Cosmologic philosophy: probably less multiverses (Introduction)

by David Turell , Monday, July 30, 2018, 20:26 (2089 days ago) @ David Turell

Recent studies have found that many possible universes are really not possible because parts of many of them are not compatible:

https://www.scientificamerican.com/article/string-theory-may-create-far-fewer-universes...

"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.

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"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.

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"If it is true string theory cannot accommodate stable dark energy, that may be a reason to doubt string theory. But to Vafa it is a reason to doubt dark energy—that is, dark energy in its most popular form, called a cosmological constant. The idea originated in 1917 with Einstein and was revived in 1998 when astronomers discovered that not only is spacetime expanding—the rate of that expansion is picking up. The cosmological constant would be a form of energy in the vacuum of space that never changes and counteracts the inward pull of gravity. But it is not the only possible explanation for the accelerating universe. An alternative is “quintessence,” a field pervading spacetime that can evolve.

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"String theory is incredibly appealing to many scientists because it is “beautiful”—its equations are satisfying and its proposed explanations elegant. But so far it lacks any experimental evidence supporting it—and even worse, any reasonable prospects for gathering such evidence. Yet even the suggestion string theory may not be able to accommodate the kind of dark energy we see in the cosmos around us does not dissuade some. “String theory is so rich and beautiful and so correct in almost all the things that it’s taught us that it’s hard to believe that the mistake is in string theory and not in us,” Sethi says. But perhaps chasing after beauty is not a good way to find the right theory of the universe. “Mathematics is full of amazing and beautiful things, and most of them do not describe the world,” physicist Sabine Hossenfelder of the Frankfurt Institute for Advanced Studies wrote in her recent book, Lost in Math: How Beauty Leads Physics Astray (Basic Books, 2018)."

Comment: Same old problem. String theory is not testable. But the debate rages.