text: When laser cooling of atoms was first proposed in 1975, the only cooling mechanism considered was Doppler cooling. As such the limit on the temperature was predicted to be the Doppler limit:
  Here k is the Boltzmann constant, T is the temperature of the atoms, and Γ is the inverse of the excited state's radiative lifetime.
  Early experiments seemed to be in agreement with this limit. However, in 1988 experiments began to report temperatures below the Doppler limit. These observations would take the theory of PG cooling to explain.