flatness problem One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.
fluorescence The absorption of a photon of one energy, or wavelength, and re-emission of one or more photons at lower energies, or longer wavelengths.
force Action of an object that causes momentum to change. The rate at which the momentum changes is numerically equal to the force.
frequency The number of wave crests passing any given point in a given period of time.
fusion Mechanism of energy generation in the core of the Sun in which light nuclei are combined, or fused, into heavier ones, releasing energy in the process.