Important Dark Matter Survey Results

The Dark Energy Survey is one of the coolest science projects ever. 

Goal:  measure everything there is, the whole universe.

Reason:  we know essentially nothing about Dark Energy and Dark Matter—which is 97% of the universe.

How can we not do this exploration?

This summer the DES folks report an important new large scale map of the sky (i.e., everything we can see from here [2]. The map represents evidence of gravitational lensing to infer the mass of dark matter between us and the source. 

I don’t really understand the details.  The data is complex and noisy, and the inference of interest involves some complicated geometry that I haven’t really mastered, to say the least.  Much of the paper is about comparing the strengths and weaknesses of alternative methods of estimating these effects.

But I do understand this:

As so often happens in astronomy and science in general, when we actually observe and measure nature it doesn’t quite fit our theories.  (This is particularly frequent in astronomy, which has a very low data to theory ratio.)

In this case, the distribution of Dark Matter does not seem to match theoretical estimates [1].  If these discrepancies hold up, this would have to mean that the theory is wrong. 

This is both exciting and scary.  It’s also what science is about, no?

Now, there is still plenty of room for doubt about these results.  For one thing, the alternative methods of estimation give different results.  And both the theory and the data analysis depend on very large scale computations, which are always vulnerable to error and mathematical biases.  A difference of one percent could certainly be partly due to computational and/or data errors that have accumulated.

But, considering how little we understand about Dark Matter, there is certainly room to wonder if there is something missing from the theoretical models.  And if so, it would be really, really important and cool.

Well done, all.  And let’s try to get more and better maps to try to pin this down better.

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1. Pallab Ghosh, New dark matter map reveals cosmic mystery, in BBC News – Science & Environment, May 27, 2021. https://www.bbc.com/news/science-environment-57244708
2. N. Jeffrey, M. Gatti, C. Chang, L. Whiteway, U. Demirbozan, A. Kovacs, G. Pollina, D. Bacon, N. Hamaus, T. Kacprzak, O. Lahav, F. Lanusse, B. Mawdsley, S. Nadathur, J. L. Starck, P. Vielzeuf, D. Zeurcher, A. Alarcon, A. Amon, K. Bechtol, G. M. Bernstein, A. Campos, A. Carnero Rosell, M. Carrasco Kind, R. Cawthon, R. Chen, A. Choi, J. Cordero, C. Davis, J. DeRose, C. Doux, A. Drlica-Wagner, K. Eckert, F. Elsner, J. Elvin-Poole, S. Everett, A. Ferté, G. Giannini, D. Gruen, R. A. Gruendl, I. Harrison, W. G. Hartley, K. Herner, E. M. Huff, D. Huterer, N. Kuropatkin, M. Jarvis, P. F. Leget, N. MacCrann, J. McCullough, J. Muir, J. Myles, A. Navarro-Alsina, S. Pandey, J. Prat, M. Raveri, R. P. Rollins, A. J. Ross, E. S. Rykoff, C. Sánchez, L. F. Secco, I. Sevilla-Noarbe, E. Sheldon, T. Shin, M. A. Troxel, I. Tutusaus, T. N. Varga, B. Yanny, B. Yin, Y. Zhang, J. Zuntz, T. M. C. Abbott, M. Aguena, S. Allam, F. Andrade-Oliveira, M. R. Becker, E. Bertin, S. Bhargava, D. Brooks, D. L. Burke, J. Carretero, F. J. Castander, C. Conselice, M. Costanzi, M. Crocce, L. N. da Costa, M. E. S. Pereira, J. De Vicente, S. Desai, H. T. Diehl, J. P. Dietrich, P. Doel, I. Ferrero, B. Flaugher, P. Fosalba, J. García-Bellido, E. Gaztanaga, D. W. Gerdes, T. Giannantonio, J. Gschwend, G. Gutierrez, S. R. Hinton, D. L. Hollowood, B. Hoyle, B. Jain, D. J. James, M. Lima, M. A. G. Maia, M. March, J. L. Marshall, P. Melchior, F. Menanteau, R. Miquel, J. J. Mohr, R. Morgan, R. L. C. Ogando, A. Palmese, F. Paz-Chinchón, A. A. Plazas, M. Rodriguez-Monroy, A. Roodman, E. Sanchez, V. Scarpine, S. Serrano, M. Smith, M. Soares-Santos, E. Suchyta, G. Tarle, D. Thomas, C. To and J. Weller, Dark Energy Survey Year 3 results: curved-sky weak lensing mass map reconstruction. arXiv  2105.13539, 2021. https://arxiv.org/abs/2105.13539

