More from the Dark Energy Survey

To much media interest the Dark Energy Survey team yesterday released 11 new papers based on the analysis of their 3-year data. You can find the papers together with short descriptions here. There’s even a little video about the Dark Energy Survey here:

The official press release summarizes the results as follows:

Scientists measured that the way matter is distributed throughout the universe is consistent with predictions in the standard cosmological model, the best current model of the universe.

This contrasts a bit with the BBC’s version:

The results are a surprise because they show that it is slightly smoother and more spread out than the current best theories predict.

The observation appears to stray from Einstein’s theory of general relativity – posing a conundrum for researchers.

The reason for this appears to be that the BBC story focusses on the weak lensing paper (found here; I’ll add a link to the arXiv version if and when it appears there). The abstract is here:

The parameter S₈ is a (slightly) rescaled version of the more familiar parameter σ₈  – which quantifies the matter-density fluctuations on a scale of 8 h^-1 Mpc – as defined in the abstract; cosmic shear is particularly sensitive to this parameter.

The key figure showing the alleged “tension” with Planck is here:

The companion paper referred to in the above abstract (found here has an abstract that concludes with the words (my emphasis).

We find a 2.3σ difference between our S₈ result and that of Planck (2018), indicating no statistically significant tension, and additionally find our results to be in qualitative agreement with current weak lensing surveys (KiDS-1000 and HSC).

So, although certain people have decided to hype up a statistically insignificant l discrepancy, everything basically fits the standard model…

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New Publication at the Open Journal of Astrophysics

Time to announce another publication in the Open Journal of Astrophysics. This one was published yesterday, actually, but I didn’t get time to post about it until just now. It is the third paper in Volume 4 (2021) and the 34th paper in all.

The latest publication is entitled Dwarfs from the Dark (Energy Survey): a machine learning approach to classify dwarf galaxies from multi-band images and is written by Oliver Müller  of the Observatoire Astronomique de Strasbourg (France) and Eva Schnider of the University of Basel (Switzerland).

Here is a screen grab of the overlay which includes the abstract:

 

You can click on the image to make it larger should you wish to do so. You can find the arXiv version of the paper here. This one is in the Instrumentation and Methods for Astrophysics Folder, though it does overlap with Astrophysics of Galaxies too.

It seems the authors were very happy with the publication process!

I am very happy with the experience of publishing with the open-access journal @OJ_Astro. Everything went smoothly, the editor (@telescoper) was very quick to respond to questions, no article fees, and a useful referee report. Would recommend 10/10. https://t.co/B3I6XCKUAO

— Oliver Müller (@VoltarCH) March 24, 2021

Incidentally, the Scholastica platform we are using for the Open Journal of Astrophysics is continuing to develop additional facilities. The most recent one is that the Open Journal of Astrophysics now has the facility to include supplementary files (e.g. code or data sets) along with the papers we publish. If any existing authors (i.e. of papers we have already published) would like us to add supplementary files retrospectively then please contact us with a request!

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Eight Papers from the Dark Energy Survey

Just a quick post to point out the exciting news that this week a clutch of papers on cosmology using Type 1a Supernovae have been released by the Dark Energy Survey team. Naturally, all of them are on the arXiv. You can also read them here. For convenience I’ve provided links below to arXiv versions through their titles:

1. Steve: A hierarchical Bayesian model for Supernova Cosmology
2. First Cosmology Results Using Type Ia Supernovae from the Dark Energy Survey: Effects of Chromatic Corrections to Supernova Photometry on Measurements of Cosmological Parameters
3. First Cosmology Results using Type Ia Supernova from the Dark Energy Survey: Simulations to Correct Supernova Distance Biases
4. First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Photometric Pipeline and Light Curve Data Release
5. First Cosmology Results Using Type Ia Supernovae From the Dark Energy Survey: Analysis, Systematic Uncertainties, and Validation
6. First Cosmological Results using Type Ia Supernovae from the Dark Energy Survey: Measurement of the Hubble Constant
7. Cosmological Constraints from Multiple Probes in the Dark Energy Survey
8. First Cosmology Results using Type Ia Supernovae from the Dark Energy Survey: Constraints on Cosmological Parameters

Here’s a plot showing some of the cosmological constraints:

The parameter plotted on the vertical axis is the dark energy equation of state parameter, w, and w=-1 corresponds to a cosmological constant.

