66 research outputs found
Estimating CDM Particle Trajectories in the Mildly Non-Linear Regime of Structure Formation. Implications for the Density Field in Real and Redshift Space
We obtain approximations for the CDM particle trajectories starting from
Lagrangian Perturbation Theory. These estimates for the CDM trajectories result
in approximations for the density in real and redshift space, as well as for
the momentum density that are better than what standard Eulerian and Lagrangian
perturbation theory give. For the real space density, we find that our proposed
approximation gives a good cross-correlation (>95%) with the non-linear density
down to scales almost twice smaller than the non-linear scale, and six times
smaller than the corresponding scale obtained using linear theory. This allows
for a speed-up of an order of magnitude or more in the scanning of the
cosmological parameter space with N-body simulations for the scales relevant
for the baryon acoustic oscillations. Possible future applications of our
method include baryon acoustic peak reconstruction, building mock galaxy
catalogs, momentum field reconstruction.Comment: 25 pages, 11 figures; reference adde
Towards an Optimal Reconstruction of Baryon Oscillations
The Baryon Acoustic Oscillations (BAO) in the large-scale structure of the
universe leave a distinct peak in the two-point correlation function of the
matter distribution. That acoustic peak is smeared and shifted by bulk flows
and non-linear evolution. However, it has been shown that it is still possible
to sharpen the peak and remove its shift by undoing the effects of the bulk
flows. We propose an improvement to the standard acoustic peak reconstruction.
Contrary to the standard approach, the new scheme has no free parameters,
treats the large-scale modes consistently, and uses optimal filters to extract
the BAO information. At redshift of zero, the reconstructed linear matter power
spectrum leads to a markedly improved sharpening of the reconstructed acoustic
peak compared to standard reconstruction.Comment: 20 pages, 5 figures; footnote adde
The Low Redshift survey at Calar Alto (LoRCA)
The Baryon Acoustic Oscillation (BAO) feature in the power spectrum of
galaxies provides a standard ruler to measure the accelerated expansion of the
Universe. To extract all available information about dark energy, it is
necessary to measure a standard ruler in the local, z<0.2, universe where dark
energy dominates most the energy density of the Universe. Though the volume
available in the local universe is limited, it is just big enough to measure
accurately the long 100 Mpc/h wave-mode of the BAO. Using cosmological N-body
simulations and approximate methods based on Lagrangian perturbation theory, we
construct a suite of a thousand light-cones to evaluate the precision at which
one can measure the BAO standard ruler in the local universe. We find that
using the most massive galaxies on the full sky (34,000 sq. deg.), i.e. a
K(2MASS)<14 magnitude-limited sample, one can measure the BAO scale up to a
precision of 4\% and 1.2\% using reconstruction). We also find that such a
survey would help to detect the dynamics of dark energy.Therefore, we propose a
3-year long observational project, named the Low Redshift survey at Calar Alto
(LoRCA), to observe spectroscopically about 200,000 galaxies in the northern
sky to contribute to the construction of aforementioned galaxy sample. The
suite of light-cones is made available to the public.Comment: 15 pages. Accepted in MNRAS. Please visit our website:
http://lorca-survey.ft.uam.es
Distribution function approach to redshift space distortions. Part IV: perturbation theory applied to dark matter
We develop a perturbative approach to redshift space distortions (RSD) using
the phase space distribution function approach and apply it to the dark matter
redshift space power spectrum and its moments. RSD can be written as a sum over
density weighted velocity moments correlators, with the lowest order being
density, momentum density and stress energy density. We use standard and
extended perturbation theory (PT) to determine their auto and cross
correlators, comparing them to N-body simulations. We show which of the terms
can be modeled well with the standard PT and which need additional terms that
include higher order corrections which cannot be modeled in PT. Most of these
additional terms are related to the small scale velocity dispersion effects,
the so called finger of god (FoG) effects, which affect some, but not all, of
the terms in this expansion, and which can be approximately modeled using a
simple physically motivated ansatz such as the halo model. We point out that
there are several velocity dispersions that enter into the detailed RSD
analysis with very different amplitudes, which can be approximately predicted
by the halo model. In contrast to previous models our approach systematically
includes all of the terms at a given order in PT and provides a physical
interpretation for the small scale dispersion values. We investigate RSD power
spectrum as a function of \mu, the cosine of the angle between the Fourier mode
and line of sight, focusing on the lowest order powers of \mu and multipole
moments which dominate the observable RSD power spectrum. Overall we find
considerable success in modeling many, but not all, of the terms in this
expansion.Comment: 37 pages, 13 figures, published in JCA
The Effective Field Theory of Cosmological Large Scale Structures
Large scale structure surveys will likely become the next leading
cosmological probe. In our universe, matter perturbations are large on short
distances and small at long scales, i.e. strongly coupled in the UV and weakly
coupled in the IR. To make precise analytical predictions on large scales, we
develop an effective field theory formulated in terms of an IR effective fluid
characterized by several parameters, such as speed of sound and viscosity.
These parameters, determined by the UV physics described by the Boltzmann
equation, are measured from N-body simulations. We find that the speed of sound
of the effective fluid is c_s^2 10^(-6) and that the viscosity contributions
are of the same order. The fluid describes all the relevant physics at long
scales k and permits a manifestly convergent perturbative expansion in the size
of the matter perturbations \delta(k) for all the observables. As an example,
we calculate the correction to the power spectrum at order \delta(k)^4. The
predictions of the effective field theory are found to be in much better
agreement with observation than standard cosmological perturbation theory,
already reaching percent precision at this order up to a relatively short scale
k \sim 0.24 h/Mpc.Comment: v2: typos corrected, JHEP published versio
Primordial magnetic fields from second-order cosmological perturbations: Tight coupling approximation
We explore the possibility of generating large-scale magnetic fields from
second-order cosmological perturbations during the pre-recombination era. The
key process for this is Thomson scattering between the photons and the charged
particles within the cosmic plasma. To tame the multi-component interacting
fluid system, we employ the tight coupling approximation. It is shown that the
source term for the magnetic field is given by the vorticity, which signals the
intrinsically second-order quantities, and the product of the first order
perturbations. The vorticity itself is sourced by the product of the
first-order quantities in the vorticity evolution equation. The magnetic fields
generated by this process are estimated to be Gauss on the
horizon scale at the recombination epoch. Although our rough estimate suggests
that the current generation mechanism can work even on smaller scales, more
careful investigation is needed to make clear whether it indeed works in a wide
range of spatial scales.Comment: 10pages, minor corrections, accepted for publication in Class. Quant.
Gra
Nonlinear Power Spectrum from Resummed Perturbation Theory: a Leap Beyond the BAO Scale
A new computational scheme for the nonlinear cosmological matter power
spectrum (PS) is presented. Our method is based on evolution equations in time,
which can be cast in a form extremely convenient for fast numerical
evaluations. A nonlinear PS is obtained in a time comparable to that needed for
a simple 1-loop computation, and the numerical implementation is very simple.
Our results agree with N-body simulations at the percent level in the BAO range
of scales, and at the few-percent level up to h/Mpc at ,
thereby opening the possibility of applying this tool to scales interesting for
weak lensing. We clarify the approximations inherent to this approach as well
as its relations to previous ones, such as the Time Renormalization Group, and
the multi-point propagator expansion. We discuss possible lines of improvements
of the method and its intrinsic limitations by multi streaming at small scales
and low redshifts.Comment: Discussion of the small scale regime dramatically reorganized. Two
appendices added. Results unchange
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