# Universe corresponding to our current views may not be possible.

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## The new physical hypothesis challenges the leading "theory of everything"

On June 25, physicist Timm Vrase [Timm Wrase], who lives in Vienna, woke up and sleepily went through the list of recently published physical works. One headline struck him so much that he dropped all the rest of his sleep.

The work of the outstanding string theory specialist Kamran Wafa from Harvard, performed jointly with his colleagues, hypothesized the existence of a simple formula that determines which universes are allowed to exist and which are not, in accordance with string theory. String theory, the leading candidate for the “ theory of everything, ” stitching together gravity and quantum physics, defines all matter and interactions in the form of vibrations of tiny energy threads. The theory allows about 10

^{500}solutions: a huge and diverse "landscape" of possible universes. String theory specialists, such as Vraze and Wafa, have tried for years to place our Universe somewhere in this landscape of possibilities.

But for the time being, Wafa and his colleagues suggested that, on the landscape of string theory, universes like ours — more precisely, the way we had imagined it — cannot exist. If the theory is correct, as Vraze and other physicists immediately understood, then either our Universe is not at all what it should be, or the string theory is wrong.

After taking his son to a kindergarten, Vraze went to work at the Vienna Institute of Technology, where his colleagues vigorously discussed the same work. On the same day, Wafa, while in Okinawa in Japan, presented this theory at the Strings 2018 conference, which was followed by physicists from all over the world. The controversy unfolded both at the conference and elsewhere. “Many immediately said:“ This is probably a mistake, ”others said:“ Yes, I have been saying this for so many years, ”there were other, intermediate reactions,” says Vraze. He adds that there was also confusion, but “and, of course, great interest. Because if this hypothesis is true, it will have many huge consequences for cosmology. ”

The researchers sat down at work, trying to test the hypothesis and study its consequences. Vraz already wrote two worksone of which can lead to a refinement of the hypothesis, and made it, mainly being on vacation with the family. He recalls thinking: “It’s so interesting, I have to work it out and study it more deeply.”

The proposed formula, which appeared in the work of June 25 for the authorship of Wafa, Georges Obied, Hiroshi Ooguri, and Lev Spodynicko, and then studied more deeply in a subsequent work published two days later, authored by Wafa, Obied, Praetek Agrawala and Pla Steinhardt, in fact, says that with the expansion of the Universe, the vacuum energy density of empty space should decrease faster than a certain value. The rule should work in all simple models of universes based on string theory. But it contradicts two common opinions about the real Universe: it makes impossible both the generally accepted idea of its current expansion and the leading model of its explosive birth.

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Since 1998, observations from telescopes have shown that space is expanding faster and faster, from which it follows that the vacuum of empty space must be fueled by a dose of gravitationally repulsive "dark energy." In addition, apparently, the amount of dark energy poured into empty space remains constant (as far as can be judged).

But the new hypothesis states that the vacuum energy of the Universe should decrease.

Wafa and his colleagues prove that universes with stable and positive vacuum energy, known as de Sitter universescan not exist. Since the discovery of dark energy in 1998, string theory experts have been struggling to construct a convincing string model of de Sitter universes. But if Wafa is right, such attempts are doomed to be bogged down with logical inconsistencies; The de Sitter universes are not in this landscape, but in “swamps.” “I call things that look consistent, but that have contradictions, swamps,” he explained recently. “They are very similar to the landscape, they can deceive you. It seems to you that you will be able to construct them, but in reality this is not the case. ”

According to this “de Sitter swamp hypothesis,” in all possible, logical universes, the vacuum energy must either fall like a ball rolling downhill or must come to a stable negative value. (The so-called anti-Sitter universes, with stable and negative values of vacuum energy, are easy to construct in string theory).

If this assumption is true, it means that the density of dark energy in our Universe cannot be constant, and should take the form of a so-called. " quintessence"- energy source, gradually decreasing over tens of billions of years. Now it is expected to launch several experiments in which with the help of telescopes it will be established with better accuracy whether the Universe expands at a constant speed, or accelerated - that is, whether a proportional amount appears with new space A new dark energy, or cosmic acceleration is gradually changing according to quintessence models. The discovery of quintessence would revolutionize fundamental physics and cosmology, and rewrite b past and future of the cosmos. The universe with quintessential would not be broken big gap , but would gradually slow down, and, according to most models, the result would have been to expand and would have shrunk in more compressed or Big Bounce.

Steinhardt, a cosmologist from Princeton and one of Wafa’s co-authors, says that in the next few years “all eyes will be focused” on the results of such experiments as dark energy observation , the wide-range infrared telescope , and the Euclidean telescope from which it will be clear whether dark energy density. “If it becomes clear that the picture contradicts quintessence,” says Steinhardt, “it will mean that either the idea with the swamp is wrong, or the string theory is wrong, or both of them are wrong — in general, something should be wrong.”

