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From N-body simulation, the authors find that concentration-mass relation displays a turnover for group scale dark matter haloes, for the case of WDM particles with masses of the order ~0.25 keV. This may be interpreted as a hint for top-down structure formation on small scales. Is there any reionization mechanism in this scenario?

1)Because the size of the simulated boxes were ten times smaller in previous studies. 2)Weak lensing at scale below ~1arcsec could work. Their results might be helpful for estimating non-linear power spectrum based on a certain halo model.

Cooool. Was Malin there this morning? This could be right up her street, with her flexion stuff! Also, weak lensing at < 1" sounds a bit like strong lensing to me - I'll read the paper in detail and see if there's anything we can already say from our (admittedly modest) SDSS samples. Thanks!

I've not read the whole thing (it's 44 pages!) but Yu Lu is, IMO, doing the Right Thing in this field - he takes a Semi Analytic Model of galaxy formation, and *actually fits* all the parameters to the data (in this case, the observed K-band luminosity function). Some parameters are well-constrained (implying we may have learnt something about galaxies), while others show strong degeneracies, indicating what new physics needs to be included. Seems like the models in most of the parameter volume fail to predict some other datasets, giving more clues on how to improve the model. The key thing is that by doing the inference properly, Lu has elevated SAM study to a quantified learning process.

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.

A star with a mid-IR excess (circumstellar disk?) that vanishes in a 2-year interval.

This paper is by David Wallace (a philosopher in Oxford, not the novelist). The idea seems to be to talk through some of the statements that are made about the treatment of entropy in gravitation. I found this to be a useful exercise, and there are some interesting thoughts in here, even if the cosmology is a bit hit and miss. In particular he points out that the formation of structure in the Universe does not necessarily imply that gravitational fields in the Universe have to carry large amounts of entropy at late times.

Good question. So far the only compelling definitions of gravitational entropy have been in stationary space-times (those that admit a time-like Killing vector). There are various suggestions for how to define entropy in other situations, most notably Penrose's Weyl curvature hypothesis, but nothing concrete has yet emerged.

Is there not a definition of gravitational entropy from the holographic principle?

Boylan-Kolchin et al identify a new problem with CDM at sub-galactic scales: the Aquarius simulated MW galaxy halos have subhalos that are about 5 times more massive than the actual dwarf satellites we see. Are we underestimating the MW satellites' masses somehow? Or is their something wrong with the simulations? Or both? Anyway, as Phil B said: add it to the list of things to investigate about CDM :-)

The observational counterpart? http://arxiv.org/abs/1111.2611

Interesting: Strigari & Wechsler prefer to state the problem as the sims predicting 25-75 times as many subhalos at the Fornax mass scale as are observed in the MW system - and in the paper you posted they look at thousands of MW analogs in the SDSS survey and find that the MW is not atypical. This strengthens MBK's conclusion, that there is a problem with CDM - although note that S&W put the emphasis on galaxy formation not being well understood at this mass scale. They imagine that there really are all those dark Fornaxes out there! Pretty cool - now, if we could just see them somehow...