8,810 research outputs found

### The early reionization with the primordial magnetic fields

The early reionization of the intergalactic medium, which is favored from the
WMAP temperature-polarization cross-correlations, contests the validity of the
standard scenario of structure formation in the cold dark matter cosmogony. It
is difficult to achieve early enough star formation without rather extreme
assumptions such as very high escape fraction of ionizing photons from
proto-galaxies or a top-heavy initial mass function. Here we propose an
alternative scenario that is additional fluctuations on small scales induced by
primordial magnetic fields trigger the early structure formation. We found that
ionizing photons from Population III stars formed in dark haloes can easily
reionize the universe by $z \simeq 15$ if the strength of primordial magnetic
fields is larger than $0.6 \times 10^{-9}$Gauss.Comment: 8 pages, 5 figures. accepted for publication in MNRA

### Patchy He II reionization and the physical state of the IGM

We present a Monte-Carlo model of He II reionization by QSOs and its effect
on the thermal state of the clumpy intergalactic medium (IGM). The model
assumes that patchy reionization develops as a result of the discrete
distribution of QSOs. It includes various recipes for the propagation of the
ionizing photons, and treats photo-heating self-consistently. The model
provides the fraction of He III, the mean temperature in the IGM, and the He II
mean optical depth -- all as a function of redshift. It also predicts the
evolution of the local temperature versus density relation during reionization.
Our findings are as follows: The fraction of He III increases gradually until
it becomes close to unity at $z\sim 2.8-3.0$. The He II mean optical depth
decreases from $\tau\sim 10$ at $z\geq 3.5$ to $\tau\leq 0.5$ at $z\leq 2.5$.
The mean temperature rises gradually between $z\sim 4$ and $z\sim 3$ and
declines slowly at lower redshifts. The model predicts a flattening of the
temperature-density relation with significant increase in the scatter during
reionization at $z\sim 3$. Towards the end of reionization the scatter is
reduced and a tight relation is re-established. This scatter should be
incorporated in the analysis of the Ly$\alpha$ forest at $z\leq 3$. Comparison
with observational results of the optical depth and the mean temperature at
moderate redshifts constrains several key physical parameters.Comment: 18 pages, 9 figures; Changed content. Accepted for publication in
MNRA

### Scalability of spin FPGA: A Reconfigurable Architecture based on spin MOSFET

Scalability of Field Programmable Gate Array (FPGA) using spin MOSFET (spin
FPGA) with magnetocurrent (MC) ratio in the range of 100% to 1000% is discussed
for the first time. Area and speed of million-gate spin FPGA are numerically
benchmarked with CMOS FPGA for 22nm, 32nm and 45nm technologies including 20%
transistor size variation. We show that area is reduced and speed is increased
in spin FPGA owing to the nonvolatile memory function of spin MOSFET.Comment: 3 pages, 7 figure

### {\delta}N formalism

Precise understanding of nonlinear evolution of cosmological perturbations
during inflation is necessary for the correct interpretation of measurements of
non-Gaussian correlations in the cosmic microwave background and the
large-scale structure of the universe. The "{\delta}N formalism" is a popular
and powerful technique for computing non-linear evolution of cosmological
perturbations on large scales. In particular, it enables us to compute the
curvature perturbation, {\zeta}, on large scales without actually solving
perturbed field equations. However, people often wonder why this is the case.
In order for this approach to be valid, the perturbed Hamiltonian constraint
and matter-field equations on large scales must, with a suitable choice of
coordinates, take on the same forms as the corresponding unperturbed equations.
We find that this is possible when (1) the unperturbed metric is given by a
homogeneous and isotropic Friedmann-Lema\^itre-Robertson-Walker metric; and (2)
on large scales and with a suitable choice of coordinates, one can ignore the
shift vector (g0i) as well as time-dependence of tensor perturbations to
gij/a2(t) of the perturbed metric. While the first condition has to be assumed
a priori, the second condition can be met when (3) the anisotropic stress
becomes negligible on large scales. However, in order to explicitly show that
the second condition follows from the third condition, one has to use
gravitational field equations, and thus this statement may depend on the
details of theory of gravitation. Finally, as the {\delta}N formalism uses only
the Hamiltonian constraint and matter-field equations, it does not a priori
respect the momentum constraint. We show that the violation of the momentum
constraint only yields a decaying mode solution for {\zeta}, and the violation
vanishes when the slow-roll conditions are satisfied.Comment: 10 page

### The Degrees of Freedom of Partial Least Squares Regression

The derivation of statistical properties for Partial Least Squares regression
can be a challenging task. The reason is that the construction of latent
components from the predictor variables also depends on the response variable.
While this typically leads to good performance and interpretable models in
practice, it makes the statistical analysis more involved. In this work, we
study the intrinsic complexity of Partial Least Squares Regression. Our
contribution is an unbiased estimate of its Degrees of Freedom. It is defined
as the trace of the first derivative of the fitted values, seen as a function
of the response. We establish two equivalent representations that rely on the
close connection of Partial Least Squares to matrix decompositions and Krylov
subspace techniques. We show that the Degrees of Freedom depend on the
collinearity of the predictor variables: The lower the collinearity is, the
higher the Degrees of Freedom are. In particular, they are typically higher
than the naive approach that defines the Degrees of Freedom as the number of
components. Further, we illustrate how the Degrees of Freedom approach can be
used for the comparison of different regression methods. In the experimental
section, we show that our Degrees of Freedom estimate in combination with
information criteria is useful for model selection.Comment: to appear in the Journal of the American Statistical Associatio

### Systematic limits on sin^2{2theta_{13}} in neutrino oscillation experiments with multi-reactors

Sensitivities to sin^2{2theta_{13}} without statistical errors (``systematic
limit'') are investigated in neutrino oscillation experiments with multiple
reactors. Using an analytical approach, we show that the systematic limit on
sin^2{2theta_{13}} is dominated by the uncorrelated systematic error sigma_u of
the detector. Even in an experiment with multi-detectors and multi-reactors, it
turns out that most of the systematic errors including the one due to the
nature of multiple sources is canceled as in the case with a single reactor
plus two detectors, if the near detectors are placed suitably. The case of the
KASKA plan (7 reactors and 3 detectors) is investigated in detail, and it is
explicitly shown that it does not suffer from the extra uncertainty due to
multiple reactors.Comment: 26 pages, 10 eps-files, revtex

### Neutron Response Matrix for Unfolding NE-213 Scintillator Measurements in 6 < En < 34 MeV

開始ページ、終了ページ: 冊子体のページ付

- …