Cosmic age problem
The cosmic age problem was a historical problem in astronomy concerning the age of the universe. The problem was that at various times in the 20th century, the universe was estimated to be younger than the oldest observed stars. Estimates of the universe's age came from measurements of the current expansion rate of the universe, the Hubble constant 
  
    
      
        
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  , as well as cosmological models relating 
  
    
      
        
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   to the universe's matter and energy contents (see the Friedmann equations). Issues with measuring 
  
    
      
        
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   as well as not knowing about the existence of dark energy led to spurious estimates of the age. Additionally, objects such as galaxies, stars, and planets could not have existed in the extreme temperatures and densities shortly after the Big Bang.