9 research outputs found
Boltzmann hierarchy for the cosmic microwave background at second order including photon polarization
Non-gaussianity and B-mode polarization are particularly interesting features
of the cosmic microwave background, as -- at least in the standard model of
cosmology -- their only sources to first order in cosmological perturbation
theory are primordial, possibly generated during inflation. If the primordial
sources are small, the question arises how large is the non-gaussianity and
B-mode background induced in second-order from the initially gaussian and
scalar perturbations. In this paper we derive the Boltzmann hierarchy for the
microwave background photon phase-space distributions at second order in
cosmological perturbation theory including the complete polarization
information, providing the basis for further numerical studies. As an aside we
note that the second-order collision term contains new sources of B-mode
polarization and that no polarization persists in the tight-coupling limit.Comment: LaTeX, 33 page
B polarization of cosmic background radiation from second-order scattering sources
B-mode polarization of the cosmic background radiation is induced from purely
scalar primordial sources at second order in perturbations of the homogeneous,
isotropic universe. We calculate the B-mode angular power spectrum C_l^{BB}
sourced by the second-order scattering term in the full second-order Boltzmann
equations for the polarized radiation phase-space density, which have recently
become available. We find that at l\approx 200 the second-order effect is
comparable to the first-order effect for a tensor-to-scalar ratio of r=10^{-6},
and to about 2\cdot 10^{-4} at l\approx 1000. It is always negligible relative
to the weak-lensing induced contribution.Comment: 32 page
Hunting for Primordial Non-Gaussianity in the Cosmic Microwave Background
Since the first limit on the (local) primordial non-Gaussianity parameter,
fNL, was obtained from COBE data in 2002, observations of the CMB have been
playing a central role in constraining the amplitudes of various forms of
non-Gaussianity in primordial fluctuations. The current 68% limit from the
7-year WMAP data is fNL=32+/-21, and the Planck satellite is expected to reduce
the uncertainty by a factor of four in a few years from now. If fNL>>1 is found
by Planck with high statistical significance, all single-field models of
inflation would be ruled out. Moreover, if the Planck satellite finds fNL=30,
then it would be able to test a broad class of multi-field models using the
four-point function (trispectrum) test of tauNL>=(6fNL/5)^2. In this article,
we review the methods (optimal estimator), results (WMAP 7-year), and
challenges (secondary anisotropy, second-order effect, and foreground) of
measuring primordial non-Gaussianity from the CMB data, present a science case
for the trispectrum, and conclude with future prospects.Comment: 33 pages, 4 figures. Invited review, accepted for publication in the
CQG special issue on nonlinear cosmological perturbations. (v2) References
added. More clarifications are added to the second-order effect and the
multi-field consistency relation, tauNL>=(6fNL/5)^2
The intrinsic B-mode polarisation of the cosmic mMicrowave background
We estimate the B-polarisation induced in the Cosmic Microwave Background by the non-linear evolution of density perturbations. Using the second-order Boltzmann code SONG, our analysis incorporates, for the first time, all physical effects at recombination. We also include novel contributions from the redshift part of the Boltzmann equation and from the bolometric definition of the temperature in the presence of polarisation. The remaining line-of-sight terms (lensing and time-delay) have previously been studied and must be calculated non-perturbatively. The intrinsic B-mode polarisation is present independent of the initial conditions and might contaminate the signal from primordial gravitational waves. We find this contamination to be comparable to a primordial tensor-to-scalar ratio of râ10â7 at the angular scale ââ100, where the primordial signal peaks, and râ5â
10â5 at ââ700, where the intrinsic signal peaks. Therefore, we conclude that the intrinsic B-polarisation from second-order effects is not likely to contaminate future searches of primordial gravitational waves
Leptogenesis from oscillations of heavy neutrinos with large mixing angles
via the seesaw mechanism and the baryon asymmetry of the Universe via leptogenesis. If the mass of the heavy neutrinos is below the electroweak scale, they may be found at the LHC, BELLE II, NA62, the proposed SHiP experiment or a future high-energy collider. In this mass range, the baryon asymmetry is generated via CP -violating oscillations of the heavy neutrinos during their production. We study the generation of the baryon asymmetry of the Universe in this scenario from first principles of non-equilibrium quantum field theory, including spectator processes and feedback effects. We eliminate several uncertainties from previous calcula-tions and find that the baryon asymmetry of the Universe can be explained with larger heavy neutrino mixing angles, increasing the chance for an experimental discovery. For the limiting cases of fast and strongly overdamped oscillations of right-handed neutrinos, the generation of the baryon asymmetry can be calculated analytically up to corrections of order one