for an arbitrary background configuration. As an application, we discuss
the Goldberger-Wise mechanism for the stabilization of the radion in the
Randall-Sundrum type models. The existing studies, limited to small amplitude
of the bulk scalar field, are characterized by a radion mass which is
significantly below the physical scale at the observable brane. We extend them
beyond the small backreaction regime. For intermediate amplitudes, the radion
mass approaches the electroweak scale, while its coupling to the observable
brane remains nearly constant. At very high amplitudes, the radion mass instead
decreases, while the coupling sharply increases. Severe experimental
constraints are expected in this regime.Comment: 20 pages, 6 figure
Chaos and Preheating
We show evidence for a relationship between chaos and parametric resonance
both in a classical system and in the semiclassical process of particle
creation. We apply our considerations in a toy model for preheating after
inflation.Comment: 7 pages, 9 figures; uses epsfig and revtex v3.1. Matches version
accepted for publication in Phys. Rev.
Some Remarks on Oscillating Inflation
In a recent paper Damour and Mukhanov describe a scenario where inflation may
continue during the oscillatory phase. This effect is possible because the
scalar field spends a significant fraction of each period of oscillation on the
upper part of the potential. Such additional period of inflation could push
perturbations after the slow roll regime to observable scales. Although in this
work we show that the small region of the Damour-Mukhanov parameter q gives the
main contribution to oscillating inflation, it was not satisfactory understood
until now. Furthermore, it gives an expression for the energy density spectrum
of perturbations, which is well behaved in the whole physical range of q .Comment: 4 pages including figures caption, 3 ps-figures. To appear in Phys.
Rev.
Dangerous Angular KK/Glueball Relics in String Theory Cosmology
The presence of Kaluza-Klein particles in the universe is a potential
manifestation of string theory cosmology. In general, they can be present in
the high temperature bath of the early universe. In particular examples, string
theory inflation often ends with brane-antibrane annihilation followed by the
energy cascading through massive closed string loops to KK modes which then
decay into lighter standard model particles. However, massive KK modes in the
early universe may become dangerous cosmological relics if the inner manifold
contains warped throat(s) with approximate isometries. In the complimentary
picture, in the AdS/CFT dual gauge theory with extra symmetries, massive
glueballs of various spins become the dangerous cosmological relics. The decay
of these angular KK modes/glueballs, located around the tip of the throat, is
caused by isometry breaking which results from gluing the throat to the compact
CY manifold. We address the problem of these angular KK particles/glueballs,
studying their interactions and decay channels, from the theory side, and the
resulting cosmological constraints on the warped compactification parameters,
from the phenomenology side. The abundance and decay time of the long-lived
non-relativistic angular KK modes depend strongly on the parameters of the
warped geometry, so that observational constraints rule out a significant
fraction of the parameter space. In particular, the coupling of the angular KK
particles can be weaker than gravitational.Comment: 58 pages, 11 figures, published versio
Onward migration: an introduction
In August 2016, Maslax Moxamed, a 19-year-old man from Somalia, arrived in Italy. He spent two months in Rome, first in via Cupa in the San Lorenzo district, where he met volunteers at the Baobab experience, an occupied social centre that had been turned into a reception centre for migrants in 2015 at the height of the ârefugee crisisâ (Dines et al., 2018). After eviction in September 2016, he followed volunteers to the various makeshift reception points around the city. His migratory project does not, however, involve spending the rest of his life in Italy. He decides to move on and one evening in October takes a bus to Milan from Piazzale Tiburtino, from where he reaches Belgium with a Sudanese friend, Azou. However, on 31 January 2017, he landed again at Rome's Fiumicino airport. The Belgian authorities sent him back to Italy as a result of the Dublin regulation, European legislation which states that asylum applications are examined by the first country of entry.
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Reheating and thermalization in a simple scalar model
We consider a simple model for the Universe reheating, which consists of a
single self--interacting scalar field in Minkowskian space--time. Making use of
the existence of an additional small parameter proportional to the amplitude of
the initial spatially homogeneous field oscillations, we show that the behavior
of the field can be found reliably. We describe the evolution of the system
from the homogeneous oscillations to the moment when thermalization is
completed. We compare our results with the Hartree--Fock approximation and
argue that some properties found for this model may be the common features of
realistic theories.Comment: Some changes in Introduction and Discussion, comparison with the
Hartree--Fock results added. 37 pages, 2 postscript figures attache
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