Oxford astro-ph Coffee

The Cosmic Censorship hypothesis suggests that gravitational singularities will always be hidden from the view of an external observer by an event horizon. It's sometimes given the status of a 'fundamental principle of the universe', though there's no particularly solid reasoning behind this (as far as I'm aware, at least). This short paper explores a gedanken experiment that attempts to violate cosmic censorship.

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?

Abstract: In the standard cosmological framework, the Hubble diagram is interpreted by assuming that the light emitted by standard candles propagates in a spatially homogeneous and isotropic spacetime. However, the light from "point sources" -- such as supernovae -- probes the universe on scales where the homogeneity principle is no longer valid.

I'll let Phil do the talking on this one!

There's been a growing awareness that the 'free' functions used to parameterise modified gravity aren't really as arbitrary as one might first think. This methods paper suggests how to use these ideas - although the forecast isn't all that encouraging. Still, the proof of the pudding is yet to come.

Perhaps an ACT team member could give us a verdict on this?

Modern MCMC method for cosmological parameter estimation. "While Metropolis-Hastings is constrained by overheads, CosmoHammer is able to accelerate the sampling process from a wall time of 30 hours on a single machine to 16 minutes by the efficient use of 2048 cores. Such short wall times for complex data sets opens possibilities for extensive model testing and control of systematics."

Discovery of a group of quasars which appear to form a structure over 1 Gpc across. This is much larger than any previously-suggested homogeneity scale in LambdaCDM.

"We argue using simple models that all successful practical uses of probabilities originate in quantum fluctuations in the microscopic physical world around us, often propagated to macroscopic scales. Thus we claim there is no physically verified fully classical theory of probability. We comment on the general implications of this view, and specifically question the application of classical probability theory to cosmology in cases where key questions are known to have no quantum answer."

BAO detected at z > 2 using BOSS quasars