Dark Matter Decays and Slows the Universe's Growth express
Analysis of fσ8 and weak lensing data suggests that the linear growth rate at z≲1 may be lower than ΛCDM predictions. The iDCDM (interacting decaying cold dark matter) model is built, where dark matter particles experience friction with dark radiation born from their decay. The friction does not weaken but grows toward late times. The model adds two parameters, keeping background evolution, primordial nucleosynthesis, and the CMB unchanged, but leads to a step-like suppression of the growth factor f(k,z) — a target for DESI, Euclid, and Rubin. With current data, iDCDM is preferred over ΛCDM: Δχ² from -2.7 to -7.6 depending on the redshift scaling of friction and neutrino mass. Definitive tests will require k- and z-resolved measurements of structure growth.
The Universe grows like a metropolis: galaxies skyscrapers are erected on an invisible scaffold of dark matter—its gravity pulling in construction material. Meanwhile, dark energy makes the city itself continuously expand. Previously, it was assumed the scaffold compacts at a steady rate, despite the expansion of space. But recent measurements show that in recent epochs, the pace of construction has slowed down.
Astronomers suspect the issue lies with the scaffold itself: it gradually decays, emitting a 'dust' of lightweight particles. This dust, like a headwind, presses on the remaining dark matter and hinders further building. The most surprising thing is that the slowdown didn't start right away, but only a few billion years ago, as if the construction material suddenly started losing strength. The model predicts a clear signal—a characteristic break in the growth chart of galaxies on certain scales. Projects Euclid and Rubin will soon be able to test this hypothesis.
🎯 Dark radiation interacts so weakly with ordinary matter that it passes through Earth almost unnoticed.