Dark Energy Survey Data Available

If the fate of the Antarctic ice is the single most important question about our own planet, looking outward, the most important question surely must be “What is Dark Energy?”

For the past decade, the Dark Energy Survey has begun to measure fast swaths of the visible sky, with the goal to better understand DE.  The DES is an awesome project, and a world-wide collaboration: the paper that ‘splains the data dump has 200 authors listed.

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I’m particularly fond of this project not only because of the shear romantic appeal (we basically have no idea about the physics 95% of our universe), but also because the data is collected every night in Chile, and shot up the spine of the Americas to the National Center for Supercomputing Applications, my old institution. (I used to have an office just down the hall from the team who built that part of the data system.)

After the first three years of data collection, the DES has just dropped a huge public “Data Release 1”.  Come and get it!

I haven’t really looked at the data in any detail, though I can confirm that it is definitely open to the public.

I’ll note that this is yet another example of the challenges of “citizen science”. Anyone can have this data, and can do whatever they want with it.  Should we expect a flood of cool discoveries from the Internet “crowd”?  I wouldn’t bet on it.

The data is not pretty pictures, and doing science with it requires quite a bit of technical knowledge.  In fact, just understanding how the data was created requires a ton of background. The researchers have gone to a lot of work to create solid, useful data [1].

This just goes to show that real science (as opposed to Hollywood or Washington science) isn’t just looking at a screen and saying, “aha”.  Making data available is great, but it neither makes scientists redundant, nor necessarily generates more knowledge.

