Radiation emitted from a dark sector, potentially affecting the universe's expansion.Dark radiation is a hypothetical form of radiation that originates from a 'dark sector' — a set of particles and forces that interact very weakly with ordinary matter, much like dark matter. Unlike dark matter, which is thought to be slow-moving (cold), dark radiation would consist of particles moving at or near the speed of light. It is a key ingredient in some cosmological models because it can influence the expansion rate of the early universe, leaving subtle imprints on the cosmic microwave background and the formation of large-scale structures.
The concept gained prominence in the 2000s and 2010s, particularly after the Planck satellite measurements of the cosmic microwave background showed a slight (though not definitive) preference for extra radiation. It is an active area of research, with no single discoverer.
In the standard picture, the universe is filled with photons (ordinary radiation) and neutrinos (which act as a form of radiation). Dark radiation would add an extra component of relativistic particles that contribute to the total energy density. This extra energy density changes the rate at which the universe expanded in its first moments, which in turn affects the acoustic oscillations seen in the cosmic microwave background and the growth of galaxy clusters. The amount of dark radiation is often parameterized as the 'effective number of neutrino species' (N_eff), with any deviation from the standard value hinting at new physics.
Dark radiation is primarily a theoretical tool used to constrain models of dark matter and new particles, such as axions or sterile neutrinos, by comparing predictions with cosmological observations.
If dark radiation exists, it would have diluted the density of dark matter in the early universe, meaning that dark matter might be slightly more abundant today than it would be otherwise.
