Bernard Schutz, Gravity from the Ground Up: An Introductory Guide to Gravity and General Relativity, Cambridge University Press, 2003, p. 363, ISBN978-0-521-45506-0.
Mark H. Jones e Robert J. Lambourne, An Introduction to Galaxies and Cosmology, Cambridge University Press, 2004, p. 244, ISBN978-0-521-83738-5.