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Posted Oct 8, 2018
"The discovery of dark energy has dramatically changed the way we think about the laws of nature," said Edward Witten, one of the world's leading theoreticians at the Institute for Advanced Study in Princeton, New Jersey.
In string theory, a paradigm shift could be imminent. In June, a team of string theorists from Harvard and Caltech released a new conjecture that seemed revolutionary: string theory would be fundamentally inconsistent with our current understanding of black energy – but only that energy can explain the ## 147 ## Accelerated expansion of our universe.
Timm Wrase of the Vienna University of Technology quickly became aware of the astonishing notion of this hypothesis: it seemed incompatible with the existence of the Higgs particle. His calculations, which he made with theorists from Columbia University in New York and from the University of Heidelberg, have now been published in Physical Review D. Challenging discussions on the strings and the Dark energy are currently being organized around the world. Wrase hopes that this will lead to further advances in this area of research.
Astronomers have thought that the expansion of the universe is regulated by both the force of gravity and a mysterious dark energy, as shown in the picture above. In the design of this artist, dark energy is represented by the purple grid at the top and the gravity by the green grid at the bottom. (NASA / JPL-Caltech)
Much hope has been placed in the string theory. It is supposed to explain how gravitation is related to quantum physics and how we can understand the laws of nature, which describe the physical world as a whole, from the smallest particles to the largest structure of the cosmos.
ESA Euclid's mission is expected to provide a better understanding of the fundamental issues of string theory. Credit: ESA / C. Carreau and CERN / J. Ellis, CC BY-SA 3.0
ESA Euclid's mission is expected to provide a better understanding of the fundamental issues of string theory.
Often, string theory has been accused of simply providing abstract mathematical results and making too few predictions that can be verified by an experiment. Now, however, the string theory community around the world is discussing an issue closely related to cosmic experiments measuring the expansion of the universe. In 2011, the Nobel Prize in Physics was awarded for the discovery that the universe is not growing bigger, but that this expansion is actually accelerating.
This phenomenon can only be explained by the hypothesis of an additional black energy, hitherto unknown. This idea came originally from Albert Einstein, who added it to his theory of general relativity as a "cosmological constant." Einstein really did this to build a non-expanding universe. When Hubble discovered in 1929 that the universe was expanding, Einstein described this change in his equations as the greatest fault of his life. But with the discovery of the accelerated expansion of the cosmos, the cosmological constant has been reintroduced as dark energy in the current standard model of cosmology.
"For a long time, we thought that such dark energy could be well integrated with string theory," says Wrase. String theory assumes that there are additional particles, hitherto unknown, that can be described as fields.
These fields have a minimal energy state, much like an apple in a bowl. It will always be at the bottom, at the lowest point of the bowl. Everywhere else, his energy would be higher. If we want to move it, we have to exercise energy. But that does not mean that the apple at the lowest point has no energy. You can put the bowl with the apple on the floor or on the top of the table. There, the apple has more energy but still can not move because it is still in a state of minimal energy.
"In string theory, there are fields that could explain black energy in the same way: locally, they are in a state of minimal energy, but their energy always has a value greater than zero" says Wrase. "So, these fields would provide the so-called dark energy, with which we could explain the accelerated expansion of the universe."
The constant was actually nil for profound reasons which, once revealed, would allow physicists to better understand what they call "the void", that is, the structure of reality. But what if the answer is not really zero?
Enter Cumrun Vafa from Harvard University, one of the world's most renowned string theorists, who published an article on June 25, raising many eyebrows. He suggested that such "cup-shaped" positive energy fields are not possible in string theory.
Wrase quickly understood the implications of this statement: "If this is true, the accelerated expansion of the universe, as we have imagined it to the present day, is not possible, "he says. "Accelerated expansion should then be described by a field having very different properties, such as an inclined plane on which a ball rolls downhill, losing potential energy."
But in this case, the amount of dark energy in the universe would change over time and the accelerated expansion of the universe could stop one day. Gravity could then gather all the materials and assemble everything at a point similar to that of the Big Bang.
But Tim Wrase, who had already dealt with similar issues in his doctoral dissertation, felt that this idea could not be the truth as a whole either. "The conjecture of Cumrun Vafa, which forbids certain types of fields, would also prohibit things we already know to exist," he explains.
Wrase was able to show that the Higgs field also has properties that should actually be prohibited by Vafa's conjecture – and the Higgs field is considered a proven fact. For his discovery, the 2013 Nobel Prize in Physics was awarded. Wrase has uploaded his results to the Arxiv pre-print website, prompting a great deal of discussion in the string theory community. The work has now been peer reviewed and published in the journal Physical Review.
"This controversy is a good thing for string theory," writes Timm Wrase. "Suddenly, many people have completely new ideas that nobody had thought of before." Wrase and his team are now studying which areas are allowed in string theory and when they violate Vafa's conjectures. "Maybe this brings us to exciting new insights into the nature of dark energy – it would be a great success," writes Wrase.
The hypotheses that emerge will (at least in part) soon be tested experimentally. Over the next few years, the accelerated expansion of the universe will be measured with unparalleled accuracy.
The Daily Galaxy via the Vienna University of Technology
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