Weekend Feature: NASA’s New Search for Invisible Dimensions of the Universe

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A new theory that revolutionizes the way we think about early universe cosmology says that 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 beauty of this new theory is that, unlike string theory, it can be tested with the next generation of space telescopes, NASA's LISA (Laser Interferometer Space Antenna) that will be able to scan the universe for gravitational waves and use them to study the astrophysics of dark objects that are invisible to all other telescopes, according to a new study March 11 in Physical Review Letters.

LISA is a joint NASA–ESA mission to observe astrophysical and cosmological sources of gravitational waves of low frequencies (0.03 mHz to 0.1 Hz, corresponding to oscillation periods of about 10 hours to 10 seconds). This frequency band contains the emission from massive black-hole binaries that form after galactic mergers; the "song" of compact stellar remnants as they slowly spiral to their final fate in the black holes at the centers of galaxies; and, according to NASA, the "chorus of millions of compact binaries in our own Galaxy; and possibly the faint whispers of waves generated shortly after the Big Bang."

LISA consists of three identical spacecraft flying in a triangular constellation, with equal arms of 5 million kilometers each. As gravitational waves from distant sources reach LISA, they warp space-time, stretching and compressing the triangle. Thus, by precisely monitoring the separation between the spacecraft, we can measure the waves; and by studying the shape and timing of the waves we can learn about the nature and evolution of the systems that emitted them.
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, but can’t explain why the expansion of the universe is accelerating 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 these new theories are yet to be supported by experimental proof.

“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. “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.

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 scientists claim to have already seen such signatures in cosmic ray collisions high in the upper atmosphere. But those results may be difficult to interpret, because different theoretical models have different predictions.

“This is a surefire way,” Mureika told wired.com. “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.

“Detecting Vanishing Dimensions via Primordial Gravitational Wave Astronomy.” Jonas Mureika and Dejan Stojkovic. Physical Review Letters, Vol. 106 No.10, Week of March 11, 2011. DOI: 10.1103/PhysRevLett.106.101101.

“Vanishing Dimensions and Planar Events at the LHC.” Luis Anchordoqui, De Chang Dai, Malcolm Fairbairn, Greg Landsberg, and Dejan Stojkovic. Published online on arXiv.org.

The Daily Galaxy via Physical Review Letters and wired.com

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