Group items tagged

The statistical anisotropy of the mean of the CMB temperature fluctuations is tested. The naive inflationary prediction is that the mean a_lm's are zero, but the authors find a deviation from this expectation for l=221 - 240.

Interpolation schemes for modelling the PSF, including some fun stuff like Gaussian Processes.

Now we need to get our paper out ASAP and start doing comparisons! :-) Re: "optimal" interpolation: I think they do a nice job of showing (and agreeing with us ;-) that you do best by having a very flexible, multi-scale model for the underlying maps. We also incorporate information about the statistics of the atmospheric PSF pattern in our prior; Kriging is more local, in that the covariance function is estimated only from the data in hand. Which makes me wonder what happens when the data sparseness goes up...

"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."

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."

They claim a ~100x speed increase when using GPUs over a single processor. Execution time on the GPUs is comparable with a 128-processor MPI implementation.

A sampler, new in both algorithm and code, from Hogg and co. They claim speed ups over Metropolis Hastings and easy shared-memory parallelism using an ensemble method.

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.

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?

This work claims a 4-sigma detection of variation in the fine-structure constant over the sky and, furthermore, if a dipole form is assumed, a preference (also at 4-sigma) for evolution over cosmic time.

A long paper on measures in Cosmology, which I haven't read in its entireity yet. However, I found the final comment in the abstract quite provocative and interesting: "In a universe where the second law of thermodynamics holds, one cannot make use of our knowledge of the present state of the universe to "retrodict" the likelihood of past conditions." This is due to laws being time symmetric, while in practice we have the second law. In practice we *must* resort to Occam's razor and/or beauty arguments. Later: "if one wishes to make an argument in favor of inflation having occurred in the early universe, this argument must be based upon its being a simple and/or elegant hypothesis that accounts for observed phenomena. Any argument about the "likelihood" of inflation based upon the Liouville (or other) measure on phase space will require a justification for the use of this measure." Sometimes, beauty and Occam's razor are incompatible, or rather scale dependent (as in the case of the string landscape), and from a purely philosophical point of view I don't feel entriely comfortable with either being used as a criterion of truth when trying to discover things about the universe. However, this paper makes this seem inevitable. Certainly food for thought.

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.

Astrophysics > Cosmology and Extragalactic Astrophysics Title: Dipoles in the Sky (Submitted on 29 May 2012) Abstract: We perform observational tests of statistical isotropy using data from large-scale structure surveys spanning a wide range of wavelengths.