Day 39

Wednesday.  Drama in IEU.  It's all my fault for stirring things up where I shouldn't.  Sorry Teppei, hope I didn't cause you too much trouble.


1107.1503
Dark goo: bulk viscosity as an alternative to dark energy
Gagnon, Lesgourgues


For a sufficiently large bulk viscosity, the effective pressure becomes negative and could mimic DE equation of state.  Microscopic model: self-interacting spin-zero particles (bulk viscosity known).  BG equations and linear perturbations to this model studied.  Can get DE behaviour for reasonable values of the two parameters of the model (mass and coupling).  Linear perturbations well-behaved, no fine-tuning.  Conditions when hydrodynamics holds: spin-zero particles must be in local equilibrium today.


* how exactly does the viscosity turn into negative pressure?


1107.1656
A weak lensing view on primordial non-Gaussianities
Schaefer, Grassi, Gerstenlauer, Byrnes


WL bispectrum signature of primordial non-Gaussianities: local, orthogonal, equilateral.  S/N in Euclid--pretty large on fNL (200 for local), misestimations of fNL if wrong model chosen, probability of noticing such a mistake, degeneracies with other cosmological parameters (Omega_m only), subtraction of structure-generated bispectrum.  If prior on wCDM parameters are available from Planck/Euclid, structure-formation bispectrum can be predicted accurately for subtraction; any residual would influence estimation of fNL to a minor degree.  Monte-Carlo technique is the key.  WL probes smaller scales than the CMB and hence provide additional constraint on non-Gaussianities, although they are not as sensitive to the primordial non-Gaussianities as the CMB


* is the structure-formation bispectrum a noise on top of the primordial non-Gaussianity bispectrum?  Yes (probably).
* Interesting, would like to read this.