Dark Matter Models Reassessed on a Common Naturalness Scale
A new analysis compared a dozen dark matter proposals using the same quantitative measure of fine-tuning. It found that both particle dark matter and primordial black hole scenarios include natural and highly tuned examples, challenging the common assumption that black holes are the more contrived option. The study argues that naturalness depends more on how a model generates dark matter than on its broad category.
Profumo, a UC Santa Cruz physics professor and deputy director for theory at the Santa Cruz Institute for Particle Physics, applied the Barbieri-Giudice measure to twelve established scenarios. This tool gauges how much a model's predicted dark matter abundance shifts when one underlying parameter is slightly changed.
The set spanned the long-studied WIMP and several mechanisms for forming primordial black holes in the early universe. One black hole model, in which collapsing domain wall networks create black holes, ranked among the most natural, matching the least tuned particle proposals. The findings appear in Physical Review D.
The study may influence how physicists and funding bodies weigh competing dark matter searches, since it suggests no broad category is inherently more plausible. Researchers deciding which experiments to prioritize could find the shared scale useful, though a single fine-tuning metric cannot settle which theory is correct. For the public, the work may modestly shift how science coverage frames black holes versus particles as dark matter candidates, without changing any observational evidence.