The quest to unravel the true nature of the universe dates back millennia, but scientific advances in recent decades have helped revolutionize our understanding of this perplexing structure we inhabit. And, as recent discoveries have found, the universe truly is a structure with connections to be mapped.
These efforts have led to the establishment of a standard cosmological model (or ΛCDM model) that neatly accounts for a variety of cosmic phenomena from the expansion of the universe to the distribution of galaxies. But for all its elegance, the standard cosmological model can’t explain everything. For instance, the unexpected “coherent motion” of dwarf galaxies that orbit larger host galaxies, such as the Milky Way, remains “a persistent challenge” to the model, according to a study published on Friday in the journal Astronomy & Astrophysics.
“Our expectation from the cosmological model, from what we understand about how the whole cosmos evolved, is that satellite galaxies should be more or less randomly distributed on the hosts and should also move more or less randomly,” said Marcel Pawlowski, a Schwarzschild Fellow at the Leibniz-Institute for Astrophysics and author of the study, in a call.
It was therefore a “big surprise,” added Pawlowski, that satellite galaxies around Centaurus A, a galaxy located 13 million light years away, are kinematically synced into a disk-like alignment rather than the messy swam predicted by the standard cosmological model.
Pawlowski and his colleagues, including University of Strasbourg astronomer Oliver Müller, argued that this “observational evidence suggests that something is wrong with standard cosmological simulations” in a 2018 paper in Science.
The new study, led by Müller, reports expanded observations of Centaurus A captured by the specialized MUSE instrument at the Very Large Telescope in Chile. Previously, the team had reported that 14 out of 16 satellites of Centaurus A shared this unexpected disk alignment.
MUSE was able to expand that ratio to 21 out of 28 satellites, showing that the anomaly still showed up in larger populations of dwarf galaxies.
“Ultimately, I think now we have convinced people that in Centaurus A, at least, there is some coherent motion and there is this co-rotating plane of satellites,” said Müller in the same call, a discrepancy that the study deems “one of the most severe small-scale challenges” to the standard cosmological model.
This odd coherence, known as the plane-of-satellites problem, can also be observed in our own galaxy, the Milky Way, as well as the nearest galaxy, Andromeda. In simulations based on the assumptions of the standard cosmological model, coherences of this nature should only occur in 0.2 percent of galactic satellite systems, and yet observations keep suggesting that they are apparently commonplace.
The reason for this discrepancy is unclear, but it might be related to the “cosmic web,” an enormous structure made of threads of dark matter that links galaxies across the universe. The orbits of satellite galaxies are thought to be guided by these filaments, which may help explain some of the enigmatic behavior seen in nearby galactic systems.
“There's definitely a preference for these satellite planes to align with larger scales, which would indicate that maybe the filamentary structure, this cosmic web, somehow plays into that,” Pawlowski said.
It’s also possible that the problem is a local cosmic phenomenon that might be unique to our corner of the universe. “It would be a worthwhile test to see whether there's something special about where we are, which has not been tested so far,” Pawlowski added.
The standard cosmological model is extremely well-corroborated, so any evidence that defies it is bound to spark controversy within scientific circles. Müller and Pawlowski, along with their colleagues, have already experienced their fair share of pushback over their research —and were bracing for a new onslaught with the publication of this study.
When the research went live on Friday, Müller only half-jokingly asked Pawlowski: When will the first angry emails come in?
“That was actually what I was expecting,” Müller said. “But the feedback was amazing. We got quite nice emails from our competitors that said: ‘okay, this really now looks exciting and interesting.’”
“I've been working on these kinds of topics for 10 years or so, and I've definitely noticed a change,” Pawlowski noted, referring to a growing acknowledgement, among his peers, that these satellites are a real thorn in the side of our basic understanding of the cosmos.
The team had hoped to build on this work with even more observations, but the coronavirus pandemic disrupted their plans. As Müller succinctly put it: “2020 being 2020, it didn’t happen.”
Now that 2020 is in the rear view mirror, the team aims to resume the hunt for more co-orbiting satellites within the Milky Way, Andromeda, and Centaurus A. They also intend to build a statistical survey of satellites in galaxies further afield, which could help resolve—or perhaps deepen—the mystery of the plane-of-satellites problem
“The cosmological model is really successful on large scales, and this is a serious problem we don't really have a good solution to,” Pawlowski concluded.