String Theorists' Heads Bobble Over Potential Dark Energy Wobble

This seemed about as good an image as any for a story about string theory. (Image: Daniel Schwen, Wikimedia Commons)

Harvard physicist Cumrun Vafa is one of string theory’s strongest proponents. But this winter, other string theorists have been reeling from his latest conjecture, which might invalidate their ideas built on a decade-long assumption that dark energy is constant. Vafa’s work implies that dark energy’s value changes.

A non-constant dark energy is a consequence of Vafa and his collaborators’ attempts to apply string theory to a universe like ours — one where the very vacuum of space appears to have some innate energy. If his conjecture (and string theory itself) is true, dark energy, the mysterious stuff that seems to make up more than 70 per cent of the universe’s total mass and energy and fuels its accelerated expansion, should be a changing force.

But a constant dark energy has long served as a backbone to many string theorists’ ideas — paradoxically, a non-constant dark energy could be a mark of the theory’s success.

“This would be the first time we got something new out of string theory that you could measure,” scientist Timm Wrase from the Institute for Theoretical Physics at TU Wien in Austria told Gizmodo. “But I don’t know whether it will happen.”

Let’s start from the beginning: We live in a universe that seems to follow rules. On the grandest scale, large objects follow the rules of general relativity, where they interact with one another through the force of gravity. On the smallest scale, subatomic particles follow the rules of quantum mechanics and quantum field theory, interacting via force fields that manifest themselves as force-carrying particles.

But the maths falls apart when you try to explain general relativity as an enormous extension of quantum field theory. A grander theory — string theory — attempts to unite the two, where each particle is actually a tiny string whose vibrations in a higher-dimensional space encode the properties that scientists observe.

“Theory” is sort of a misnomer — it’s more like an overarching mathematical framework from which scientists can derive theories about our universe as well as a mind-boggling number of other permitted universes. String theorists hope that our universe is among these possibilities. Others think string theory is incorrect to its very core, though I won’t get into that in this story.

String theories must explain a universe like ours in every way in order to be correct, of course. Our universe appears to consist of about four per cent matter (the stuff that makes up everything we see), 25 per cent mysterious dark matter, and the rest, since the 1998 observation of the universe’s expansion accelerating, “dark energy”.

String theorists have operated under the assumption that dark energy’s strength doesn’t change, and their theories have evolved in kind. But Vafa and his coauthors conjectured in a paper this winter that in order to exist by the rules of string theory, our universe must have a dark energy field whose value is decreasing.

“Whether dark energy changes or not is a serious thing,” Vafa told Gizmodo.

If dark energy’s value changed, well, that would be a big deal for those whose theories rely on the assumption that it was instead an innate constant. “We may have to go back to the basics,” Wrase said. It would also change our understanding of the universe’s evolution — both in the past and future.

Vafa’s conjecture was originally quite strong and led to “huge excitement,” said Wrase (you can read about that here), serving as a call-to-action for string theorists who felt the framework was under threat.

Some called it outright wrong — Stanford physicist Eva Silverstein told Quanta that the conjecture was rooted in speculation and that it cited “highly dubious” analyses. Others are using the paper as an opportunity to ensure that their theories can indeed describe a universe like ours.

Wrase and others have since presented a critical look at Vafa and his group’s work, most recently published in Physical Review D. Wrase’s paper finds that some of our own universe’s theorised properties, specifically those regarding the Higgs boson’s associated field, contradict some of the mathematics in the conjecture.

Think back to precalculus — the initial conjecture essentially said that the behaviour the physical field governing dark energy derived from a mathematical function with no maxima or minima, a line on a graph with no peaks or valleys. Wrase found that the presence of the force field associated with the Higgs boson necessitated a peak in that function.

But Wrase’s paper doesn’t rule out Vafa’s idea — Vafa felt it sharpened the conjecture to better apply to the universe we live in. There are other such papers that tighten the conjecture as well, and Vafa agreed with these refinements.

The real excitement comes from how soon we might know whether Vafa’s work has produced a testable prediction of string theory — which would be a first. Experiments such as the Dark Energy Survey or the upcoming WFIRST telescope could possibly detect whether dark energy is constant or changing over time, and could perhaps do so within the next few years.

So, is a paradigm-shifting discovery on the horizon? “Most scientists wouldn’t say the conjecture is wrong or right,” Wrase said. Vafa himself said that of course he could be wrong, which would also say something important about string theory.

“But it might also be spectacular if [Vafa] is right,” said Wrase. “It would be the biggest thing ever in string theory — to make a measurable prediction.”

[PRD]

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