Archive for DES

Cosmological Results from the Dark Energy Survey

Posted in The Universe and Stuff with tags , , , , , on August 4, 2017 by telescoper

At last the Dark Energy Survey has produced its first cosmological results. The actual papers have not yet hit the arXiv but they have been announced at a meeting in the USA and are linked to from this page.

I’ll jump straight to this one, which shows the joint constraints on S8 which is related to σ8 (a measure of the level of fluctuations in the cosmological mass distribution) via S8= σ8m/0.3)0.5 against the cosmological density parameter, Ωm.

These constraints, derived using DES Y1 measurements of galaxy clustering, galaxy-galaxy lensing, and weak lensing cosmic shear are compared with those obtained from the cosmic microwave background using Planck data, and also combined with them to produce a joint constraint. Following usual practice, the contours are 68% and 95%  posterior probability regions.

The central values of DES and Planck values are different, but the discrepancy is only marginal. Compare this with a an equivalent diagram from a paper I discussed last year.

The KIDS analysis used to produce this plot uses only weak lensing tomography, so you can see that using additional measures reduces the viable region in this parameter space.

It’s great to see new data coming in, but at first sight it seems it is tending to confirm the predictions of the standard cosmological model, rather than providing evidence of departures from it.

Incidentally, this little video shows the extent to which the Dark Energy Survey is a global project, including some of my former colleagues at the University of Sussex!

 

Dark Matter from the Dark Energy Survey

Posted in The Universe and Stuff with tags , , , on April 14, 2015 by telescoper

I’m a bit late onto this story which has already been quite active in the media today, and has generated an associated flurry of activity on social media, but I thought it was still worth passing it on via the medium of this blog. The Dark Energy Survey has just released a number of papers onto the arXiv, the most interesting of which (to me) is entitled Wide-Field Lensing Mass Maps from DES Science Verification Data. The abstract reads as follows (the link was added by me):

Weak gravitational lensing allows one to reconstruct the spatial distribution of the projected mass density across the sky. These “mass maps” provide a powerful tool for studying cosmology as they probe both luminous and dark matter. In this paper, we present a weak lensing mass map reconstructed from shear measurements in a 139 deg^2 area from the Dark Energy Survey (DES) Science Verification (SV) data overlapping with the South Pole Telescope survey. We compare the distribution of mass with that of the foreground distribution of galaxies and clusters. The overdensities in the reconstructed map correlate well with the distribution of optically detected clusters. Cross-correlating the mass map with the foreground galaxies from the same DES SV data gives results consistent with mock catalogs that include the primary sources of statistical uncertainties in the galaxy, lensing, and photo-z catalogs. The statistical significance of the cross-correlation is at the 6.8 sigma level with 20 arcminute smoothing. A major goal of this study is to investigate systematic effects arising from a variety of sources, including PSF and photo-z uncertainties. We make maps derived from twenty variables that may characterize systematics and find the principal components. We find that the contribution of systematics to the lensing mass maps is generally within measurement uncertainties. We test and validate our results with mock catalogs from N-body simulations. In this work, we analyze less than 3% of the final area that will be mapped by the DES; the tools and analysis techniques developed in this paper can be applied to forthcoming larger datasets from the survey.

This is by no means a final result from the Dark Energy Survey, as it was basically put together in order to test the telescope, but it is interesting from the point of view that it represents a kind of proof of concept. Here is one of the key figures from the paper which shows a reconstruction of the mass distribution of the Universe (dominated by dark matter) obtained indirectly by the Dark Energy Survey using distortions of galaxy images produced by gravitational lensing by foreground objects, onto which the positions of large galaxy clusters seen in direct observations have been plotted. Although this is just a small part of the planned DES study (it covers only 0.4% of the sky) it does seem to indicate that the strong concentrations of dark matter (red) do corrrelate with the positions of concentrations of galaxy clusters.

DES_MAP

It all seems to work, so hopefully we can look forward to lots of interesting science results in future!

P.S. When I first saw the map it looked like a map of the North of England Midlands and I was surprised to see that the survey showed such strong support for the Greens…