160 research outputs found
CMB anisotropies from patchy reionisation and diffuse Sunyaev-Zel'dovich effects
Anisotropies in the Cosmic Microwave Background (CMB) can be induced during
the later stages of cosmic evolution, and in particular during and after the
Epoch of Reionisation. Inhomogeneities in the ionised fraction, but also in the
baryon density, in the velocity fields and in the gravitational potentials are
expected to generate correlated CMB perturbations. We present a complete
relativistic treatment of all these effects, up to second order in perturbation
theory, that we solve using the numerical Boltzmann code SONG. The physical
origin and relevance of all second order terms are carefully discussed. In
addition to collisional and gravitational contributions, we identify the
diffuse analogue of the blurring and kinetic Sunyaev-Zel'dovich (SZ) effects.
Our approach naturally includes the correlations between the imprint from
patchy reionisation and the diffuse SZ effects thereby allowing us to derive
reliable estimates of the induced temperature and polarisation CMB angular
power spectra. In particular, we show that the B-modes generated at
intermediate length-scales (l~100) have the same amplitude as the B-modes
coming from primordial gravitational waves with a tensor-to-scalar ratio
r=10^{-4}.Comment: 27 pages, 7 figures, uses jcappub. Matches published versio
Suitable Initial Conditions for Newtonian Simulations with Massive Neutrinos
Initial conditions for cosmological N-body simulations are usually calculated
by rescaling the present day linear power spectrum obtained from an
Einstein-Boltzmann solver to the initial time employing the scale-independent
matter growth function. For the baseline Lambda-CDM model, this has been shown
to be consistent with General Relativity (GR) even in the presence of
relativistic species such as photons. We show that this approach is not
feasible in cosmologies with massive neutrinos and present an alternative
method employing the Newtonian motion gauge framework.Comment: 10 pages, 4 figure
The large-scale general-relativistic correction for Newtonian mocks
We clarify the subtle issue of finding the correct mapping of Newtonian
simulations to light-cone observables at very large distance scales. A faithful
general-relativistic interpretation specifies a gauge, i.e. a chart that
relates the simulation data to points of the space-time manifold. It has
already been pointed out that the implicit gauge choice of Newtonian
simulations is indeed different from the Poisson gauge that is commonly adopted
for relativistic calculations, the difference being most significant at large
scales. It is therefore inconsistent, for example, to predict weak-lensing
observables from simulations unless this gauge issue is properly accounted for.
Using perturbation theory as well as fully relativistic N-body simulations we
quantify the systematic error introduced this way, and we discuss several
solutions that would render the calculations relativistically self-consistent.Comment: 10 pages, 5 figures; v2: minor revision with additional content,
matches accepted manuscrip
A precise numerical estimation of the magnetic field generated around recombination
We investigate the generation of magnetic fields from non-linear effects
around recombination. As tight-coupling is gradually lost when approaching
, the velocity difference between photons and baryons starts to
increase, leading to an increasing Compton drag of the photons on the
electrons. The protons are then forced to follow the electrons due to the
electric field created by the charge displacement; the same field, following
Maxwell's laws, eventually induces a magnetic field on cosmological scales.
Since scalar perturbations do not generate any magnetic field as they are
curl-free, one has to resort to second-order perturbation theory to compute the
magnetic field generated by this effect. We reinvestigate this problem
numerically using the powerful second-order Boltzmann code SONG. We show that:
i) all previous studies do not have a high enough angular resolution to reach a
precise and consistent estimation of the magnetic field spectrum; ii) the
magnetic field is generated up to ; iii) it is in practice
impossible to compute the magnetic field with a Boltzmann code for scales
smaller than . Finally we confirm that for scales of a few , this magnetic field is of order , many orders
of magnitude smaller than what is currently observed on intergalactic scales.Comment: 6 pages, 3 figure
Relativistic bias in neutrino cosmologies
Halos and galaxies are tracers of the underlying dark matter structures.
While their bias is well understood in the case of a simple Universe composed
dominantly of dark matter, the relation becomes more complex in the presence of
massive neutrinos. Indeed massive neutrinos introduce rich dynamics in the
process of structure formation leading to scale-dependent bias. We study this
process from the perspective of general relativity employing a simple spherical
collapse model. We find a characteristic signature at the neutrino
free-streaming scale in addition to a large-scale feature from general
relativity. The scale-dependent halo bias opposes the suppression in the matter
distribution due to neutrino free-streaming and leads to corrections of a few
percent in the halo power spectrum. It is not only sensitive to the sum of the
neutrino-masses, but respond to the individual masses. Accurate models for the
neutrino bias are a crucial ingredient for the future data analysis and play an
important role in constraining the neutrino masses.Comment: 20 pages, 10 figure
General relativistic corrections to -body simulations and the Zel'dovich approximation
The initial conditions for Newtonian -body simulations are usually
generated by applying the Zel'dovich approximation to the initial displacements
of the particles using an initial power spectrum of density fluctuations
generated by an Einstein-Boltzmann solver. We show that in most gauges the
initial displacements generated in this way receive a first-order relativistic
correction. We define a new gauge, the -body gauge, in which this
relativistic correction vanishes and show that a conventional Newtonian
-body simulation includes all first-order relativistic contributions (in the
absence of radiation) if we identify the coordinates in Newtonian simulations
with those in the relativistic -body gauge.Comment: 5 pages, 3 figures, revised text and figures, matches published
version in PR
Relativistic initial conditions for N-body simulations
Initial conditions for (Newtonian) cosmological N-body simulations are
usually set by re-scaling the present-day power spectrum obtained from linear
(relativistic) Boltzmann codes to the desired initial redshift of the
simulation. This back-scaling method can account for the effect of
inhomogeneous residual thermal radiation at early times, which is absent in the
Newtonian simulations. We analyse this procedure from a fully relativistic
perspective, employing the recently-proposed Newtonian motion gauge framework.
We find that N-body simulations for LambdaCDM cosmology starting from
back-scaled initial conditions can be self-consistently embedded in a
relativistic space-time with first-order metric potentials calculated using a
linear Boltzmann code. This space-time coincides with a simple "N-body gauge"
for z<50 for all observable modes. Care must be taken, however, when simulating
non-standard cosmologies. As an example, we analyse the back-scaling method in
a cosmology with decaying dark matter, and show that metric perturbations
become large at early times in the back-scaling approach, indicating a
breakdown of the perturbative description. We suggest a suitable "forwards
approach" for such cases.Comment: 20 pages, 8 figure
Impact of polarization on the intrinsic cosmic microwave background bispectrum
We compute the cosmic microwave background (CMB) bispectrum induced by the evolution of the primordial density perturbations, including for the first time both temperature and polarization using a second-order Boltzmann code. We show that including polarization can increase the signal-to-noise by a factor 4 with respect to temperature alone. We find the expected signal-to-noise for this intrinsic bispectrum of S=N ¼ 3.8; 2.9; 1.6 and 0.5 for an ideal experiment with an angular resolution of lmax ¼ 3000, the proposed CMB surveys PRISM and COrE, and Planck’s polarized data, respectively; the bulk of this signal comes from E-mode polarization and from squeezed configurations. We discuss how CMB lensing is expected to reduce these estimates as it suppresses the bispectrum for squeezed configurations and contributes to the noise in the estimator. We find that the presence of the intrinsic bispectrum will bias a measurement of primordial non-Gaussianity of local type by fintr NL ¼ 0.66 for an ideal experiment with lmax ¼ 3000. Finally, we verify the robustness of our results by recovering the analytic approximation for the squeezed-limit bispectrum in the general polarized case
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