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1. T. M. C. Abbott, F. B. Abdalla, S. Allam, A. Amara, J. Annis, J. Asorey, S. Avila, O. Ballester, M. Banerji, W. Barkhouse, L. Baruah, M. Baumer, K. Bechtol, M . R. Becker, A. Benoit-Lévy, G. M. Bernstein, E. Bertin, J. Blazek, S. Bocquet, D. Brooks, D. Brout, E. Buckley-Geer, D. L. Burke, V. Busti, R. Campisano, L. Cardiel-Sas, A. C arnero Rosell, M. Carrasco Kind, J. Carretero, F. J. Castander, R. Cawthon, C. Chang, C. Conselice, G. Costa, M. Crocce, C. E. Cunha, C. B. D’Andrea, L. N. da Costa, R. Das, G. Daues, T. M. Davis, C. Davis, J. De Vicente, D. L. DePoy, J. DeRose, S. Desai, H. T. Diehl, J. P. Dietrich, S. Dodelson, P. Doel, A. Drlica-Wagner, T. F. Eifler, A. E. Elliott, A. E. Evrard, A. Farahi, A. Fausti Neto, E. Fernandez, D. A. Finley, M. Fitzpatrick, B. Flaugher, R. J. Foley, P. Fosalba, D. N. Friedel, J. Frieman, J. García-Bellido, E. Gaz tanaga, D. W. Gerdes, T. Giannantonio, M. S. S. Gill, K. Glazebrook, D. A. Goldstein, M. Gower, D. Gruen, R. A. Gruendl, J. Gschwend, R. R. Gupta, G. Gutierrez, S. Hamilton, W. G. Hartley, S. R. Hinton, J. M. Hislop, D. Hollowood, K. Honscheid, B. Hoyle, D. Huterer, B. Jain, D. J. James, T. Jeltema, M. W. G. Johnson, M. D. Johnson, S. Juneau, T. Kacpr zak, S. Kent, G. Khullar, M. Klein, A. Kovacs, A. M. G. Koziol, E. Krause, A. Kremin, R. Kron, K. Kuehn, S. Kuhlmann, N. Kuropatkin, O. Lahav, J. Lasker, T. S. Li, R. T. Li, A. R. Liddle, M. Lima, H. Lin, P. López-Reyes, N. MacCrann, M. A. G. Maia, J. D. Maloney, M. Manera, M. March, J. Marriner, J. L. Marshall, P. Martini, T. McClintock, T. McKay, R . G. McMahon, P. Melchior, F. Menanteau, C. J. Miller, R. Miquel, J. J. Mohr, E. Morganson, J. Mould, E. Neilsen, R. C. Nichol, D. Nidever, R. Nikutta, F. Nogueira, B. Nord, P. Nugent, L. Nunes, R. L. C. Ogando, L. Old, K. Olsen, A. B. Pace, A. Palmese, F. Paz-Chinchón, H. V. Peiris, W. J. Percival, D. Petravick, A. A. Plazas, J. Poh, C. Pond, A. Por redon, A. Pujol, A. Refregier, K. Reil, P. M. Ricker, R. P. Rollins, A. K. Romer, A. Roodman, P. Rooney, A. J. Ross, E. S. Rykoff, M. Sako, E. Sanchez, M. L. Sanchez, B. Santiago, A. Saro, V. Scarpine, D. Scolnic, A. Scott, S. Serrano, I. Sevilla-Noarbe, E. Sheldon, N. Shipp, M.L. Silveira, R. C. Smith, J. A. Smith, M. Smith, M. Soares-Santos, F. Sobre ira, J. Song, A. Stebbins, E. Suchyta, M. Sullivan, M. E. C. Swanson, G. Tarle, J. Thaler, D. Thomas, R. C. Thomas, M. A. Troxel, D. L. Tucker, V. Vikram, A. K. Vivas, A. R. Wal ker, R. H. Wechsler, J. Weller, W. Wester, R. C. Wolf, H. Wu, B. Yanny, A. Zenteno, Y. Zhang and J. Zuntz, The Dark Energy Survey Data Release 1. The DES Collaboration, 2018. https://arxiv.org/abs/1801.03181

 

 

Dark Energy Survey Reports

The Universe we live in has a lot of matter and energy in it. We can see and measure matter and energy, but it is now clear that we can see only about 2% of what is there. We can tell that there is a lot of matter and energy out there that we simply cannot measure, it is called Dark Matter and Dark Energy. The universe is about 26% Dark Matter, and about 70% Dark Energy. What is all this Dark stuff? No one knows, but we sure need to find out, right?

Since 2013, a global collaboration of astronomers has been systematically surveying the sky to confirm the existence and assess the amount of Dark Energy. The Dark Energy Survey  is a heroic project, using a large telescope facility in the high desert of the Andes to spot and measure supernova. A massive amount of data is collected each night and stored in digital images.

The data is transferred through an optic fiber channel that runs up the spine of the America’s to Illinois, right down the street from where I sit. The data is organized into the archive, which is analyzed by science teams around the globe. It takes hours to transfer each night’s (irreplaceable) data to the data center, every day.

This summer, the DES is releasing a burst of ten papers to report the first year’s results [1]. (It has taken several years to analyze the first year’s data.  Unlike Hollywood movies, real life data analysis is hard work and takes time.)

The details of these analyses are largely beyond my own understanding, though I understand very well the scale of the computation and the system engineering this has required: this project is trying to measure the whole sky, and is looking for brief events that must be zoomed into. “Challenging” doesn’t begin to describe it.

Glancing through the papers, it is clear that this massive effort is yielding pretty solid results. To pick one paper arbitrarily, “Dark Energy Survey Year 1 Results: Cosmological Constraints from Cosmic Shear” discusses one important thrust of the research, attempting to document the actual expansion of the Universe, and to improve estimates for the infamous cosmological constant that represents the “anti gravity” effects of Dark Matter and Energy.

