In this book, Aczel proposes that Einstein's Cosmological Constant, discarded and by the genius himself considered his greatest blunder, is in fact an integral part of the equation that defines the nature of the universe, its past and its future. Some mysterious force is accelerating the expansion of the universe, pushing out on space, countering gravity and making the universe accelerate towards infinity. Aczel argues that in addition to the four known forces: gravity, electromagnetism, the weak and the strong nuclear forces, there is a fifth: the cosmological constant which is the quintessence of the universe. He spoke to many experts in the fields of mathematics, physics, astronomy and cosmology and integrated the ideas of prominent scientists like Eddington, Penrose and Grossman. The chapters deal with stuff like Euclid's Riddle, Riemann's Metric, the expansion of space, the nature of matter and the geometry of the universe but it also serves as a type of biography of Einstein and a history of the development of his theories. There are quotes from Einstein' work and the text is enlivened by portraits, photographs and illustrations. Although an engaging and thought provoking text, it is sometimes difficult to grasp all of the intricacies as there are many formulas that a non-mathematician would not understand. Nevertheless a uniquely stimulating work that concludes with a helpful bibliography and thorough index. I also recommend Marcus Chown's The Universe Next Door, Mark Ward's Universality: Beyond Chaos and Martin J. Rees' Before The Beginning: Our Universe And Others.