Simulations Suggest Two-State Dark Matter Could Explain Giant Early Cluster El Gordo

Seven billion light-years from Earth sits a galaxy cluster that astronomers did not expect to exist. Nicknamed El Gordo, the Fat One, it is made of two regions in the middle of colliding with each other, with a combined mass of more than two quadrillion times the mass of the Sun. What makes it strange is its size for its era: it is extraordinarily large for that period of the universe.

That raises a hard question about how such a massive structure formed so early. El Gordo is surprising enough that some astronomers hold it up as a counterexample to the standard model of cosmology. It is difficult to explain how cold dark matter in an inflationary universe could build a cluster this large this quickly. A new study, reported by Brian Koberlein of Universe Today, examines whether dark matter that interacts with itself could explain it.

Dark matter makes up the bulk of the mass in the cosmos. It does not interact strongly with light, if at all, which is why it is called dark, and for the same reason it does not interact strongly with ordinary matter, since most matter interactions are electromagnetic. Whether dark matter can interact strongly with itself is one of the field's open questions. The most popular view is that it cannot, because observations show dark matter is mostly cold, meaning its particles carry little kinetic energy. If dark matter particles could collide, they would stay warm, just as ordinary matter heats up when it collides.

Self-interacting dark matter could, in principle, help build an early cluster like El Gordo, but it would also produce plenty of warm dark matter. The new study proposes a way around that. Ordinary matter mostly interacts through elastic collisions, like air molecules in a room bouncing off each other without losing much energy. If collisions were inelastic and lost energy with every bounce, a room would rapidly freeze. Most self-interacting dark matter models assume an elastic interaction, and the results of tuning those models to speed up galactic clustering have been mixed.

The paper, by R. Valdarnini and published in The Astrophysical Journal, instead considers a dark matter model with a ground state and an excited state. Most collisions would be elastic. But at the right energy, a collision could kick a dark matter particle into its excited state, which removes kinetic energy from the collision. In that picture, dark matter particles can clump together and cool down at the same time.

Using N-body simulations of the colliding regions of El Gordo, Valdarnini found that the two-state model, with both elastic and inelastic collisions, was the best fit to what astronomers observe. It fit better than both noninteracting dark matter and models with elastic collisions only.

There is a catch. Astronomers are still uncertain about El Gordo's exact mass and the speed at which its two regions are colliding. If the cluster sits at the low end of those ranges, the two-state model would not fit the data. As Koberlein put it, this is a possible solution rather than a definite one.

The result does not settle anything about what dark matter is. It shows that a more complex dark sector than the standard picture assumes can account for one of the most awkward objects in the early universe, and that the answer may hinge on better measurements of the cluster itself. Until dark matter particles are observed directly, as Koberlein wrote, all sorts of possibilities remain on the table.