OUTSIDE the Great Gate of Trinity College, Cambridge, is a tree reputed to be descended from the one that dropped the apple on Isaac Newton's head. The result of the concussion, the legend goes, was Newton's law of universal gravitation: The same force that pulls things to Earth also binds the planets around the sun. So began an era of cosmic mergers and acquisitions that led, three centuries later, to Stephen Hawking's famous prediction that the end of physics -- the Theory of Everything -- was in sight.
Hawking, as his press agents never tire of reminding us, holds Newton's old chair as Lucasian Professor of Mathematics at Cambridge University. By the time he assumed the post, an all-encompassing physics indeed seemed at hand. Electricity had been united with magnetism, and electromagnetism with the weak force that causes nuclear decays. For their next act, physicists planned to combine this electroweak force with the strong nuclear force that glues quarks together. That would still leave gravity, but Hawking bet, in his inaugural Lucasian lecture in April 1980, that by the year 2000 it too would be absorbed into the mix. All these fundamental forces would be reduced to facets of a single phenomenon -- supergravity -- present at the moment of creation.
Sitting in the audience that day was a graduate student, Neil Turok, who went on to get a PhD in theoretical physics. He didn't suspect at the time that he would be swept up by another convergence -- between his newly chosen field and cosmology.
It was a natural connection: Testing an idea like supergravity required a particle accelerator literally the size of the universe -- and the "experiment" had already been done: In the searing temperatures of the Big Bang, all the forces would still have been fused together. Figure out the Bang and you could reverse-engineer the Theory of Everything.
That's not how things turned out. In "Endless Universe: Beyond the Big Bang," Turok and Paul J. Steinhardt, a Princeton physicist, describe how they devoted years to the quest, only to find themselves veering from the pack with a new theory -- a radical model of the universe in which there is no beginning explosion but, rather, an endless cycle of cosmic thunderclaps.
Part of the book's appeal is the way it intertwines the two scientists' stories. Around the time Turok is listening to Hawking's Lucasian lecture, we see Steinhardt, then a junior fellow at Harvard, trying to make sense of a seminar he'd just heard by Alan Guth, who was putting forth a wild idea called cosmological inflation, a modification of the Big Bang.
Unadorned, the Big Bang theory has trouble explaining some striking features of the universe, like why it is smooth instead of lumpy: To us, the starry sky may appear disorderly, but from a god's-eye view matter is distributed, for no obvious reason, more or less homogeneously. Guth proposed that an early burst of breakneck expansion, lasting a tiny fraction of a second, had straightened out the kinks. Inflation also promised to explain why the universe seems to have a mostly uniform temperature and why its overall geometry is "flat" and not curved.
Guth's idea needed some work, and Steinhardt became one of its principal architects. Turok and others also made contributions, and through all these efforts the inflationary model of the Big Bang entered the mainstream of science. That's when Steinhardt and Turok began to have second thoughts.
The problem had much to do with aesthetics. So simple in its original form, the Big Bang theory was starting to look to them like a kludge. Not only did you have to posit this incongruous early spurt of inflation but also other bric-a-brac -- like dark matter (invisible stuff that helped galaxies form) and dark energy (an anti-gravitational force that explains the continuing acceleration of the cosmic expansion). Were these features or bugs? For that matter, why had there been a Big Bang in the first place? Emerging from a 1999 lecture at Cambridge University on superstrings, Steinhardt and Turok began brainstorming their way into an alternative to inflation that drew on the weirdest new ideas in high-energy physics.
BY now, the supergravity theory Hawking had longed for had been overtaken by another approach to unification, in which everything was made of strings vibrating in multiple dimensions -- not three dimensions but nine or 10. String theory itself had recently undergone elaboration, and physicists were now speaking of exotic objects called branes (short for membranes) accompanying the strings in the cosmic orchestra. In the most ambitious schemes, our universe itself was a giant flapping brane -- one of many floating like jellyfish in what amounted to a universe of universes.