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Valkenburg et al look at the way inhomogeneities in the universe introduce apparent uncertainty in dark energy measurements, if one assumes a homogeneous world model when interpreting distance measurements. They also point out that cosmic variance will lead to bias in w(a). Modeling the structure in the universe is important! My question: is weak lensing immune from these worries, since it involves treatment of all the inhomogeneities?

"An expanding fluid leaves its equilibrium state; the energy density decreases and the pressure also decreases. In the absence of bulk viscosity, the fluid relaxes instantaneously and pressure and density are related by the equation of state. Bulk viscosity dampens this behavior by introducing a finite relaxation timescale, hence producing a shift between the equation of state pressure and the true pressure. We note that for a large enough ζ, the effective pressure becomes negative and could mimic a dark energy behavior." In particular, authors Gagnon & Lesgourges predict effective neutrino number around 3.04, w0 around -0.9, and wa around 0.1. Something to shoot for!

A neat paper demonstrating constraints on faster than light neutrinos using cosmological bounds on the number of effective relativistic species at BBN and at the CMB. For a simple Lorentzian tachyon there is an imaginary mass \mu, and an anergy dependent speed v(\mu,E)>1 (where c=1). The bounds on N_eff translate to bounds on the mass, and therefore bounds on the speed at different energies for this type of super-luminal neutrino. From the CMB the speed at OPERA energies (\sim GeV) is bounded to be v-1<10^{-23}ish, whereas OPERA claimed v-1\sim 10^{-5}. The constraint at \sim MeV is also tighter than SNe1987A constraints. These constarints further rule out explanation of the OPERA results with this type of neutrino. Even though I would not think OPERA is explained by anything like this anyway, I still think it is a simple and neat way to use cosmology. It would probably make a good problem sheet/exam question!

BAO detected at z > 2 using BOSS quasars

More dipoles!

Calculation to 2nd order of the effects of inhomogeneities on averaged observables.

The authors construct exact instanton solutions for tunneling over very small barriers in the presence of gravity, and demonstrate matching between previous results, and with the flat potential and no gravity case. Confusingly, it seems that for consistency one should include the tunneling effect along side the rolling of a field on a flat potential, even when there is no barrier. I'm not sure quite what this means operationally, but I think it may have effects for models of quintessence where the asymptotic future is a big crunch. Here it seems we may not be able to consider simple scalar field rolling, but may also have to include the instanton effects. More excuses to go back and read Coleman-DeLuccia again are always good.

Hints of something interesting or data artefact? Also: http://arxiv.org/pdf/1207.4781v1.pdf

This set of three papers (the link is to the first one, by Nikhil Padmanabhan) describes a factor of two improvement in the SDSS DR7 BAO distance estimate, just by improving the data analysis. Basically, non-linear gravitational collapse causes the usual BAO feature in the galaxy correlation function to appear smoothed out: it can be partially sharpened back up by using the Zel'dovich approximation to reconstruct the density field given the redshift and position data. The result is an increase in cosmological parameter accuracy roughly equivalent to surveying 3-4 times more sky. Software is vital!

It is interesting to see, in the third paper, how the constraints on H_0 \Omega_m space are robust to different scenarios of curvature and dark energy, and compatible with direct measurements of H_0. But also, as expected, this is effected by what one assumes about the neutrinos.