We may be more in the dark about dark matter than previously thought, according to a new analysis of distant galaxy clusters.
Yale astrophysicist Priyamvada Natarajan, a leading theorist on the nature of black holes and dark matter, says new observational data conflicts with certain assumptions about cold dark matter (CDM) - unseen, slow-moving particles that are inferred by their effect on gravity - and may prompt a fundamental rethinking of dark matter by scientists.
The analysis suggests there are either two types of dark matter or the presence of an entirely new type of particle affecting the innermost, densest pockets of galaxy clusters - the universe's largest collections of galaxies held together by dark matter.
"Both possibilities require an intellectual expansion of sorts," said Natarajan, the Joseph S. and Sophia S. Fruton Professor of Astronomy & Physics in Yale's Faculty of Arts and Sciences, and principal author of the new study, which is published in The Astrophysical Journal Letters. "It's a moment when we have to open our minds and change our notion of what works. I personally find that very exciting."
Though still undiscovered, dark matter is thought to be the scaffolding upon which structures within the universe - including planets, stars, and galaxies - are built. Astronomers have spent nearly two decades simulating the effects of dark matter on galaxy formation, mergers, and clusters, based upon the Lambda CDM cosmological model, the standard model for understanding the evolution of the universe.
In their new study, Natarajan and co-authors Barry T. Chiang and Isaque Dutra, both Ph.D. students in the Yale Graduate School of Arts and Sciences, use a technique called gravitational lensing to collect data from three galaxy clusters and compare it with the standard CDM model. What they learned may prove instrumental in either revising the standard model or sending the study of dark matter physics in an entirely new direction.
Natarajan, Chiang, and Dutra spoke with Yale News about the new research and what it may portend.
What led you to look for discrepancies in the standard model for dark matter?
Priyamvada Natarajan: In 2017, I led a group that looked at one galaxy cluster. There were hints then that there may be anomalies. Nearly a decade later, we've seen an incredible leap in the amount of precise data available to study from the Hubble Space Telescope and the James Webb Space Telescope.
We were able to look at three exceptionally well-studied, massive lensing clusters - MACS J0416, MACS J1206, and MACS J1149. These clusters have the deep data-imaging, extensive spectroscopy, and high-fidelity lens models needed for a stringent comparison with theory.
Barry T. Chiang: It's remarkably exciting to have been involved in this project and start, quite literally, seeing the unseen by confronting dark matter theories with cutting-edge observational data and simulations.
You mentioned "lensing" - that would be gravitational lensing. Why is that an advantageous method for looking at dark matter?
Natarajan: It is uniquely powerful. Lensing, which results from gravity bending light, offers a unique way to map out the distribution of all matter, both dark and visible. The presence of matter itself causes the bending of light, so unlike other probes, lensing doesn't care what the dynamics of a particular region might be, be it a calm or turbulent region of the universe - and these are complex regions. The inner parts of the galaxies we're interested in are filled with black holes, jets, and stars. There is a lot of action going on.
Lensing is the perfect way to stress test the standard model of cosmology.
What are you stress testing for, exactly?
Natarajan: According to the standard paradigm, every massive galaxy cluster sits inside a halo - an enormous, invisible envelope of dark matter. The constituent dark matter particles aggregate only via gravity. But this halo is not perfectly smooth. It contains many smaller clumps of dark matter called sub-halos, that are the surviving remnants that fell into the gravitational grip of the cluster over cosmic time. These sub-halos, in turn, host the visible galaxies held within the gravitational grip of the larger cluster.
We examined four independent properties of sub-halos, comparing our observational data with carefully selected analogs found in simulations of the standard model.
What did you find?
Natarajan: The model remains spectacularly successful in explaining the large-scale structure of the universe: the cosmic web, the clustering of galaxies, and the distribution of matter on vast scales. But inside galaxy clusters, clumps of dark matter appear to behave one way in their outer regions and another way in their dense inner cores, pointing to an anomaly that may be telling us something about the very nature of dark matter itself.
What is going on inside the galaxies in the inner part of clusters?
Natarajan: These inner regions of cluster galaxies are telling a different story than expected. The distribution of dark matter clumps does not match the modeling at all. The matter content of the observed sub-halos is far more concentrated toward the centers of the clusters. Additionally, standard model simulations do not produce enough sub-halos in this innermost region.
Even more striking is the signal from galaxy-galaxy strong lensing. Here, individual cluster galaxies - and the dark matter sub-halos around them - act as smaller lenses embedded within the larger cluster lens. Observed clusters produce these galaxy-scale lensing events far more abundantly than standard model simulations predict, by nearly an order of magnitude. To explain this excess lensing, the centers of the sub-halos must be much denser and more concentrated than ordinary simulations allow. Their inner profiles resemble what might be produced if dark matter particles can self-interact and undergo extreme collapse, causing a real pile-up in the innermost regions of sub-halos.
Isaque Dutra: This lensing excess is striking because it is not merely mapping where dark matter is - it may be revealing what dark matter is as its resolution requires self-interactions beyond gravity for the dark matter particle.
Where does all of this leave the search for dark matter, moving forward?
Natarajan: The implication is not that the standard model has failed wholesale. It is subtler and far more interesting.
The model passes some of the most stringent cluster-lensing tests, while failing others in a coherent, scale-dependent pattern. Sub-halos have therefore become precision probes of the dark sector. They may be telling us, for the first time, not merely where dark matter is, but how it behaves.
Either we need to refine the current model and perhaps accommodate a second particle, one that self-interacts, or perhaps more excitingly we may just be seeing the first small hints that point the way to an entirely new kind of particle. Time will tell.
