Scale factor (cosmology)
The expansion of the universe is parametrized by a dimensionless scale factor 
  
    
      
        a
      
    
    {\displaystyle a}
  . Also known as the cosmic scale factor or sometimes the Robertson Walker scale factor, this is a key parameter of the  Friedmann equations. In the early stages of the Big Bang, most of the energy was in the form of radiation, and that radiation was the dominant influence on the expansion of the universe. Later, with cooling from the expansion the roles of matter and radiation changed and the universe entered a matter-dominated era. Recent results suggest that we have already entered an era dominated by dark energy, but examination of the roles of matter and radiation are most important for understanding the early universe.