136 research outputs found
The radiative transfer for polarized radiation at second order in cosmological perturbations
This article investigates the full Boltzmann equation up to second order in
the cosmological perturbations. Describing the distribution of polarized
radiation by using a tensor valued distribution function, the second order
Boltzmann equation, including polarization, is derived without relying on the
Stokes parameters.Comment: 4 pages, no figure; replaced to match published versio
General formulation of general-relativistic higher-order gauge-invariant perturbation theory
Gauge-invariant treatments of general-relativistic higher-order perturbations
on generic background spacetime is proposed. After reviewing the general
framework of the second-order gauge-invariant perturbation theory, we show the
fact that the linear-order metric perturbation is decomposed into
gauge-invariant and gauge-variant parts, which was the important premis of this
general framework. This means that the development the higher-order
gauge-invariant perturbation theory on generic background spacetime is
possible. A remaining issue to be resolve is also disscussed.Comment: 4 pages, no figure. (v3) some explanations are added and a reference
is adde
On the non-Gaussianity from Recombination
The non-linear effects operating at the recombination epoch generate a
non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant
because it represents a major part of the second-order radiation transfer
function which must be determined in order to have a complete control of both
the primordial and non-primordial part of non-Gaussianity in the CMB
anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB
arising from the recombination epoch which shows up mainly in the equilateral
configuration. We find that it causes a contamination to the possible
measurement of the equilateral primordial bispectrum shifting the minimum
detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL=
O(10) for an experiment like Planck.Comment: LaTeX file; 11 pages. v2: Typos corrected; references added; comments
about the effective non-linearity parameter added in Sec. IV; comments added
in the conclusions of Sec. IV. v3: References added; some clarifications
added as footnotes 4 and 6, and in Sec. 3. Matches version accepted for
publication in JCA
The CMB bispectrum in the squeezed limit
The CMB bispectrum generated by second-order effects at recombination can be
calculated analytically when one of the three modes has a wavelength much
longer than the other two and is outside the horizon at recombination. This was
pointed out in \cite{Creminelli:2004pv} and here we correct their results. We
derive a simple formula for the bispectrum, , where is the
short scale spectrum and the relative orientation between the long and
the short modes. This formula is exact and takes into account all effects at
recombination, including recombination-lensing, but neglects all late-time
effects such as ISW-lensing. The induced bispectrum in the squeezed limit is
small and will negligibly contaminate the Planck search for a local primordial
signal: this will be biased only by . The above
analytic formula includes the primordial non-Gaussianity of any single-field
model. It also represents a consistency check for second-order Boltzmann codes:
we find substantial agreement with the CMBquick code.Comment: 20 pages, 4 figures, corrected typos to match published version on
JCA
Non-Gaussianity in the Cosmic Microwave Background Anisotropies at Recombination in the Squeezed limit
We estimate analytically the second-order cosmic microwave background
temperature anisotropies at the recombination epoch in the squeezed limit and
we deduce the contamination to the primordial local non-Gaussianity. We find
that the level of contamination corresponds to f_NL^{con}=O(1) which is below
the sensitivity of present experiments and smaller than the value O(5) recently
claimed in the literature.Comment: LaTeX file; 15 pages. Slightly revised version. Main result unchange
Aberration of gravitational waveforms by peculiar velocity
One key prediction of General Relativity is that gravitational waves areemitted with a pure spin-2 polarisation. Any extra polarisation mode, spin-1 orspin-0, is consequently considered a smoking gun for deviations from GeneralRelativity. In this paper, we show that the velocity of merging binaries withrespect to the observer gives rise to spin-1 polarisation in the observer frameeven in the context of General Relativity. These are pure projection effects,proportional to the plus and cross polarisations in the source frame, hencethey do not correspond to new degrees of freedom. We demonstrate that thespin-1 modes can always be rewritten as pure spin-2 modes coming from anaberrated direction. Since gravitational waves are not isotropically emittedaround binary systems, this aberration modifies the apparent orientation of thebinary system with respect to the observer: the system appears slightly rotateddue to the source velocity. Fortunately, this bias does not propagate to otherparameters of the system (and therefore does not spoil tests of GeneralRelativity), since the impact of the velocity can be fully reabsorbed into neworientation angles.<br
Anisotropic Inflation from Charged Scalar Fields
We consider models of inflation with U(1) gauge fields and charged scalar
fields including symmetry breaking potential, chaotic inflation and hybrid
inflation. We show that there exist attractor solutions where the anisotropies
produced during inflation becomes comparable to the slow-roll parameters. In
the models where the inflaton field is a charged scalar field the gauge field
becomes highly oscillatory at the end of inflation ending inflation quickly.
Furthermore, in charged hybrid inflation the onset of waterfall phase
transition at the end of inflation is affected significantly by the evolution
of the background gauge field. Rapid oscillations of the gauge field and its
coupling to inflaton can have interesting effects on preheating and
non-Gaussianities.Comment: minor changes, references added, figures are modified, conforms JCAP
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
Perturbations of generic Kasner spacetimes and their stability
This article investigates the stability of a generic Kasner spacetime to
linear perturbations, both at late and early times. It demonstrates that the
perturbation of the Weyl tensor diverges at late time in all cases but in the
particular one in which the Kasner spacetime is the product of a
two-dimensional Milne spacetime and a two-dimensional Euclidean space. At early
times, the perturbation of the Weyl tensor also diverges unless one imposes a
condition on the perturbations so as to avoid the most divergent modes to be
excited.Comment: 17 pages, 6 figure
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