Oxford astro-ph Coffee

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!

K.A. Meissner, P. Nurowski, B. Ruszczycki

Tim Clifton's epic review

Not read it yet though ;-)

The authors claim a detection of a missing dark satellite in a galaxy-galaxy strong lens system B1938+666. The claimed perturber is located at the brightest region in the lensed image.

A good overview of the current status of weak lensing shape measurement software.

Intended for the next lensing lunch by the way.

The current upper limit on the tensor-to-scalar ratio r is ~0.2, and should improve by an order of magnitude with future experiments. These authors claim that measuring the curl component of the lensing power spectrum of 21cm emission could yield incredible constraints r~10^(-9) (though this headline figure corresponds to their most optimistic case). The angular resolution of our planned 21cm telescopes is the crucial quantity in determining these constraints.

This is a very thought-provoking paper. At z=50 - the redshifted wavelength of HI is 21 cm * 51 which is 21m [about the length of a cricket wicket]. To image fluctuations you would need to space antennae by about half a wavelength, or by ~10 m. So far, so good, LOFAR is trying this already, filling many cricket pitches worth of land with antennae in Northern Holland. The UK even has its own little version at Chilbolton near Winchester. So far, so good. However, to get to r~10^-9 you need (apparently) to get to l_ max ~ 10^7, or an angular resolution of about 0.01 arcsec (Better than Hubble Space Telescope resolution). This means that, according to lambda/D, the total size of your 21-cm instrument has to have a diameter of ~ [21 m] / (10^-7), or ~10-times larger than the Earth. Of course, the atmosphere is getting close to opaque at these wavelengths, and the radio frequency interference is so bad that you'd want to put such an instrument on the back side of the moon. Unfortunately, the moon isn't large enough either, so you'd have to launch (or remotely deploy) something ~10-times the size of the Earth into deep space. This might be quite expensive, but in the SKA project we have most of the machinery to simulate such an instrument if any of you theorists out there are interested in collaboration.

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?

"We extrapolate a dark matter (DM) density in the solar neighborhood of 0+-1 mM_sun pc^-3, and all the current models of a spherical DM halo are excluded at a confidence level higher than 4sigma."

An explanation of the anomalously low measurement of the local dark matter density that we discussed a few weeks ago?

"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!

One of the interesting things about this paper, is that they have a modular code, StringFast which they release here: http://www.astro.ubc.ca/people/scott/stringfast.html

Nice to see our (Morganson et al, ref 17) lensing limit on the string tension (from not finding any sets of split pairs of faint blue galaxy images in the HST archive) is still competitive - its equal to the CMB limit they take as their best case scenario :-) Mind you, we could only rule out long straight strings.We could probably put some limit on their wiggliness parametr too I suppose - but its nice that the CMB power doesn't care about wiggliness or anything.

A recent-historical analysis of cosmological parameter estimation. "Of the 28 measurements of Omega_Lambda in our sample published since 2003, only 2 are more than 1 sigma from the WMAP results. Wider use of blind analyses in cosmology could help to avoid this."

Their detection of confirmation bias (aka unconscious experimenter bias, or groupthink) may not be so significant: most if not all of those Omega_Lambda measurements will have used WMAP CMB priors. Next step would be to try and correct for that. Their warning for future analyses is spot on though: parameter estimation needs to be done blind.

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...

Rather than being split up into two components of different aged stellar populations with distinct scale heights, a thin and thick disc, the Milky Way seems to have "a continuous and monotonic distribution of disk thicknesses".

That's not what Lucio Meyer said!

Discovery of a very distant group of galaxies believed to be living in a M > 10^14 solar mass halo.

This is a dramatic illustration of the fact that any amount of observational data, necessarily restricted to the past lightcone and necessarily with non-zero errors, cannot predict anything mathematically about the future even one hour hence without further assumptions. It is also a display of the difference between mathematics and physics: the physicist necessarily employs intuition about the real world.

I'm confused by the final statement that we can't predict anything without further assumptions. It's a function of the complexity of the model, surely. If we have a simple model so that all the relevant constants are fixed by the observations, then they uniquely predict the future. However, if your model is complicated and has parameters unconstrained by experiment, then you can choose them to give "ripping" cosmologies within the hour. This is why we like to choose models that make sense (which is our intuition, as they say, for example to not have silly things like phantom fields, and why we choose to work within rigorous frameworks and seek to embed or motivate models within them), and also why we do model comparison. It may be possible to have some rip within the hour, but we can quantify how unlikely that is given the data, or how unlikely it is within the landscape. The statement they give is very deep sounding, but I think it has very little content.

A new method to determine the luminosity-distance/redshift relation from gravitational waves, without the need to find an EM counterpart? I would love someone with more NS knowledge to explain the details!

There is a discrepancy between cluster Y_SZ measurements made by Planck and AMI. Yikes!