125 research outputs found
The Inflationary Paradigm: Predictions for CMB
We review why the search for a causal explanation of the large scale
properties of the universe supports the idea that an extended period of
accelerated expansion, called inflation, preceded primordial nucleosynthesis.
As a consequence of inflation, all pre-existing classical structures are washed
out, and the primordial density fluctuations (the seeds of the large scale
structures) result from the amplification of vacuum quantum fluctuations. The
properties of the spectrum are derived and compared to those of the spectrum of
CMB anisotropies. The agreement is striking.Comment: 15 pages, 2 figures, review articl
A sub-horizon framework for probing the relationship between the cosmological matter distribution and metric perturbations
The relationship between the metric and nonrelativistic matter distribution
depends on the theory of gravity and additional fields, providing a possible
way of distinguishing competing theories. With the assumption that the geometry
and kinematics of the homogeneous universe have been measured to sufficient
accuracy, we present a procedure for understanding and testing the relationship
between the cosmological matter distribution and metric perturbations (along
with their respective evolution) using the ratio of the physical size of the
perturbation to the size of the horizon as our small expansion parameter. We
expand around Newtonian gravity on linear, subhorizon scales with coefficient
functions in front of the expansion parameter. Our framework relies on an
ansatz which ensures that (i) the Poisson equation is recovered on small scales
(ii) the metric variables (and any additional fields) are generated and
supported by the nonrelativistic matter overdensity. The scales for which our
framework is intended are small enough so that cosmic variance does not
significantly limit the accuracy of the measurements and large enough to avoid
complications from nonlinear effects and baryon cooling. The coefficient
functions provide a general framework for contrasting the consequences of
Lambda CDM and its alternatives. We calculate the coefficient functions for
general relativity with a cosmological constant and dark matter, GR with dark
matter and quintessence, scalar-tensor theories, f(R) gravity and braneworld
models. We identify a possibly unique signature of braneworld models.
Constraining the coefficient functions provides a streamlined approach for
testing gravity in a scale dependent manner. We briefly discuss the
observations best suited for an application of our framework.Comment: Updated references and minor changes to match the published version
in MNRA
The overdensity and masses of the friends-of-friends halos and universality of the halo mass function
The friends-of-friends algorithm (hereafter, FOF) is a percolation algorithm
which is routinely used to identify dark matter halos from N-body simulations.
We use results from percolation theory to show that the boundary of FOF halos
does not correspond to a single density threshold but to a range of densities
close to a critical value that depends upon the linking length parameter, b. We
show that for the commonly used choice of b = 0.2, this critical density is
equal to 81.62 times the mean matter density. Consequently, halos identified by
the FOF algorithm enclose an average overdensity which depends on their density
profile (concentration) and therefore changes with halo mass contrary to the
popular belief that the average overdensity is ~180. We derive an analytical
expression for the overdensity as a function of the linking length parameter b
and the concentration of the halo. Results of tests carried out using simulated
and actual FOF halos identified in cosmological simulations show excellent
agreement with our analytical prediction. We also find that the mass of the
halo that the FOF algorithm selects crucially depends upon mass resolution. We
find a percolation theory motivated formula that is able to accurately correct
for the dependence on number of particles for the mock realizations of
spherical and triaxial Navarro-Frenk-White halos. However, we show that this
correction breaks down when applied to the real cosmological FOF halos due to
presence of substructures. Given that abundance of substructure depends on
redshift and cosmology, we expect that the resolution effects due to
substructure on the FOF mass and halo mass function will also depend on
redshift and cosmology and will be difficult to correct for in general.
Finally, we discuss the implications of our results for the universality of the
mass function.Comment: 19 pages, 17 figures, submitted to ApJ supplemen
Bacteria in milk from anterior and posterior mammary glands in sows affected and unaffected by postpartum dysgalactia syndrome (PPDS)
<p>Abstract</p> <p>Background</p> <p>The performance of piglet weight gain is strongly dependent on the sow's ability to meet the demand for adequate milk. Postparturient disorders, especially those subsumed under the term postpartum dysgalactia syndrome (PPDS), can alter or reduce the milk production sensitively, resulting in starving piglets. The aim of this study was to gather further information about the prevalence of different bacterial species in the anterior and posterior mammary glands of sows with respect to the clinical appearance of PPDS.</p> <p>Methods</p> <p>In this study, the health status of 56 sows after farrowing was determined with special regard to mastitis and dysgalactia. Pooled milk samples from anterior and posterior glands were taken from both affected and non-affected animals and analysed bacteriologically for the presence of a wide spectrum of different pathogens.</p> <p>Results</p> <p>Mainly <it>Escherichia coli</it>, staphylococci and streptococci were detected in high percentages but without significant differences in healthy and diseased animals and anterior and posterior glands. However, the large percentages of coliform bacteria suggested a transmission route via faecal contamination.</p> <p>Conclusion</p> <p>In this study, the prevalence of different bacteria in anterior and posterior glands in PPDS positive and negative sows was analysed. No significant differences in bacteria of healthy and diseased sows were assessed. Therefore, the development of clinical PPDS and actual infection seems to be largely dependant on individual resistance in single sows.</p
A limit on the detectability of the energy scale of inflation
We show that the polarization of the cosmic microwave background can be used
