Equation of state (cosmology)
In cosmology, the equation of state of a perfect fluid is characterized by a dimensionless number 
  
    
      
        w
      
    
    {\displaystyle w}
  , equal to the ratio of its pressure 
  
    
      
        p
      
    
    {\displaystyle p}
   to its energy density 
  
    
      
        ρ
      
    
    {\displaystyle \rho }
  : 

It is closely related to the thermodynamic equation of state and ideal gas law.