Harvard physicist Qumran Wafa is one of the strongest proponents of string theory. But this summer, other string theorists lashed out at his latest proposal, which could discredit their ideas based on the decade-long assumption that dark energy is constant (constant). Wafa's work implies that the meaning of dark energy is changing. Fickle dark energy is a consequence of Wafa and his collaborators' attempt to apply string theory to a universe like ours, where the vacuum of space itself has some inherent energy.
If his hypothesis (and string theory itself) is true, dark energy, this mysterious substance, which accounts for more than 70% of the total mass and energy of the universe and which accelerates its expansion, will be the force of change. But persistent dark energy has long served as the basis for many ideas in string theory - so it is paradoxical that it is fickle dark energy that can lead to the theory's success.
Dark energy and string theory
“For the first time, we could learn something from string theory that can be measured,” says scientist Timm Wreiss of the Institute for Theoretical Physics at the Vienna University of Technology in Austria. "But I don't know if this will actually happen or not."
Let's start from the beginning: we live in a universe that seems to follow the rules. On the largest scales, large objects follow the rules of general relativity, interacting with each other through the force of gravity. On the smallest scale, subatomic particles follow the rules of quantum mechanics and quantum field theory, interacting through force fields that manifest themselves as force-carrying particles. But the mathematics doesn't add up when you try to explain general relativity as a huge extension of quantum field theory. A grander theory, string theory, tries to combine general relativity with quantum mechanics, and in it, each particle is represented by a tiny string, the vibrations of which in a more multidimensional space encode the properties that scientists observe.
However, theory is not entirely accurate. Rather, it is an overarching mathematical foundation, a framework from which scientists can derive theories about our universe, as well as the vast number of other permitted universes. String theorists hope our universe is one of those possibilities. Others believe that string theory is fundamentally wrong, but that's not the point now.
String theories have to explain a universe like ours in every aspect to be considered correct. Our universe, apparently, consists of 4% of matter (the substance that we see), 25% of mysterious dark matter, and the rest, as shown by observations in 1988, falls on "dark energy." String theorists have worked under the assumption that the strength of dark energy does not change, and their theories have evolved. But Wafa and his co-authors suggested in a paper this summer that in order to exist according to the rules of string theory, our universe must have a dark energy field whose value is decreasing.
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If the value of dark energy changes, this will be important for those whose theories rely on the assumption that dark energy is constant. “Maybe we need to get back to basics,” says Wreiss. It would also change the understanding of the evolution of the universe - both in the past and in the future.
Wafa's hypothesis was initially quite powerful and led to "tremendous excitement," says Wreiss. It served as a call to action for string theorists who felt the framework was under threat. Some immediately said it was nonsense - Stanford physicist Eva Silverstein told Quanta that the assumption was based on other assumptions, and the analysis was "highly questionable." Others use this document as an opportunity to verify that their theories can indeed describe a universe like ours.
Wreiss and others criticized the work of Wafa and his group, and their opinions were published in Physical Review D. Wreiss's work established that some of the alleged properties of our own universe, in particular those associated with the field of the Higgs boson, contradict essentially some mathematical assumptions. For example, the original assumption was that the behavior of the physical field governing dark energy is derived from a mathematical function with no highs and no lows, a line on a graph without peaks or lows. Vreiss found that the presence of a force field associated with the Higgs boson requires a peak in this function.
But Vreiss's work does not rule out Wafa's idea - Wafa simply refined the assumption so that it would be better applied to the universe we live in. There are other similar works, and Wafa agrees with the clarifications.
What's really interesting is that we may soon find out if Wafa's work offers an experimentally tested prediction of string theory. This would be the first provable consequence of string theory. Some experiments could test whether dark energy changes over time or remains constant, and may do so over the next few years.
So is a paradigm shift looming on the horizon? “Most scientists won't say this hypothesis is true or false,” says Wreiss. Wafa himself believes that he can of course be wrong, and this also speaks of the importance of string theory. “But if Wafa is right?” Vreiss says. "That would be the biggest thing in string theory to make a measurable prediction."
Ilya Khel