The new hypothesis of the swamp expresses no less sharp doubts about the generally accepted history of the birth of the Universe: the theory of the Big Bang and cosmic inflation. According to this theory, a tiny particle of space-time, containing a huge amount of energy, quickly expanded and formed a macroscopic Universe in which we live. This theory, among other things, was invented to explain exactly how the universe became so huge, smooth and flat.

But the hypothetical inflationary energy field, which was supposed to fuel cosmic inflation, is not combined with the Wafa formula. To satisfy this formula, the energy of the inflation field had to be exhausted very quickly in order to get a smooth and flat Universe, as he and his colleagues explained. Therefore, their hypothesis contradicts many popular models of cosmic inflation. In the coming years, telescopes such as the Simonsov Observatory will look for final signs of cosmic inflation, comparing this theory with competing ones.

At this time, string theory experts, who usually act as a united front, disagreed about the hypothesis. Eva Silverstein, a professor of physics at Stanford University, the leader of the project on creating string models of inflation, considers it very likely that this theory will be wrong. Her husband Professor Stanford Shamit Kachru also thinks so; he is the first letter K in KKLT, the famous work of 2003, known by the initials of its authors, in which they proposed a set of string ingredients that can be used to create desiters universes. The Wafa Formula says that Silverstein and Kachru designs won't work. "These hypotheses have besieged our family," Silverstein jokes. But from her point of view, the models of accelerated expansion have lost nothing after the publication of new works. "They are, in fact, just claiming

Matthew Kleban, a string theory specialist and cosmologist at New York University, is also working on inflation string models. He emphasizes that the new swamp theory is very speculative, and is a classic example of the behavior of a drunk who searched for keys under a lamp, because it was brighter there, since most of the landscape of string theory has yet to be studied. But he admits that on the basis of existing evidence, the hypothesis may be true. “It may be true about string theory, and then it turns out that string theory does not describe the world,” says Kleban. And, perhaps, “dark energy refuted it. And it will obviously be very interesting. ”

Whether de Sitter’s swamp theory and future experiments will be able to disprove string theory will be revealed later. The discovery made in the 2000s, according to which string theory has about

^{10,500}solutions, has killed the hope that it can uniquely and inevitably predict the future of our Universe. The theory looks like it can support almost any observation, which is why it is very difficult to experimentally test or refute it.

In 2005, Wafa and a whole network of collaborators began to reflect on how to reduce this number of possibilities, marking out the fundamental properties of nature, which in any case should be true. For example, their theory of weak gravitysuggests that gravity must be the weakest of interactions in any logical universe. Theoretical universes that do not meet these requirements are thrown from the landscape into the swamp. Many of these swampy assumptions coped well with the attacking arguments, and some "are now on a very solid foundation," says Hiroshi Ooguri, a theoretical physicist at the California Institute of Technology, one of Wafa’s first colleagues on the wetland hypothesis. For example, the theory of weak gravity gained so much evidence that it is believed that it is generally correct, regardless of whether string theory turns out to be a suitable theory of gravity.

Intuition about where the landscape ends and the marsh begins, appears thanks to decades of attempts to build string models of universes. The main obstacle of this project is that string theory predicts the existence of 10 dimensions of space-time, which is much more than the four visible. String theory specialists suggest that the six additional dimensions must be small — tightly folded at each point. The landscape is filled with all possible ways to customize these additional dimensions. But although a huge number of opportunities, researchers, for example, Wafa, have discovered the emergence of generalized principles. For example, twisted measurements usually tend to shrink gravitationally, and such fields as electromagnetic, tend to push everything to the side. In simple, stable configurations, these effects are balanced by the negative energy of the vacuum, which leads to the emergence of anti-Sitter universes. Turning vacuum energy into positive is very difficult. “Usually in physics there are simple examples of phenomena of a general kind,” said Wafa. “But this does not apply to de Sitter’s model.”

The work of KKLT, sponsored by Kachru, Renata Kallosh, Andrei Linde and Sandip Trivedi, offers string traps such as fluxes, instantons and anti-d-branes, which in theory can be tools for tuning positive and constant vacuum energy. However, these structures are complex, and over the years, possible instabilities have been found in them. Although Kachru says that he has no “serious doubts,” many researchers began to suspect that the KKLT script still does not provide any stable de Sitter universes.

Wafa believes that a consistent search for a model of a uniquely stable De Sitter universe was long overdue. His hypothesis first of all should emphasize the importance of this problem. From his point of view, string theory experts do not feel sufficiently motivated to figure out whether string theory is capable of describing our world, instead of taking the point of view that, since the string landscape is huge, there will be a place for us, although no one knows exactly where. “Most of the representatives of the string theory community are still on the side of the existence of de Sitter constructions,” he says, “because everyone thinks: Look, since we live in a de Sitter universe with positive energy; therefore, we'd better look for examples of this type. ”

His hypothesis prompted the community to take action, and researchers, such as Vraze, began searching for counterexamples of stable DeSitter universes, while others play with little-studied string models of universes with quintessence. “In any case, I would be interested to know whether the hypothesis is correct or not,” said Wafa. - The question is what we should do. And we can achieve progress only by finding evidence in favor of or against the theory. ”

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