From nice friendly volcanoes — by which I mean deadly liquid mounds of moving rock — we now jump back out to the farther corners of the universe and look at the vast number of the galaxies between here and there. The Dark Energy Survey has released 29 new scientific papers from their collaboration of 400 scientists spread across 25 institutions in seven countries. This mound of papers looks at the distribution of 226 million galaxies, their morphologies, and the structures they combine to form throughout a seven billion light-year deep cone of the universe.

To be honest, with this many papers by this many authors, I’m not sure there is a galaxy-focused researcher who isn’t either on one of these papers or reviewed one of these papers.

Over the seven billion years observed in this survey, the distribution of galaxies became more and more clustered, with the degree of clustering reflecting the amount of dark matter present. By combining this galaxy data with Planck’s data on the clumpiness of material emitting the cosmic microwave background, they are able to see how our universe’s structure has evolved. Comparison of data with models indicates our standard model of how the universe is evolving does a pretty good job reflecting reality, but reality is, on average, a little less clumpy than predicted.

Why? No idea. These publications only look at one-sixteenth of the entire sky. The Dark Energy Survey has twice again as much data that they are still processing, and it will be interesting to see if this slight lack of clumpiness is seen in the additional data.

Part of what makes me so curious about what’s in that additional data is the discovery of a massive arch of galaxies spotted by graduate student Alexia Lopez. There is more than one way to spot a galaxy; while it’s great to see them in their round, spiral, or lumpy beauty, sometimes they are just too faint to really see. To find those half-hidden systems, Lopez looked at the light of extremely distant quasars to see if intervening galaxies absorbed some of the light. 

Galaxies commonly have clouds of magnesium, and that gas can get excited and absorb light, creating precisely spaced dark bands saying, “There is magnesium-II here.” The color at which that pattern of dark lines appears identifies the distance to these otherwise invisible galaxies. In this way, one set of dark lines at a time in quasar after quasar, Lopez mapped galaxies, and she found a super weird arc that is huge and not supposed to be there according to the Cosmological Principle.

This Arc is one-fifteenth the radius of the observable universe. It is the single largest structure known, and it tells us that our ideas for how big an average chunk of the universe must be just got bigger.

Putting these two stories together: the Dark Energy Survey found less structure and Lopez and her team found too much. This is consistent with the lumpiness not being what we thought and means we need so much more data to see if there are more arcs and more non-lumpy masses waiting to redefine the average scale size of the universe.

More Information

NOIRLab press release

Fermilab press release

“Dark Energy Survey Year 3 Results: Photometric Data Set for Cosmology,” I. Sevilla-Noarbe et al., 2021 May 17, The Astrophysical Journal Supplement

UCLAN press release