In standard cosmological calculations, "matter" means any constituent of the universe whose energy density scales with the inverse cube of the scale factor, i.e., ρ ∝ a . This is in contrast to "radiation", which scales as the inverse fourth power of the scale factor ρ ∝ a , and a cosmological constant, which does not change with respect to a (ρ ∝ a). The different scaling factors for matter and radiation are a consequence of radiation redshift : For example, after gradually doubling the diameter of the observable Universe via cosmic expansion of General Relativity, the scale, a, has doubled. The energy of the cosmic microwave background radiation has been halved (because the wavelength of each photon has doubled); the energy of ultra-relativistic particles, such as early-era standard-model neutrinos, is similarly halved. The cosmological constant, as an intrinsic property of space, has a constant energy density regardless of the volume under consideration.