On the Entropy Increase in the Black Hole Formation
  The entropy increases enormously when a star collapses into a black hole. This entropy increase is interpreted as the decrease of the temperature due to the increase of the volume. Through these investigation the microscopic states of the black hole entropy is naturally understood as the statistical states of the enlarged phase space. 
Tetsuya Hara, Keita Sakai, and Daigo Kajiura