Image-Courtesy NASA/WMAP
The 13.7 billion year life of the universe, from Big Bang to the expanding
 universe driven by dark energy.

FEB 25, 2011

Dark energy may move from the depths of space and time into the laboratory to warp the action of individual atoms in a new experiment proposed last week. The universe-expanding powewer of the mysterious energy may finally be observed on earth with sensitive atom-measuring instruments to follow its tracks.

Current observations support dark energy as a strictly cosmological phenomenon, credited with driving the accelerating expansion of space. Astronomers traditionally examine the tremendous distance between events such as exploding stars to measure the effects of dark energy. But this new model may let them look much closer to home.

“Is the dark energy density uniform in all regions of space?” asked physicist Martin Perl, professor emeritus at Stanford Linear Accelerator Laboratory and one of the researchers proposing the experiment. “That’s what we’re looking for.”

Perl, who won the Nobel Prize in 1995 for his discovery of a fundamental particle of matter called the tau lepton, proposes that dark energy might exert its influence differently throughout the universe. This could change the current cosmic framework for studying the mysterious force.

“The observations are that it acts only on very large scales, and that it is constant throughout the universe,” said Richard Massey, an astronomy fellow at the Royal Observatory in Edinburgh.

But dark energy, believed to fill more than 70 percent of the universe, is an incredible unknown, never directly observed and little understood by scientists.

“It’s the most common ingredient in the universe,” said Massey, responding to the experiment proposal. “But we don’t know anything about it. It’s kind of embarrassing, really.”

“There’s a joint tug of war in the universe,” Massey said. “We blew apart at the Big Bang and are expanding. Mass is trying to pull us together.”

Gravity causes that more familiar tug on mass, the one binding humans to the earth and the earth to its orbit around the sun.
A Dark Energy Theory Correlated With Laboratory Simulations And Astronomical Observations   Dark Matter and Dark Energy: A Challenge for Modern Cosmology (Astrophysics and Space Science Library)   SuperNovae Observations and Dark Energy Models: Creating the link between observables and theory    KEK Cosmophysics Group Inaugural Conference "Accelerators in the Universe": Interplay between High Energy Physics and Cosmophysics (AIP Conference Proceedings / Astronomy and Astrophysics)   Sources and Detection of Dark Matter and Dark Energy in the Universe: Fourth International Symposium Held at Marina del Rey, CA, USA February 23-25, 2000
“Dark energy is doing the opposite: pushing everything apart,” Massey said.

Perl and his colleagues suggest that the dark contents of the vacuum, the name given to cryptic forces such as dark energy, may not behave the same between galaxies as between atoms. They hope to utilize that difference to make their observations.

The proposed experiment uses the extreme precision of atom interferometers, devices that measure subtle deviations in atomic motion, to detect this theoretical dark influence.

“I realized that it could be very sensitive,” Perl said. “More sensitive than any other present method to detect any kind of force that could be exerted on an atom.”

The basic setup is simple and precise: Drop a super cooled atom into a 1.5-meter vacuum chamber, fire a laser at the top to excite the atom into a split quantum state of extreme sensitivity, then fire a second laser at the bottom to recombine the atom.

The traveling atom is susceptible to all kinds of stimuli, from subtle magnetic fluctuations to the vibrations of the earth. To balance out interference and account for the tug of gravity, the experiment calls for a second identical interferometer to operate at precisely the same instant.

By comparing the two atomic journeys the physicists may detect the influence of new, never before observed forces, Perl said. “It’s a beautiful technique, very delicate.”

The atoms in the experiment will behave like two ping pong balls in free fall, weak to the whimsy of the wind. If only one suddenly and inexplicably changes course before reaching the bottom, something must have struck the ball. Further tests and precise measurements may determine exactly what moved one but not the other.

The interferometers work in a similar fashion, though virtually anything can shift an atom’s course.

The experimental chambers use the earth’s own orbital momentum to move the atom interferometers through space. The planet itself will cause the atoms to pass through the theoretical lumps of dark energy, causing deviation in one measurement but not the other.

“But you have to assume that the dark energy is not uniform,” Perl said. “If it’s uniform, then atom interferometry doesn’t work. You’re always looking for the atoms to follow different paths.”

The study hinges upon one other major assumption: that dark contents of the vacuum exert a new, non-gravitational influence on matter. Neither of those is evidenced by cosmological observations.

“Those two hinges are a bit squeaky,” Massey said. “Basically, I wouldn’t expect it to show anything.”

“I’m very impressed with the experimental setup,” Massey added. “It will be useful for all kinds of fundamental physics.”

“It’s a bit of a shot in the dark, in case we’re going down the wrong track with our theoretical models,” said Fergus Simpson, an astrophysicist at the University of Edinburgh. “But perhaps there is some serendipitous signal that we can pick up if we try something a bit different on the laboratory scale.”

“But certainly I would be very skeptical and very surprised if they found any interesting result,” Simpson said.

Dark energy observations usually rely upon astronomical observations, which support a homogenous force driving the universe’s expansion.

“If I were still teaching, the first thing I would do is teach the cosmological constant" drawn from Einstein's equations, Perl said, referring to a dominant mathematical explanation of dark energy’s push.

But this experiment may challenge the elegance of Einstein’s century-old work and its harmony with new discoveries in cosmology and dark energy research.

Embracing assumptions and conventions can lead to a dead end in science. And as evidenced by Einstein, novel approaches are often useful in unanticipated ways.

“Look, we’re just at the beginning of science – physics, biology everywhere,” Perl said. “In 100 years they’ll look back on us and say, ‘These guys and gals, they worked pretty hard. But they didn’t know very much.’”

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