The cosmological kitchen is full of invisible ingredients. Dark matter, like an invisible scaffold, keeps galaxies from flying apart, while dark energy, like yeast in dough, makes space expand ever faster. But what if these two components don't just coexist but actively exchange energy?
Such a model, with an exponential potential and a coupling between dark energy and dark matter, was tested by a team of astrophysicists. To do so, they turned to data from three major cosmological surveys. The first is the cosmic microwave background radiation recorded by the Planck satellite. The second is baryon acoustic oscillations (sound waves from the early universe, frozen into the distribution of galaxies) from the DESI survey. The third is supernova observations from the DES-Dovekie project. The result surprised them: when the sum of neutrino masses (light ghost-like particles) is fixed at 0.06 eV, the coupling between the dark components shows up with a confidence of more than three sigma.
The most intriguing part is the direction of energy flow. The data favor a scenario where the coupling parameter of dark energy is negative: energy flows from dark matter to dark energy. In this case, the effective equation of state (EoS)—a quantity describing the 'stiffness' of dark energy — crosses the value minus one. This puts it into the phantom divide — a region where dark energy becomes even more aggressive than Einstein's cosmological constant. If, however, neutrino masses are allowed to take negative effective values (itself a puzzle), the distinction between models with and without the crossing disappears — both fit the observations equally well.
Negative effective neutrino mass is not an error but a mathematical trick that sometimes appears in complex cosmological models. In reality, the rest mass of a particle cannot be negative, but in equations it might signal that we are missing some new physics.
In Peter Watts's novel Blindsight, an alien life form uses dark matter as a habitat — perhaps energy flows between dark sectors will one day help us glimpse such a hidden world.