A refreshingly simple new idea has emerged in the complicated world of high energy physics. It proposes that the early universe was a one-dimensional line. Not an exploding sphere, not a chaotic ball of fire. Just a simple line of pure energy.
Over time, as that line grew, it crisscrossed and intersected itself more and more, gradually forming a tightly interwoven fabric, which, at large distances, appeared as a 2-D plane. More time passed and the 2-D universe expanded and twisted about, eventually creating a web the 3-D universe we see today.
This concept, called "vanishing dimensions" to describe what happens the farther one looks back in time, has been gaining traction within the high energy physics community in recent months. If correct, it promises to bridge the gap between quantum mechanics the physics of the very small and general relativity the physics of space-time. It would also make sense of the properties of a hypothetical elementary particle called the Higgs boson. And best of all, it would do so with elegant simplicity.
"In the last 30 years, [physicists] were trying to make our theories more complicated by introducing more particles, more dimensions," said Dejan Stojkovic, a physicist at the University of Buffalo who researches vanishing dimensions, said. "We decided to go the other way and make theories less complicated in the high energy realm. At high energy [in the early universe], we are changing the background on which the standard model of particle physics is formulated. In 1-D, the problem greatly simplifies."
According to the theory, for the first thousand-trillionth of a second after the Big Bang, up until the moment when the universe cooled to an average temperature of 100 teraelectronvolts (TeV are actually a measure of energy, but energy and temperature correspond), it was a 1-D line.
What would the young universe have been like?
Life on a line
"In 1-D, there's a new sense of unification," Stojkovic told Life's Little Mysteries. "Right now, you see the diverse world because you're in 3-D. When you go down to 1-D, things become much simpler. Properties that distinguish all the different particles don't exist anymore, so they all become alike. There is no rotation. All you have is forward and backward, and energy moving in either direction."
"As time goes on, the 1-D string universe evolves, intersecting itself many times to build a fabric," he said. The second dimension is built, and later, the third, in the same way that a 2-D sheet of paper can be folded to make a pop-up book. [Does the Universe Have an Edge? ]
But Stojkovic hasn't yet identified the mechanism that causes the universe to evolve as time passes. "We need to explain what caused the evolution from different energies to happen. You need a precise model that starts with a string and creates higher dimensions as it evolves in time to create the space-time we see today." In its skeletal form, Stojkovic calls vanishing dimensions a framework rather than a theory. "As a framework, it's beautiful. But we need to work out the details," he said.
Put to the test
Unlike string theory, a similarly beautiful conceit that describes the architecture of the universe, the vanishing dimensions framework may be verifiable through experimentation: This month, Stojkovic and Jonas Mureika, a physicist at Loyola Marymount University in Los Angeles, have published the first peer-reviewed article on the topic in the prestigious journal Physical Review Letters, and in it they lay out an experiment designed to test whether the early universe really was one-dimensional.
The experiment involves gravity waves faint oscillations thought to emanate from massive objects and travel through space-time. Gravity waves have never been detected, but their existence is predicted by the standard model of particle physics, and physicists hope to observe them within the next decade using a network of satellites in space. [Is There Gravity In Space? ]
Gravity waves carry an energy signature of the objects that created them. If Stojkovic is right, then no gravity waves should exist from before the time the universe became three-dimensional.
"Gravity waves don't travel in less than three spatial dimensions," Stojkovic told Life's Little Mysteries. "If you go down to two dimensions, gravity waves don't exist. Neither do they exist in one dimension."
"If our proposal is correct, the crossover from 2-D to 3-D happened when the energy of the universe cooled to 1 TeV," Stojkovic said. That happened one-trillionth of a second after the Big Bang . "When the early universe was 1 TeV hot, it transitioned from 2-D to 3-D, and at that point gravity waves began to be produced only after that crossover, not before," he said. An absence of gravity waves with associated energies greater than 1 TeV would give this theory weight.
When future satellites measure the frequencies (and corresponding energies) of gravity waves, Stojkovic hopes that they'll see a frequency cutoff. "There would be a cutoff in frequencies above which you don't measure gravity waves, corresponding to the transition from 2-D to 3-D," Stojkovic said. If these instruments identify the cutoff that Stojkovic predicts, vanishing dimensions will get a big boost.
Some physicists object to the premise of the experimental test; namely, that gravity waves will cut off above a certain frequency. "There is gravitational radiation at all frequencies," high energy physicists Thomas Sotiriou, at the University of Cambridge, and Silke Weinfurtner, at the SISSA Institute in Italy, wrote in an email. "This is not to say that this gravitational radiation will not carry some imprint of the vanishing dimensions," they explained but not in the way Stojkovic and Mureika have laid out. "It would not be a generic absence of any radiation over a certain frequency, as Stojkovic and Mureika suggest."
Sotiriou and Weinfurtner also object to the lack of an underlying mechanism to explain the evolution of the universe and the emergence of dimensions. "The [PRL] Letter by Stojkovic et al. is quite vague," they wrote. "They refer to vanishing dimensions at high energies and in the context of gravity but they practically say nothing specific about the mechanism via which this would be achieved."
"The idea of vanishing dimensions is quite interesting and potentially fruitful, as long as one clarifies exactly what is meant by 'vanishing dimensions.' Without a concrete, mathematically well-defined model of how dimensions will vanish, one cannot say much," Sotirious and Weinfurtner wrote. Along with Matt Visser of Victoria University in New Zealand, have presented their views on vanishing dimensions in an article posted to the physics arXiv.
Cosmic ray hints
Vague as the concept may be, there may be one hint of evidence in favor of vanishing dimensions already. "When cosmic rays collide with particles in the atmosphere, this creates a shower of other particles," Stojkovic said. "That shower looks like a cone. And as you can imagine, a cross-sectional slice of the cone looks like a circle." [What Are Cosmic Rays?]
"Well, it looks like the highest-energy cosmic ray collisions are instead planar, meaning they happen in 2-D rather than 3-D," he said. Dimensions seem to vanish for particle collisions that are as energetic as the early universe. In two dimensions, "a cosmic ray hits a particle, then creates a shower of particles that travel out in a circle. A slice of the circle looks like a line, and that's what detectors very high up in the atmospheres have seen."
Experiments at the Large Hadron Collider ought to be able to probe high-enough energies to see the same 2-D realm, he said. "The LHC should see the same alignment. The particle events should align on a plane."
If that happens, the new vanishing dimensions framework will gain more traction, and the beautifully simple picture of the early universe will come into greater focus.
Follow Natalie Wolchover on Twitter @nattyover.
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Natalie Wolchover was a staff writer for Live Science from 2010 to 2012 and is currently a senior physics writer and editor for Quanta Magazine. She holds a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Along with the staff of Quanta, Wolchover won the 2022 Pulitzer Prize for explanatory writing for her work on the building of the James Webb Space Telescope. Her work has also appeared in the The Best American Science and Nature Writing and The Best Writing on Mathematics, Nature, The New Yorker and Popular Science. She was the 2016 winner of the Evert Clark/Seth Payne Award, an annual prize for young science journalists, as well as the winner of the 2017 Science Communication Award for the American Institute of Physics.