Dark matter is commonly described as an invisible substance that interacts primarily through gravity. However, scientists are investigating a more complex possibility: dark matter particles may also be connected by a hidden force that affects how they cluster and shape the Universe.
A new study published in the Journal of Cosmology and Astroparticle Physics (JCAP) explores what could happen if dark matter experienced an additional attractive force. The results are unexpectedly counterintuitive. Although the hidden force helps dark matter form clumps more efficiently, it generally does not speed up the growth of cosmic structure. Instead, it may suppress the formation of galaxies and other large-scale structures.
Why Scientists Are Investigating a Dark Force
Interest in a possible “dark force” has grown as increasingly precise observations of the Universe reveal small tensions within the standard model of cosmology.
Measurements of cosmic expansion and the growth of galaxies and other large-scale structures do not always produce perfectly matching results. Some observations of the distant Universe suggest that cosmic expansion may have been slower in the past than the standard cosmological model predicts.
Meanwhile, studies of the cosmic microwave background have suggested that matter may be more strongly clustered across the largest scales of the Universe than expected.
These differences are relatively small, but they have prompted researchers to consider whether the standard cosmological model may be missing an important physical process.
One possibility is that dark matter particles experience an additional force that ordinary matter cannot detect. Because this interaction would operate only within the dark matter sector, scientists refer to it as a “dark force.” Such an interaction could influence both cosmic expansion and the formation of galaxies, galaxy clusters, and other large-scale structures.
“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 and the study’s corresponding author. “That leaves open the possibility that dark matter might have additional interactions that are hidden from ordinary matter.”
Testing a Hidden Force Beyond Gravity
The research team analyzed theoretical models in which dark matter particles interact through a long-range force in addition to gravity.
Using theoretical calculations and cosmological observations, the scientists examined how this hidden interaction could affect the history of cosmic expansion and the development of large-scale structure.
At first glance, the expected outcome seems obvious. If dark matter particles attract one another through an additional force, they should gather into dense clumps more quickly. Stronger clustering might also appear to explain observations indicating that the Universe contains more concentrated structures than predicted.
“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.”
A Counterintuitive Effect on Cosmic Structure Growth
In the models studied by the researchers, the extra force does make dark matter cluster more efficiently. However, it also changes the way dark matter responds as the Universe expands.
The interaction causes dark matter particles to effectively lose mass over time. This reduction weakens their gravitational influence, counteracting the stronger attraction created by the hidden force.
As a result, more efficient dark matter clustering does not produce a stronger gravitational imprint on the cosmic microwave background. In many cases, the combined effect actually slows the growth of cosmic structure.
What the Findings Could Mean for DESI and Dark Energy Models
The results may have important consequences for theories that extend beyond conventional dark matter models. Some explanations for recent observations from the Dark Energy Spectroscopic Instrument (DESI) involve similar interactions between dark matter particles.
According to the researchers, the newly identified mechanism could affect many of these more complex theories. Any model involving a hidden attractive force may need to account for the possibility that dark matter becomes effectively lighter as the Universe evolves.
Future observations from advanced space telescopes, ground-based observatories, and large cosmological surveys could help scientists determine whether dark matter has hidden interactions and identify which models are consistent with the evolution of the Universe.
“The Universe is often more subtle than our intuition,” says Weiner. “That’s exactly why we have to keep testing these ideas.”
Source: www.sciencedaily.com