But now, Vafa and his colleagues were conjecturing that in the string landscape, universes like ours—or what ours is thought to be like—don’t exist. If the conjecture is correct, Wrase and other string theorists immediately realized, the cosmos must either be profoundly different than previously supposed or string theory must be wrong. [...]
The conjectured formula—posed in the June 25 paper by Vafa, Georges Obied, Hirosi Ooguri, and Lev Spodyneiko, and further explored in a second paper released two days later by Vafa, Obied, Prateek Agrawal, and Paul Steinhardt—says, simply, that as the universe expands, the density of energy in the vacuum of empty space must decrease faster than a certain rate. The rule appears to be true in all simple string-theory-based models of universes. But it violates two widespread beliefs about the actual universe: It deems impossible both the accepted picture of the universe’s present-day expansion and the leading model of its explosive birth. [...]
The conjecture, if true, would mean the density of dark energy in our universe cannot be constant, but must instead take a form called “quintessence”—an energy source that will gradually diminish over tens of billions of years. Several telescope experiments are underway now to more precisely probe whether the universe is expanding with a constant rate of acceleration, which would mean that as new space is created, a proportionate amount of new dark energy arises with it, or whether the cosmic acceleration is gradually changing, as in quintessence models. A discovery of quintessence would revolutionize fundamental physics and cosmology, including rewriting the cosmos’ history and future. Instead of tearing apart in a Big Rip, a quintessent universe would gradually decelerate, and in most models would eventually stop expanding and contract in either a Big Crunch or Big Bounce. [...]
Vafa thinks a concerted search for definitely stable de Sitter universe models is long overdue. His conjecture is, above all, intended to press the issue. In his view, string theorists have not felt sufficiently motivated to figure out whether string theory really is capable of describing our world, instead taking the attitude that because the string landscape is huge, there must be a place in it for us, even if no one knows where. “The bulk of the community in string theory still sides on the side of de Sitter constructions” existing, he says, “because the belief is, ‘Look, we live in a de Sitter universe with positive energy; therefore we better have examples of that type.’”
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