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.
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 detected 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. They 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.
“Cosmic voids are an increasingly favored cosmic probe and have now already been successfully used to extract cosmological information,” according to the new study.
Though the findings of the DES collaboration confirm the standard model of cosmology on the whole, Jeffrey and his colleagues 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, compared to the clumpier version predicted by models.
“It may seem a relatively small thing, but if these hints are true then it may mean there’s something wrong with Einstein’s theory of general relativity, one of the great pillars of physics,” Jeffrey told the Guardian.
However, 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.