Dark forces hinder the growth of the Universe

Dark matter is often portrayed as a cosmic loner, interacting with itself and the rest of the Universe only through gravity. But what if dark matter particles also exert a hidden force on one another?

A new study published in the Journal of Cosmology and Astroparticle Physics (JCAP) explored this possibility and uncovered an unexpected result. While an additional attractive force does help dark matter particles cluster together, it does not necessarily produce the extra growth of cosmic structure that intuition would suggest. In fact, the opposite is usually the case.

An extra "dark force”

Interest in the idea of an extra "dark force" has grown in recent years because some of the most precise observations of the Universe seem to tell slightly different stories. Measurements of how the Universe has expanded over its history and of how galaxies and cosmic structure have grown do not always fit together as neatly as expected. 

Some recent observations of the distant Universe suggest that cosmic expansion may have been slightly slower in the past than predicted by the standard cosmological model, while observations of the cosmic microwave background have long hinted that matter may be more strongly clustered on the largest cosmic scales than expected. 

While these discrepancies are small, they have prompted researchers to investigate whether some ingredient may be missing from the current cosmological model.  

One possibility is that dark matter particles interact through an additional force that acts only between them—hence the term "dark force". Such a force could influence both the expansion of the Universe and the formation of cosmic structures, potentially helping to explain some of these puzzling observations.

"What we really know about dark matter has so far been learned only through its gravitational effects,” says Zachary Weiner, a researcher at the Perimeter Institute for Theoretical Physics, corresponding author for the study. “That leaves open the possibility that dark matter might have additional interactions that are hidden from ordinary matter.”

To investigate this possibility, the researchers studied a class of theoretical models in which dark matter particles interact through an additional long-range force beyond gravity.

Using theoretical calculations and cosmological data, they examined how such a force would affect both the expansion of the Universe and the growth of large-scale cosmic structures.

Counterintuitive results

At first glance, the outcome seems obvious. If dark matter particles can attract one another through an additional force, they should clump together more efficiently. Such an effect might also help explain why some observations appear to point to a Universe that has denser structures than expected.
“The first thing you would expect is that giving dark matter an additional attractive force should make structures grow faster,” says Weiner. “But another effect comes into play at the same time.”

In the model studied by the researchers, the force does help dark matter cluster more efficiently. But the same mechanism also changes how dark matter evolves as the Universe expands: dark matter particles effectively become lighter over time. 

As a result, the extra clustering is not enough to increase the gravitational imprint of matter on the cosmic microwave background. Instead, the overall effect is usually a suppression of structure growth.  

The findings may also have implications beyond dark matter itself. Some models proposed to explain recent observations from the Dark Energy Spectroscopic Instrument (DESI) rely on the same kind of dark-matter interactions explored in this study. According to the researchers, the mechanism identified here is likely to apply to many of these more complex scenarios as well.

As increasingly precise observations arrive from new surveys and observatories, studies like this may help researchers understand which hidden properties dark matter can—and cannot—possess.

“The Universe is often more subtle than our intuition,” says Weiner. “That's exactly why we have to keep testing these ideas.”