to detect gravity waves from inflation if the energy scale of inflation is
above 3.2 times 10^15 GeV. These gravity waves generate polarization patterns
with a curl, whereas (to first order in perturbation theory) density
perturbations do not. The limiting ``noise'' arises from the second--order
generation of curl from density perturbations, or rather residuals from its
subtraction. We calculate optimal sky coverage and detectability limits as a
function of detector sensitivity and observing time.Comment: 4 pages, 3 figures, submitted to PR
A circular polarimeter for the Cosmic Microwave Background
A primordial degree of circular polarization of the Cosmic Microwave
Background is not observationally excluded. The hypothesis of primordial
dichroism can be quantitatively falsified if the plasma is magnetized prior to
photon decoupling since the initial V-mode polarization affects the evolution
of the temperature fluctuations as well as the equations for the linear
polarization. The observed values of the temperature and polarization angular
power spectra are used to infer constraints on the amplitude and on the
spectral slope of the primordial V-mode. Prior to photon decoupling magnetic
fields play the role of polarimeters insofar as they unveil the circular
dichroism by coupling the V-mode power spectrum to the remaining brightness
perturbations. Conversely, for angular scales ranging between 4 deg and 10 deg
the joined bounds on the magnitude of circular polarization and on the magnetic
field intensity suggest that direct limits on the V-mode power spectrum in the
range of 0.01 mK could directly rule out pre-decoupling magnetic fields in the
range of 10-100 nG. The frequency dependence of the signal is located, for the
present purposes, in the GHz range.Comment: 28 pages, 12 included figures
The CMBR ISW and HI 21-cm Cross-correlation Angular Power Spectrum
The late-time growth of large scale structures (LSS) is imprinted in the CMBR
anisotropy through the Integrated Sachs Wolfe (ISW) effect. This is perceived
to be a very important observational probe of dark energy. Future observations
of redshifted 21-cm radiation from the cosmological neutral hydrogen (HI)
distribution hold the potential of probing the LSS over a large redshift range.
We have investigated the possibility of detecting the ISW through
cross-correlations between the CMBR anisotropies and redshifted 21-cm
observations. Assuming that the HI traces the dark matter, we find that the
ISW-HI cross-correlation angular power spectrum at an angular multipole l is
proportional to the dark matter power spectrum evaluated at the comoving wave
number l/r, where r is the comoving distance to the redshift from which the HI
signal originated. The amplitude of the cross-correlation signal depends on
parameters related to the HI distribution and the growth of cosmological
perturbations. However the cross-correlation is extremely weak as compared to
the CMBR anisotropies and the predicted HI signal. As a consequence the
cross-correlation signal is smaller than the cosmic variance, and a
statistically significant detection is not very likely.Comment: 13 pages, 4 eps figures, submitte
The Evolution of Central Group Galaxies in Hydrodynamical Simulations
We trace the evolution of central galaxies in three ~10^13 M_sun galaxy
groups simulated at high resolution in cosmological hydrodynamical simulations.
The evolution in the group potential leads, at z=0, to central galaxies that
are massive, gas-poor early-type systems supported by stellar velocity
dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main
progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr)
galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in
extended gas disks. Our simulations thus show that star forming galaxies
observed at z~2 are likely the main progenitors of central galaxies in galaxy
groups at z=0. Their central stellar densities stay approximately constant from
z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a
stellar envelope outside the innermost ~2 kpc. Consequently the density within
the effective radius decreases by up to two orders of magnitude. Both major and
minor mergers contribute to most of the mass accreted outside the effective
radius and thus drive the evolution of the half-mass radii. In one of the three
simulated groups the short central cooling time leads to a dramatic
rejuvenation of the central group galaxy at z<1, affecting its morphology,
kinematics and colors. This episode is eventually terminated by a group-group
merger. Our simulations demonstrate that, in galaxy groups, the interplay
between halo mass assembly, galaxy merging and gas accretion has a substantial
influence on the star formation histories and z=0 morphologies of central
galaxies.[Abridged]Comment: 28 pages, 23 figures, 9 tables, accepted to APJ (revised to match
accepted version
Small-Scale Fluctuations in Cosmic X-ray Background : A Power Spectrum Approach
Equations to investigate fluctuations in cosmic X-ray background radiation
due to point-like sources at high-redshift are formulated in a systematic way.
The angular power spectrum of X-ray background fluctuations is investigated
from large-scales to small-scales in various cosmological models such as open
universe models and models with the cosmological constant, assuming a simple
evolution model of the sources. The effect of epoch-dependent bias is
demonstrated for small-angle fluctuations. The contribution from shot noise
fluctuations is also discussed.Comment: 12 pages, 4 figures, Phys.Rev.D in pres
Position-Space Description of the Cosmic Microwave Background and Its Temperature Correlation Function
We suggest that the cosmic microwave background (CMB) temperature correlation
function C(theta) as a function of angle provides a direct connection between
experimental data and the fundamental cosmological quantities. The evolution of
inhomogeneities in the prerecombination universe is studied using their Green's
functions in position space. We find that a primordial adiabatic point
perturbation propagates as a sharp-edged spherical acoustic wave. Density
singularities at its wavefronts create a feature in the CMB correlation
function distinguished by a dip at theta ~ 1.2 deg. Characteristics of the
feature are sensitive to the values of cosmological parameters, in particular
to the total and the baryon densities.Comment: The version accepted for publication in Phys. Rev. Letters. 4 pages,
3 figure
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