For those of youparticularly interested in the Hubble constant, the headline value from Paper 6 is H₀ = 67.77 +/- 1.30 km s^-1 Mpc^-1. This closer to the value obtained from Planck and in tension with other values as I’ve blogged about before, and gives me an excuse to continue my online poll:

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Mapping the Universe

Following yesterday’s post, here’s a nice visualisation of how much (and indeed how little) of the Universe the latest galaxy surveys have mapped.

In this animation the Earth is at the centre, and the dots represent observed galaxies, with distances are estimated using redshifts Every blue dot in the animation is a galaxy measured by the Dark Energy Survey. Gold dots are galaxies in the DES supernova fields (measured by OzDES) and red dots are from the Sloan Digital Sky Survey. The dark space in between the surveys is yet to be mapped….

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Cosmological Results from the Dark Energy Survey

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= σ₈ (Ω_m/0.3)^0.5 against the cosmological density parameter, Ω_m.

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!

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Dark Matter from the Dark Energy Survey

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.

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…

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Why Cosmology Isn’t Boring

As promised yesterday, here’s a copy of the slides I used for my talk to the ~150 participants of the collaboration meeting of the Dark Energy Survey that’s going on here this week at Sussex. The title is a reaction to a statement I heard that recent developments in cosmology, especially from Planck, have established that we live in a “Maximally Boring Universe”. I the talk I tried to explain why I don’t think the standard cosmology is at all boring. In fact, I think it’s only now that we can start to ask the really interesting questions.

At various points along the way I stopped to sample opinions…

I did however notice that Josh Frieman (front left) seemed to vote in favour of all the possible options on all the questions.  I think that’s taking the multiverse idea a bit too far..

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Cosmology, to be precise…

After an extremely busy morning I had the pleasant task this afternoon of talking to the participants of a collaboration meeting of the Dark Energy Survey that’s going on here at Sussex. Now there’s the even more pleasant task in front of me of having drinks and dinner with the crowd. At some point I’ll post the slides of my talk on here, but for the mean time here’s a pretty accurate summary..

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Cosmology Webcasts Coming Up…

Courtesy of freelance science writer Bruce Lieberman, whom I met briefly at the recent “Origin of the Expansion of the Universe” meeting in Flagstaff, AZ,  here’s a plug for two live webcasts on topical topics that are coming up in the next couple of weeks. On behalf of the Kavli Foundation, Bruce will be interviewing astronomers about the new Hubble XDF image (Oct. 4) and the new Dark Energy Survey camera, which just saw First Light (Oct. 11).

Live Q&A and Webcast: What Does Hubble’s Deepest Image of the Universe Reveal?

Click on the above heading for  direct link to webcast.

October 4, 11-11:30 am PDT (18-18:30 GMT; 19-19:30 BST)

Using data from the Hubble Space Telescope, a multi-national team of astronomers recently released our deepest-ever image of the
universe. Pascal Oesch, a Hubble Fellow at the University of California at Santa Cruz, and Michele Trenti, a researcher at the Kavli Institute for Cosmology, Cambridge at the University of Cambridge in the U.K., answer your questions about how the image was created and what it reveals about the early universe.

Viewers may submit questions to the two Hubble researchers via Twitter using #KavliAstro or email to info@kavlifoundation.org.

Live Q&A and Webcast: Can a New Camera Unravel the Nature of Dark Energy?

Click on the above heading for  direct link to webcast.

October 11, 9-9:30 am PDT (16-16:30 GMT; 17-17:30 BST)

Scientists have great expectations for the newly operational Dark Energy Camera, which may significantly advance our understanding of the mysterious force expanding the universe at an ever accelerating rate. Fermilab scientists Brenna Flaugher, project manager for the Dark Energy Camera, and Joshua Frieman, director of the Dark Energy Survey, answer your questions about the camera and what it’s expected to reveal.

Viewers may submit questions via Twitter using #KavliAstro or email to info@kavlifoundation.org.

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