3,872 research outputs found
Cosmological Structure Formation with Augmented Lagrangian Perturbation Theory
We present a new fast and efficient approach to model structure formation
with Augmented Lagrangian Perturbation Theory (ALPT). Our method is based on
splitting the displacement field into a long and a short-range component. The
long-range component is computed by second order LPT (2LPT). This approximation
contains a tidal nonlocal and nonlinear term. Unfortunately, 2LPT fails on
small scales due to severe shell crossing and a crude quadratic behaviour in
the low density regime. The spherical collapse (SC) approximation has been
recently reported to correct for both effects by adding an ideal collapse
truncation. However, this approach fails to reproduce the structures on large
scales where it is significantly less correlated with the N-body result than
2LPT or linear LPT (the Zeldovich approximation). We propose to combine both
approximations using for the short-range displacement field the SC solution. A
Gaussian filter with a smoothing radius r_S is used to separate between both
regimes. We use the result of 25 dark matter only N-body simulations to
benchmark at z=0 the different approximations: 1st, 2nd, 3rd order LPT, SC and
our novel combined ALPT model. This comparison demonstrates that our method
improves previous approximations at all scales showing ~25% and ~75% higher
correlation than 2LPT with the N-body solution at k = 1 and 2 h Mpc^-1,
respectively. We conduct a parameter study to determine the optimal range of
smoothing radii and find that the maximum correlation is achieved with r_S = 4
- 5 h^-1 Mpc. This structure formation approach could be used for various
purposes, such as setting-up initial conditions for N-body simulations,
generating mock galaxy catalogues, cosmic web analysis or for reconstructions
of the primordial density fluctuations.Comment: 6 pages and 4 figure
Linearisation with Cosmological Perturbation Theory
We propose a new method to linearise cosmological mass density fields using
higher order Lagrangian perturbation theory (LPT). We demonstrate that a given
density field can be expressed as the sum of a linear and a nonlinear component
which are tightly coupled to each other by the tidal field tensor within the
LPT framework. The linear component corresponds to the initial density field in
Eulerian coordinates, and its mean relation with the total field can be
approximated by a logarithm (giving theoretical support to recent attempts to
find such component). We also propose to use a combination of the linearisation
method and the continuity equation to find the mapping between Eulerian and
Lagrangian coordinates. In addition, we note that this method opens the
possibility of use directly higher order LPT on nonlinear fields. We test our
linearization scheme by applying it to the z~0.5 density field from an N-body
simulation. We find that the linearised version of the full density field can
be successfully recovered on >~5 h^{-1}Mpc, reducing the skewness and kurtosis
of the distribution by about one and two orders of magnitude, respectively.
This component can also be successfully traced back in time, converging towards
the initial unevolved density field at z~100. We anticipate a number of
applications of our results, from predicting velocity fields to estimates of
the initial conditions of the universe, passing by improved constraints on
cosmological parameters derived from galaxy clustering via reconstruction
methods.Comment: 14 pages, 8 figure
Modelling Baryon Acoustic Oscillations with Perturbation Theory and Stochastic Halo Biasing
In this work we investigate the generation of mock halo catalogues based on
perturbation theory and nonlinear stochastic biasing with the novel
PATCHY-code. In particular, we use Augmented Lagrangian Perturbation Theory
(ALPT) to generate a dark matter density field on a mesh starting from Gaussian
fluctuations and to compute the peculiar velocity field. ALPT is based on a
combination of second order LPT (2LPT) on large scales and the spherical
collapse model on smaller scales. We account for the systematic deviation of
perturbative approaches from N-body simulations together with halo biasing
adopting an exponential bias model. We then account for stochastic biasing by
defining three regimes: a low, an intermediate and a high density regime, using
a Poisson distribution in the intermediate regime and the negative binomial
distribution to model over-dispersion in the high density regime. Since we
focus in this study on massive halos, we suppress the generation of halos in
the low density regime. The various nonlinear and stochastic biasing
parameters, and density thresholds (five) are calibrated with the large
BigMultiDark N-body simulation to match the power spectrum of the corresponding
halo population. Our mock catalogues show power spectra, both in real- and
redshift-space, which are compatible with N-body simulations within about 2% up
to k ~ 1 h Mpc^-1 at z = 0.577 for a sample of halos with the typical BOSS
CMASS galaxy number density. The corresponding correlation functions are
compatible down to a few Mpc. We also find that neglecting over-dispersion in
high density regions produces power spectra with deviations of 10% at k ~ 0.4 h
Mpc^-1. These results indicate the need to account for an accurate statistical
description of the galaxy clustering for precise studies of large-scale
surveys.Comment: 5 pages, 4 figure
Itinerant-localized dual character of a strongly-correlated superfluid Bose gas in an optical lattice
We investigate a strongly-correlated Bose gas in an optical lattice.
Extending the standard-basis operator method developed by Haley and Erdos to a
boson Hubbard model, we calculate excitation spectra in the superfluid phase,
as well as in the Mott insulating phase, at T=0. In the Mott phase, the
excitation spectrum has a finite energy gap, reflecting the localized character
of atoms. In the superfluid phase, the excitation spectrum is shown to have an
itinerant-localized dual structure, where the gapless Bogoliubov mode (which
describes the itinerant character of superfluid atoms) and a band with a finite
energy gap coexist. We also show that the rf-tunneling current measurement
would give a useful information about the duality of a strongly-correlated
superfluid Bose gas near the superfluid-insulator transition.Comment: 10 pages, 4 figure
Neutrino-driven explosions twenty years after SN1987A
The neutrino-heating mechanism remains a viable possibility for the cause of
the explosion in a wide mass range of supernova progenitors. This is
demonstrated by recent two-dimensional hydrodynamic simulations with detailed,
energy-dependent neutrino transport. Neutrino-driven explosions were not only
found for stars in the range of 8-10 solar masses with ONeMg cores and in case
of the iron core collapse of a progenitor with 11 solar masses, but also for a
``typical'' progenitor model of 15 solar masses. For such more massive stars,
however, the explosion occurs significantly later than so far thought, and is
crucially supported by large-amplitude bipolar oscillations due to the
nonradial standing accretion shock instability (SASI), whose low (dipole and
quadrupole) modes can develop large growth rates in conditions where convective
instability is damped or even suppressed. The dominance of low-mode deformation
at the time of shock revival has been recognized as a possible explanation of
large pulsar kicks and of large-scale mixing phenomena observed in supernovae
like SN 1987A.Comment: 11 pages, 6 figures; review proceeding for "Supernova 1987A: 20 Years
After: Supernovae and Gamma-Ray Bursters" AIP, New York, eds. S. Immler, K.W.
Weiler, and R. McCra
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