The report itself is attributed to 135 authors from 51 institutions, and is based on observations of 36 million galaxies. The bulk of the paper describes the (complex) methods used to assemble and interpret the observational data. The results are close to earlier estimates of cosmological parameters from much smaller datasets. Results from other studies in this batch combine with these to tighten the estimated bounds on these values.

It’s all overwhelming, but as the authors dryly note, we really have no understanding of these fundamental facts yet. These are deep and fundamental mysteries, and we really need to know. The DES is an important step in understanding our universe.

“Despite the overall success of modern cosmological study, however, there remain several fundamental mysteries that enter the model as purely phenomenological parameters. These include our lack of understanding of the value of the cosmological constant or of any motivation for a different driver of cosmic acceleration.” (p.2)

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1. The Dark Energy Survey. DES Year 1 Cosmology Results: Papers. 2017, https://www.darkenergysurvey.org/des-year-1-cosmology-results-papers/.
2. The Dark Energy Survey. Home – The Dark Energy Survey. 2017, https://www.darkenergysurvey.org/.
3. M. A. Troxel, N. MacCrann, J. Zuntz, T. F. Eifler, E. Krause, S. Dodelson, D. Gruen, J. Blazek, O. Friedrich, S. Samuroff, J. Prat, L. F. Secco, C. Davis, A. Ferté, J. DeRose, A. Alarcon, A. Amara, E. Baxter, M. R. Becker, G. M. Bernstein, S. L. Bridle, R. Cawthon, C. Chang, A. Choi, J. De Vicente, A. Drlica-Wagner, J. Elvin-Poole, J. Frieman, M. Gatti, W. G. Hartley, K. Honscheid, B. Hoyle, E. M. Huff, D. Huterer, B. Jain, M. Jarvis, T. Kacprzak, D. Kirk, N. Kokron, C. Krawiec, O. Lahav, A. R. Liddle, J. Peacock, M. M. Rau, A. Refregier, R. P. Rollins, E. Rozo, E. S. Rykoff, C. Sánchez, I. Sevilla-Noarbe, E. Sheldon, A. Stebbins, T. N. Varga, P. Vielzeuf, M. Wang, R. H. Wechsler, B. Yanny, T. M. C. Abbott, F. B. Abdalla, S. Allam, J. Annis, K. Bechtol, A. Benoit-Lévy, E. Bertin, D. Brooks, E. Buckley-Geer, D. L. Burke, A. Carnero Rosell, M. Carrasco Kind, J. Carretero, F. J. Castander, M. Crocce, C. E. Cunha, C. B. D’Andrea, L. N. da Costa, D. L. DePoy, S. Desai, H. T. Diehl, J. P. Dietrich, P. Doel, E. Fernandez, B. Flaugher, P. Fosalba, J. García-Bellido, E. Gaztanaga, D. W. Gerdes, T. Giannantonio, D. A. Goldstein, R. A. Gruendl, J. Gschwend, G. Gutierrez, D. J. James, T. Jeltema, M. W. G. Johnson, M. D. Johnson, S. Kent, K. Kuehn, S. Kuhlmann, N. Kuropatkin, T. S. Li, M. Lima, H. Lin, M. A. G. Maia, M. March, J. L. Marshall, P. Martini, P. Melchior, F. Menanteau, R. Miquel, J. J. Mohr, E. Neilsen, R. C. Nichol, B. Nord, D. Petravick, A. A. Plazas, A. K. Romer, A. Roodman, M. Sako, E. Sanchez, V. Scarpine, R. Schindler, M. Schubnell, M. Smith, R. C. Smith, M. Soares-Santos, F. Sobreira, E. Suchyta, M. E. C. Swanson, G. Tarle, D. Thomas, D. L. Tucker, V. Vikram, A. R. Walker, J. Weller, Y. Zhang and (DES Collaboration), Dark Energy Survey Year 1 Results: Cosmological Constraints from Cosmic Shear. The Dark Eneergy Survey, 2017. http://www.darkenergysurvey.org/wp-content/uploads/2017/08/y1a1_cosmic_shear-1.pdf