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Dark Matter and Dark Energy: One and the Same?

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Dark matter and dark energy are two of the most vexing problems in science today. Together they dominate the universe, comprising some 96 percent of all mass and energy.

But nobody knows what either is. It's tempting to consider them products of the same unknown phenomenon, something theorist Robert Scherrer suggests. The professor of physics at Vanderbilt University says "k-essence" is behind it all.

Dark matter was invoked decades ago to explain why galaxies hold together. Given regular matter alone, galaxies might never have formed, and today they would fly apart. So there must be some unknown stuff that forms invisible clumps to act as gravitational glue.

Dark energy hit the scene in the late 1990s when astronomers discovered the universe is not just expanding, but racing out at an ever-faster pace. Some hidden force, a sort of anti-gravity, must be pushing galaxies apart from one another in this accelerated expansion.

Separate theories have been devised to try and solve each mystery.

To explain dark energy, for example, theorists have re-employed a "cosmological constant" that Einstein first introduced as a fudge factor to balance the force of gravity. Einstein called the cosmological constant a great blunder and retracted it. Yet many theorists now are comfortable re-employing it to account for the effects of dark energy. But it does not reveal what the force is.

Scherrer agrees two explanations might be necessary, but he's also bothered by that complexity.


"It is somewhat embarrassing to have two different unknown sources for the dominant forms of matter and energy in the universe," he said in an e-mail interview. "On the other hand, that may just be the way things are. We don't get to pick the universe we live in."

To explain it all in one fell swoop, Scherrer invokes an exotic form of energy called a scalar field. It's a bit like an electric or magnetic field, with energy and pressure and a magnitude. But a scalar field has no direction. A scalar field is thought to have been behind inflation, the less-than-a-second period after the Big Bang when the universe expanded many billions of times before settling into a more reasonable rate of growth.

Scherrer borrows from work by Princeton University's Paul Steinhardt, V. Slava Mukhanov at the University of Munich and Christian Armendáriz Picón of the University of Chicago, relying on a specific type of second-generation scalar field they envisioned called k-essence, short for kinetic-energy-driven quintessence.

K-essence changes behavior over time in Scherrer's model, clumping early on to help form galaxies, and now forcing the universe apart. Right now, dark matter has a density that decreases as the universe expands, he explained, while dark energy has a density that stays constant as the universe expands.

"That means that at very early times, the dark matter 'piece' of the k-essence is the dominant one," Scherrer said. "As the universe expands and the density of the dark matter 'piece' of the k-essence decreases, it eventually falls below the density of the dark energy 'piece,' and the k- essence behaves more like dark energy."

Scherrer's model -- not the first trying to tie dark energy and dark matter together -- was published July 2 in the online version of the journal Physical Review Letters.

Glaring problem

There is one glaring problem with the idea, which Scherrer admits to. It implies that we live at a very special moment in time when the energy densities of dark matter and dark energy are roughly equal.

Scientists hate coincidences.

The model also "needs some serious fine-tuning, because its 'dark matter' is not permanent but transient," said Mario Livio, senior scientist at the Space Telescope Science Institute.

"There have been quite a few attempts to find one explanation to both dark matter and dark energy," Livio told "In particular, there have been theories with modified gravity."

One example, he said, is to confine all the forces of our universe to a four-dimensional plane known as a membrane, or "brane," which is sandwiched between other branes. Then let gravity escape to a fifth dimension that's perpendicular to the plane, Livio explains. The effects of dark matter are then the gravitational influence of other branes on ours.

(Branes have also been used by Steinhardt and other colleagues to put a twist on the Big Bang, in which another brane collided with ours, releasing energy and heat and leading to the expansion of our universe.)

Connections are great. But that doesn't mean they are right. Exotic but widely popular "string theories" of the universe explain dark matter as "supersymmetric particles" that bear no relationship to dark energy, Livio points out. Serious light might be shed on dark matter around 2007, when a particle accelerator called the Large Hadron Collider will reach the necessary energies to produce supersymmetric particles, if they exist.

Dark energy, which many experts say is likely to remain mysterious for decades, might involve an outside "vacuum energy" that acts upon our universe, which many theorists suspect is just one amid many.

"In spite of the appeal of combining dark matter with dark energy," Livio says, "it is quite possible that the two things do not need to be related."


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