Hubble volume
In cosmology, a Hubble volume (named for the astronomer Edwin Hubble) or Hubble sphere, Hubble bubble, subluminal sphere, causal sphere and sphere of causality is a spherical region of the observable universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the universe. The Hubble volume is approximately equal to 1031 cubic light years (or about 1079 cubic meters). The proper radius of a Hubble sphere (known as the Hubble radius or the Hubble length) is 
  
    
      
        c
        
          /
        
        
          H
          
            0
          
        
      
    
    {\displaystyle c/H_{0}}
  , where 
  
    
      
        c
      
    
    {\displaystyle c}
   is the speed of light and 
  
    
      
        
          H
          
            0
          
        
      
    
    {\displaystyle H_{0}}
   is the Hubble constant. The surface of a Hubble sphere is called the microphysical horizon, the Hubble surface, or the Hubble limit. More generally, the term Hubble volume can be applied to any region of space with a volume of order 
  
    
      
        (
        c
        
          /
        
        
          H
          
            0
          
        
        
          )
          
            3
          
        
      
    
    {\displaystyle (c/H_{0})^{3}}
  .