The light curves for type Ia are mostly very uniform, with a consistent maximum absolute magnitude and a relatively steep decline in luminosity. Their optical energy output is driven by radioactive decay of ejected nickel-56 (half-life 6 days), which then decays to radioactive cobalt-56 (half-life 77 days). These radioisotopes excite the surrounding material to incandescence. Modern studies of cosmology rely on Ni radioactivity providing the energy for the optical brightness of supernovae of type Ia, which are the "standard candles" of cosmology but whose diagnostic 847 keV and 1,238 keV gamma rays were first detected only in 2014. The initial phases of the light curve decline steeply as the effective size of the photosphere decreases and trapped electromagnetic radiation is depleted. The light curve continues to decline in the B band while it may show a small shoulder in the visual at about 40 days, but this is only a hint of a secondary maximum that occurs in the infra-red as certain ionised heavy elements recombine to produce infra-red radiation and the ejecta become transparent to it. The visual light curve continues to decline at a rate slightly greater than the decay rate of the radioactive cobalt (which has the longer half-life and controls the later curve), because the ejected material becomes more diffuse and less able to convert the high energy radiation into visual radiation. After several months, the light curve changes its decline rate again as positron emission from the remaining cobalt-56 becomes dominant, although this portion of the light curve has been little-studied.