Perhaps it was inevitable that two of the great mysteries of physics—the nature of matter and the nature of black holes—would come crashing into each other. Yet nobody anticipated the meeting or foresaw how fruitful it would be in generating novel ideas about the workings of the universe.
In 1996 Andrew Strominger and Cumrun Vafa of Harvard University were working on the mathematics of string theory, a physics model that describes all fundamental particles as vibrating strands of energy, when they realized that a key property of certain black holes can be predicted by string equations. The researchers recognized an opportunity. So far, string theory’s predictions have proved impossible to test with laboratory technology, but the analysis of black holes could help anchor it to the real world. Such work could also clarify what happened very early in the history of the universe. This conceptual breakthrough helped spawn a startling new field of study, string cosmology.
Stephen Hawking of the University of Cambridge and other like-minded researchers quickly found grand significance in the esoterica of string theory. For instance, the theory might explain why the expansion of the universe seems to be accelerating rather than diminishing. Most cosmologists latched on to an answer rooted in Einstein’s general theory of relativity, which states that gravity could be repulsive over long distances if the universe were permeated with an exotic form of energy. Now some theorists propose instead that the cosmos is speeding up because gravity grows weaker at huge distances due to a kind of leakage into the higher dimensions anticipated by string theory.
String theory may also offer an alternative picture of the first moments of existence. According to a leading cosmological model called inflation, the entire visible universe expanded wildly just after the Big Bang, growing almost instantaneously from a speck far smaller than a proton to a ball the size of a grapefruit, when the universe was 10-35 second old. No one has satisfactorily answered why inflation occurred. String theory has recently inspired a fresh approach. Paul Steinhardt of Princeton University and others speculate that our three-dimensional universe is part of a much larger, higher-dimensional reality and that the Big Bang is the result of a collision between our three-dimensional universe and another like it. The energy of this collision could account for many of the observed features of the universe without requiring an unexplained episode of inflation.
