Dark Matter and Dark Energy: One and the Same?

Dark Matter and Dark Energy: One and the Same?

Mon Jul 12, 5:02 PM ET Add Science - Space.com to My Yahoo!

By Robert Roy Britt
Senior Science WriterSPACE.com

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.

'Embarrassing'

"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 SPACE.com. "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."

This article is part of SPACE.com's weekly Mystery Monday series.

--------------------------------------------------------------------------------
Copyright © 2004 SPACE.com.

LenderBroker wrote:

Dark Matter and Dark Energy: One and the Same?


Suggestion: Take your argument to news:sci.physics

Similar to you reasoning we can't see pink elephants traveling
faster than the speed of light either.

"The time has come," the Walrus said, "To talk of many things:
Of shoes, and ships, and sealing wax, of cabbages and kings.
And why the sea is boiling hot and whether pigs have wings."

-- L. Carroll

LenderBroker wrote:

Dark Matter and Dark Energy: One and the Same?


Suggestion: Take your argument to news:sci.physics

Similar to you reasoning we can't see pink elephants traveling
faster than the speed of light either.

"The time has come," the Walrus said, "To talk of many things:
Of shoes, and ships, and sealing wax, of cabbages and kings.
And why the sea is boiling hot and whether pigs have wings."

-- L. Carroll

Sam Wormley wrote:

LenderBroker wrote:

Dark Matter and Dark Energy: One and the Same?



Suggestion: Take your argument to news:sci.physics

What are you going on about? These appear to be articles from
space.com, not arguments...

Sam Wormley wrote:

LenderBroker wrote:

Dark Matter and Dark Energy: One and the Same?



Suggestion: Take your argument to news:sci.physics

What are you going on about? These appear to be articles from
space.com, not arguments...