Ray-Chaudhuri  by 1960, and are named after their inventors. [259] Pati-Salam model : A mainstream  Grand Unification Theory  proposed by  Jogesh Pati  in collaboration with  Abdus Salam  in 1974. [260] [261] Ivory : The use of ivory in India dates to the Indus Valley Civilization (2300-1750 BCE). [262]  Archaeological excavations have yielded