The Dark Energy Survey (DES) first year results, and a series of papers, were released on August 4, 2017. This is a massive international collaboration with over 60 institutions represented and 200 authors on the paper summarizing initial results. Over 5 years the Dark Energy Survey team plans to survey some 300 million galaxies.

The instrument is the 570-megapixel Dark Energy Camera installed on the Cerro Tololo Inter-American Observatory 4-meter Blanco Telescope.

*Image: DECam imager with CCDs (blue) in place. Credit: darkenergysurvey.org*

Over 26 million source galaxy measurements from far, far away are included in these initial results. Typical distances are several billion light-years, up to 9 billion light-years. Also included is a sample of 650,000 luminous red galaxies, lenses for the gravitational lensing, and typically these are foreground elliptical galaxies. These are at redshifts < 0.9 corresponding to up to 7 billion light-years.

They use 3 main methods to make cosmological measurements with the sample:

1. The correlations of galaxy positions (galaxy-galaxy clustering)

2. The gravitational lensing of the large sample of background galaxies by the smaller foreground population (cosmic shear)

3. The gravitational lensing of the luminous red galaxies (galaxy-galaxy lensing)

Combining these three methods provides greater interpretive power, and is very effective in eliminating nuisance parameters and systematic errors. The signals being teased out from the large samples are at only the one to ten parts in a thousand level.

They determine 7 cosmological parameters including the overall mass density (including dark matter), the baryon mass density, the neutrino mass density, the Hubble constant, and the equation of state parameter for dark energy. They also determine the spectral index and characteristic amplitude of density fluctuations.

Their results indicate Ωm of 0.28 to a few percent, indicating that the universe is 28% dark matter and 72% dark energy. They find a dark energy equation of state w = – 0.80 but with error bars such that the result is consistent with either a cosmological constant interpretation of w = -1 or a somewhat softer equation of state.

They compare the DES results with those from the Planck satellite for the cosmic microwave background and find they are statistically significant with each other and with the Λ-Cold Dark MatterΛ model (Λ, or Lambda, stands for the cosmological constant). They also compare to other galaxy correlation measurements known as BAO for Baryon Acoustic Oscillations (very large scale galaxy structure reflecting the characteristic scale of sound waves in the pre-cosmic microwave background plasma) and to Type 1a supernovae data.

This broad agreement with Planck results is a significant finding since the cosmic microwave background is at very early times, redshift z = 1100 and their galaxy sample is at more recent times, after the first five billion years had elapsed, with z < 1.4 and more typically when the universe was roughly ten billion years old.

Upon combining with Planck, BAO, and the supernovae data the best fit is Ωm of 0.30 with an error of less than 0.01, the most precise determination to date. Of this, about 0.25 is ascribed to dark matter and 0.05 to ordinary matter (baryons). And the implied dark energy fraction is 0.70.

Furthermore, the combined result for the equation of state parameter is precisely w = -1.00 with only one percent uncertainty.

The figure below is Figure 9 from the DES paper. The figure indicates, in the leftmost column the measures and error bars for the amplitude of primordial density fluctuations, in the center column the fraction of mass-energy density in matter, and in the right column the equation of state parameter w.

The DES year one results for all 3 methods are shown in the first row. The Planck plus BAO plus supernovae combined results are shown in the last row. And the middle row, the fifth row, shows all of the experiments combined, statistically. Note the values of 0.3 and – 1.0 for Ωm and w, respectively, and the extremely small error bars associated with these.

This represents continued strong support for the canonical Λ-Cold Dark Matter cosmology, with unvarying dark energy described by a cosmological constant.

They did not evaluate modifications to general relativity such as Emergent Gravity or MOND with respect to their data, but suggest they will evaluate such a possibility in the future.

**References**

https://arxiv.org/abs/1708.01530, “Dark Energy Survey Year 1 Results: Cosmological Constraints from Galaxy Clustering and Weak Lensing”, 2017, T. Abbott et al.

https://en.wikipedia.org/wiki/Weak_gravitational_lensing, Wikipedia article on weak gravitational lensing discusses galaxy-galaxy lensing and cosmic shear

October 25th, 2017 at 2:17 pm

Hi Stephen

I remember when the following data set was published: Reiss, A.G. et al. (2016) A 2.4% Determination of the Local Value of the Hubble Constant. The Astrophysical Journal. arXiv: 1604:01424

At the time there was some head-scratching in the cosmological community because the revised 1a supernova data and the Planck data did not line up, suggesting that the cosmological constant was not so constant after all. The data coming from Planck suggested a smaller cosmological constant than that of the more recent universe, as if a pro gravity force were acting in the early universe, then letting go.

Your above post states that there is “continued strong support” for an unvarying DE constant, yet table 9 suggests that the Plank data is still very much in disagreement with the other data sets.

Can you enlighten me more on this issue?

Regards

Gavin Rowland

October 25th, 2017 at 7:59 pm

So the Type 1a supernovae in our neighborhood are suggesting around 73 km/s/Mpc as H0 and the CMB Planck data around 67. And then

there is this gravitational lens technique which gives results closer to the supernovae. https://phys.org/news/2017-01-cosmic-lenses-universe-expansion.html

Just considering supernovae and Planck results I don’t see a big problem. We live in an overdense region, in the Laniakea supercluster which is 250 million light years away (5400 km/s of Hubble flow or z = .02). In our overdense portion we expect higher values of the Hubble constant, whereas Planck is averaging over the entire nearly homogeneous universe. Also there are a number of astrophysical calibrations in the supernova data so in some sense it is not as ‘clean’, although Planck has lots of calibration issues as well. I did see a paper, cannot recall, along these lines, saying there is no conflict if one considers the local overdensity issue.

The gravitational lensing of quasars makes things more interesting, I do not know how far away those are.

I doubt that the variation seen in Hubble constant from different methods is due to variations in the strength of dark energy with redshift. That is also being tested with distant galaxy BAO data with the Dark Energy Survey, as well as with Planck.

November 6th, 2017 at 11:25 am

Thanks for that Stephen. I guess we will have to see as more data narrows the constraints on models. I was quite excited when I heard that the cosmological constant was difficult to align with the Planck data because I had predicted that there should be a spatially contracting form of dark energy as well, separating from the expanding type via an inflationary force at the Big Bang. It’s something I have argued through conservation of energy considerations. I got the chance to ask Anthony Aguirre whether he knew of any reason why this couldn’t be the case & he didn’t think so.