The universe may have started out with fewer dimensions than the three we live in, and could still collapse down to one dimension at extremely high energies.
The idea could solve some of the thorniest problems in particle physics and, unlike more popular models like string theory, can be tested with the next generation of space telescopes, according to a new study March 11 in Physical Review Letters.
The problems arise from the standard model of particle physics, which successfully explains most of the universe but breaks down as it reaches the high energies that existed shortly after the Big Bang. The standard model still can’t explain why the expansion of the universe is accelerating, for example, or how to knit together the physics of extremely large and extremely small objects.
Most theoretical physicists have assumed the limitations mean some strange new physics come into play at high energies, perhaps involving exotic new particles and extra, invisible dimensions of space. But none of the new theories are supported by experimental evidence.
“I think people are too attracted to these mainstream models,” said physicist Greg Landsberg of Brown University, who was not involved in the new study. “We may not be seeing the forest behind the trees because of that. We really need a new breakthrough, new experimental data to more forward in this field.”
Last year, Landsberg and colleagues suggested a simpler way to let the standard model live on at high energies: Have a universe with vanishing dimensions. If the hot infant universe had only one spatial dimension and acquired more as it expanded and cooled, some of the most intractable problems in physics disappear.
Landsberg suggests imagining this shrinking-dimensional universe as a woven tapestry. The tapestry depicts a three-dimensional scene, with people and landscapes in realistic perspective to one another. But as you get closer, the tapestry looks more and more like a flat, two-dimensional piece of fabric. Looking under a magnifying glass reveals that the whole thing is actually a one-dimensional piece of string, folded over on itself in complicated ways.
“You can think of the universe as a very very long string that just folded as the universe expanded,” Landsberg said.
“This revolutionizes the way we think about early universe cosmology,” said theoretical physicist Jonas Mureika of Loyola Marymount University in Los Angeles, lead author of the new paper. “It turns around the paradigm.”
In the new paper, Stojkovic and Mureika propose a way to test whether the universe had fewer dimensions when it was younger and more energetic. In our three-dimensional universe, moving massive objects give off ripples in the fabric of the universe called gravitational waves. NASA is planning a space telescope called LISA (Laser Interferometer Space Antenna) to scan the universe for gravitational waves and use them to study the astrophysics of dark objects that are invisible to all other telescopes.
But in two dimensions or fewer, gravitational waves mathematically cannot exist.
“There’s no way to get around the fact that gravity waves don’t exist if you have less than three dimensions of space,” Mureika said. “They just don’t.”
That means there shouldn’t be any gravitational waves at all from before the universe went 3-D. There’s some maximum frequency, matching a certain energy and time in the universe’s history, above which LISA should see nothing.
Mureika and Stojkovic showed that the frequency cutoff is about 0.0001 Hz, right in the frequency range that LISA is designed to sense.
There may be other ways to test for vanishing dimensions. If the products of particle collisions at the Large Hadron Collider appeared to be confined to a plane instead of jetting off in all directions, it could be a sign that high-energy particles are stuck in two dimensions. Some claim to have already seen such signatures in cosmic ray collisions high in the upper atmosphere. But those results may be hard to interpret, because different theoretical models have different predictions.
“This is a surefire way,” Mureika said. “If the underlying model is correct and the vanishing dimensions scenario is real, it would be an absolute signature of it.”
The fact that the new paper is published in a peer-reviewed journal suggests the vanishing dimensions paradigm is gaining traction, Landsberg says.
“What I find particularly spectacular is that they were able to draw very concrete experimental conclusions,” he said. ”This paper shows the community has finally started appreciating this new way of thinking.”
Image: An artist’s conception of LISA. Credit: NASA