Evolving dark matter may explain cosmic acceleration. Observations support a mixed model.
For years, a stubborn puzzle has sat at the center of the standard cosmological model. The evidence clearly supports an expanding Universe, yet measurements from the early cosmos suggest a lower acceleration rate than measurements made nearby in space and time.
This mismatch is known as the Hubble tension problem, and there is still no agreed solution. Many ideas have been floated. Perhaps general relativity is incomplete. Perhaps dark matter does not exist. Perhaps the flow of time is not uniform. Some even ask whether the entire Universe rotates. Here is another possibility to consider: dark matter might change over time.
Considering dark matter evolution
Models in which dark energy evolves have been explored, but the notion that dark matter itself evolves has received far less attention. There are two main reasons. First, observations that point to dark matter are strong. They imply the presence of a substance that barely interacts with light.
The major weakness is that no one has directly detected a dark matter particle. Second, many critics of dark matter aim to remove it entirely through modified gravity ideas. To them, dark matter is not a concept to adjust but a premise to discard. That is why the proposal of evolving dark matter is intriguing.
In this work, the authors look at both evolving dark energy and evolving dark matter and argue that the latter is a much better fit to observational data. The first thing they note is that the two models are somewhat related. Since the evolution of the cosmos depends in part on the ratio of energy density to matter density, a model with constant dark matter and evolving dark energy will always appear similar to a model with evolving dark matter and constant dark energy.
Exotic dark matter and oscillations
They then go on to explore the idea of some kind of exotic dark matter. One that has a changeable equation of state (EOS). To match observation, the dark matter EOS must oscillate in time. This isn’t an outlandish notion. Neutrinos have mass and don’t interact strongly with light.
While they can’t account for all the dark matter of the Universe, they are a form of hot dark matter. And neutrinos undergo mass oscillation. Perhaps cold dark matter particles undergo a similar oscillatory effect. The authors find that the best fit to observational data is a Universe where about 15% of the cold dark matter is oscillatory and the remaining 85% is standard dark matter. This would allow for the Hubble tension to be covered while still matching the dark matter observations we have.
It should be stressed that this work presents a toy model. As the authors themselves note, the work is a broad concept that does not pin down specific constraints for dark matter particles. But this work does open the door to a broader range of dark matter models. At this point, evolving dark matter is worth considering.
Reference: “Evolving Dark Energy or Evolving Dark Matter?” by Xingang Chen and Abraham Loeb, 5 May 2025, arXiv.
DOI: 10.48550/arXiv.2505.02645
Adapted from an article originally published on Universe Today.
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1 Comment
The only thing evolving in this “dark matter” theory are the goal posts.
Much like in the “global warming” theory.