The problem with string theory, according to some physicists, is that it makes too many universes. It predicts not one but some 10500 versions of spacetime, each with their own laws of physics. But with so many universes on the table, how can the theory explain why ours has the features it does? Now some theorists suggest most—if not all—of those universes are actually forbidden, at least if we want them to have stable dark energy, the supposed force accelerating the expansion of the cosmos. To some, eliminating so many possible universes is not a drawback but a major step forward for string theory, offering new hope of making testable predictions. But others say the multiverse is here to stay, and the proposed problem with all those universes is not a problem at all.
The debate was a hot topic at the end of June in Japan, where string theorists convened for the conference Strings 2018. "This is really something new and it’s led to a controversy within the field," says Ulf Danielsson, a physicist at Uppsala University in Sweden. The conversation centers on a pair of papers* posted on the preprint server arXiv last month taking aim at the so-called "landscape" of string theory—the incomprehensible number of potential universes that result from the many different solutions to string theory’s equations that produce the ingredients of our own cosmos, including dark energy. But the vast majority of the solutions found so far are mathematically inconsistent, the papers contend, putting them not in the landscape but in the so-called "swampland" of universes that cannot actually exist. Scientists have known many solutions must fall in this swampland for years, but the idea that most, or maybe all, of the landscape solutions might live there would be a major change. In fact, it may be theoretically impossible to find a valid solution to string theory that includes stable dark energy, says Cumrun Vafa, a Harvard University physicist who led the work on the two papers...
Continue reading "String Theory May Create Far Fewer Universes Than Thought" by Clara Moskowitz in Scientific American, July 30, 2018. https://www.scientificamerican.com/article/string-theory-may-create-far-fewer-universes-than-thought/.
Also read the research papers by Prof. Vafa and collaborators* and an article in Quanta Magazine.**
* Georges Obied, Hirosi Ooguri, Lev Spodyneiko, Cumrun Vafa, " De Sitter Space and the Swampland," arXiv:1806.08362, 17 July, 2018.
Prateek Agrawal, Georges Obied, Paul J. Steinhardt, Cumrun Vafa, "On the Cosmological Implications of the String Swampland," arXiv:1806.09718. 19 July, 2018.
** Natalie Wolchover, "Dark Energy May Be Incompatible With String Theory,"Quanta Magazine, August 9, 2018. https://www.quantamagazine.org/dark-energy-may-be-incompatible-with-string-theory-20180809/. (Professor Vafa notes that a more precise title could have been "Constant Dark Energy May Be Incompatible With String Theory.")