Hidden force makes dark matter lighter, slowing cosmic structure growth
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A new theoretical study finds that an additional attractive force among dark matter particles would make them effectively lighter as the universe expands, thereby slowing, rather than accelerating, the growth of cosmic structure. The research, published in the Journal of Cosmology and Astroparticle Physics, suggests dark matter may interact through a hidden force that produces counterintuitive effects. The finding complicates efforts to explain small but persistent discrepancies in cosmological observations.
The Dark Force Hypothesis
Precise observations from the Planck satellite and other instruments have revealed tensions in the standard cosmological model. Measurements of the cosmic microwave background suggest matter may be more tightly clustered on large scales than predicted, while some probes of cosmic expansion indicate a slower rate in the distant past. These discrepancies, though small, have prompted scientists to explore whether dark matter particles might exert an extra ‘dark force’ invisible to ordinary matter. Zachary Weiner of the Perimeter Institute for Theoretical Physics, the study’s corresponding author, notes that current knowledge of dark matter comes only from its gravitational effects, leaving room for hidden interactions.
Counterintuitive Effect
The research team tested a class of models in which dark matter particles attract each other over long ranges beyond gravity. They found that while this dark force helps the particles cluster more efficiently, it also makes dark matter effectively lighter as the universe expands. This weight loss weakens the gravitational pull, so the growth of the largest cosmic structures is usually slowed rather than accelerated—the opposite of what simpler models would suggest. The effect is robust across the range of models studied, according to the paper.
Tensions Amplified
The findings add complexity to efforts aimed at resolving existing cosmological tensions. If dark matter indeed possesses a hidden force, its net impact would not be a straightforward boost to structure formation, but a more nuanced slowdown. This makes it harder to reconcile the clustered matter distribution seen in the early universe with the later evolution of galaxies and clusters. The study, published on August 2 in JCAP, does not rule out a dark force but shows that its consequences defy simple expectations.
What's Next
Upcoming surveys from the Vera C. Rubin Observatory and the Square Kilometre Array are expected to map cosmic structure with far greater precision, providing critical tests for these models. It remains unclear whether dark matter actually experiences such a force, but the study underscores the need to move beyond the simple cold dark matter paradigm.
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Hidden force makes dark matter lighter, slowing cosmic structure growth
