Our universe is woven together by a massive network of threads, made of a mysterious substance called dark matter, that creates a largely hidden superstructure known as the cosmic web.
Now, scientists have produced the largest-ever maps of these gargantuan filaments, covering about an eighth of the entire night sky from Earth and encompassing more than 200 million galaxies that were observed over 758 nights at NOIRLab's Cerro Tololo Inter-American Observatory in Chile.
These unprecedented views are the latest effort by the Dark Energy Survey (DES) collaboration, an international team made up of hundreds of scientists, which published its new findings on Thursday in a mega-batch of 30 papers.
Among the many revelations from this data dump is a huge map exposing thousands of empty voids in the cosmic web that may challenge longstanding cosmological models, including aspects of Einstein’s theory of general relativity.
The map charts out the distribution of dark matter, an unidentified material that makes up more than a quarter of the universe, in never-before-seen detail, making it “rich in information about the interaction between galaxies, clusters, and the cosmic web,” according to a DES study co-led by Niall Jeffrey, a cosmologist at École normale supérieure in France, which appeared last week in the Monthly Notices of the Royal Astronomical Society.
“To see dark matter unveiled across such a large part of the sky is exhilarating,” Jeffrey said in an email. “When we look at some of these patches in our map, we are seeing structures in the Universe that no one else has had the chance to see before. I also feel really lucky that we can share what we see with everyone in the world.”
Jeffrey’s team generated the map, which is the largest of its kind to date, using a trove of real observations that were then analyzed by sophisticated algorithms that build on, and collate, many other similar techniques.
“These results highlight expected differences in the maps constructed using the different algorithms and illustrate the advantages or disadvantages of their use in different science cases,” Jeffrey and his colleagues said in the study. “We present a comprehensive framework under which most of the convergence map-making methods described previously can be connected and compared.”
The researchers were able to create this framework thanks to a trippy method of cosmic cartography known as weak gravitational lensing. Large objects in space, including clumps or filaments of dark matter, produce gravitational fields that can distort the light emitted by galaxies, or other radiant phenomena, situated behind them from our perspective on Earth.
This lensing effect provides a means to map out the distribution of dark matter on huge scales, which would be otherwise challenging given that this unidentified material does not produce detectable light.
“Weak gravitational lensing is one of the primary cosmological probes of recent galaxy surveys,” Jeffrey and his colleagues said in the study. “By measuring the subtle distortions of galaxy shapes due to the mass distribution between the observed galaxies and us the observers, we are able to place tight constraints on the cosmological model describing the Universe,” especially “the content of matter in the Universe” and “ the level at which matter clusters,” they added.
To that point, the team observed 3,222 voids of empty space between clusters and threads in its new DES maps. These large gaps in the web can stretch across hundreds of millions of light years, yet contain only a few galaxies, or sometimes none at all. While DES surveys have identified these voids before, Jeffrey and his colleagues have characterized them in further detail.
“What we have done is seen the ‘edges’ of these voids in our dark matter map, not just their edges using the visible distribution of galaxies,” Jeffrey said.
The voids are interesting to scientists for a host of reasons, including as laboratories to test out Einstein’s theory of general relativity. For instance, some observations of voids suggest that gravity appears to operate in a slightly different way within these spaces, compared to predictions in the standard model of cosmology, which is based in part on general relativity.
“Many people are interested in learning more about cosmic voids as possible places in the Universe where gravity might act differently,” Jeffrey explained. “If we want to change Einstein’s theory of General Relativity, then we need to account for the fact that ‘nearby’ in space, General Relativity seems to work really well. Some people therefore suggest that it is inside these cosmic voids (with very low-density) where gravity could change.”
Though the findings of the DES collaboration confirm the standard model of cosmology on the whole, the researchers found that the voids they studied were larger than those mapped out in previous surveys. The findings hint that the cosmic web may be slightly smoother in its distribution, according to observations, than the clumpier version predicted by models.
“On it’s own this might not be interesting, but there are similar hints from other gravitational lensing experiments (for example, one called KiDS) that make some people start to get excited!” Jeffrey said, referring to the potentially smoother structures.
“It could be that there is something all these experiments are doing wrong, for example misunderstanding the shapes of galaxies (if we imagine we turned off lensing), or it could be early hints that something is wrong with cosmology!” he added. “The best way to find out is to continue to map out the Universe.”
Indeed, this is still just a whiff of possible discord between theory and observation, and confirming any tensions between them will require more surveys, models, and research in general.
Fortunately, the DES collaboration is already at work analyzing its newest collection of observations, which will provide an even more comprehensive look at the expansion of the universe and the cosmic web that connects it.
“Soon, in the next couple of years, cosmology is going to get a real boost,” Jeffrey concluded. “There will be the European Space Agency ‘Euclid’ mission that will photograph galaxies from space, and also the new Vera Rubin Observatory. With both of these we can make even more exquisite maps of dark matter.”
Update: This article has been updated with comments from Niall Jeffrey, a cosmologist at École normale supérieure in France who co-led one of the new DES studies