Chaplygin gas
Chaplygin gas, which occurs in certain theories of cosmology, is a hypothetical substance that satisfies an exotic equation of state in the form

  
    
      
        p
        =
        −
        A
        
          /
        
        
          ρ
          
            α
          
        
      
    
    {\displaystyle p=-A/\rho ^{\alpha }}
  ,
where 
  
    
      
        p
      
    
    {\displaystyle p}
   is the pressure, 
  
    
      
        ρ
      
    
    {\displaystyle \rho }
   is the density, with 
  
    
      
        α
        =
        1
      
    
    {\displaystyle \alpha =1}
   and 
  
    
      
        A
      
    
    {\displaystyle A}
   a positive constant. The substance is named after Sergey Chaplygin. In some models, generalized Chaplygin gas is considered, where 
  
    
      
        α
      
    
    {\displaystyle \alpha }
   is a parameter, which can take on values 
  
    
      
        0
        <
        α
        ≤
        1
      
    
    {\displaystyle 0<\alpha \leq 1}
  .