NASHVILLE, Tenn. – In the last few decades, scientists have
discovered that there is a lot more to the universe than meets the eye:
The cosmos appears to be filled with not just one, but two invisible
constituents-dark matter and dark energy-whose existence has been
proposed based solely on their gravitational effects on ordinary matter
and energy.
Now, theoretical physicist Robert J. Scherrer has come up with a
model that could cut the mystery in half by explaining dark matter and
dark energy as two aspects of a single unknown force. His model is
described in a paper titled "Purely Kinetic k Essence as Unified Dark
Matter" published online by Physical Review Letters on June 30 and
available online at http://arxiv.org/abs/astro-ph/0402316.
"One way to think of this is that the universe is filled with an
invisible fluid that exerts pressure on ordinary matter and changes the
way that the universe expands," says Scherrer, a professor of physics
at Vanderbilt University.
According to Scherrer, his model is extremely simple and avoids the
major problems that have characterized previous efforts to unify dark
matter and dark energy.
In the 1970s, astrophysicists postulated the existence of invisible
particles called dark matter in order to explain the motion of
galaxies. Based on these observations, they estimate that there must be
about 10 times as much dark matter in the universe as ordinary matter.
One possible explanation for dark matter is that it is made up of a new
type of particle (dubbed Weakly Interacting Massive Particles, or
WIMPs) that don’t emit light and barely interact with ordinary matter.
A number of experiments are searching for evidence of these particles.
As if that weren’t enough, in the 1990s along came dark energy,
which produces a repulsive force that appears to be ripping the
universe apart. Scientists invoked dark energy to explain the surprise
discovery that the rate at which the universe is expanding is not
slowing, as most cosmologists had thought, but is accelerating instead.
According to the latest estimates, dark energy makes up 75 percent of
the universe and dark matter accounts for another 23 percent, leaving
ordinary matter and energy with a distinctly minority role of only 2
percent.
Scherrer’s unifying idea is an exotic form of energy with
well-defined but complicated properties called a scalar field. In this
context, a field is a physical quantity possessing energy and pressure
that is spread throughout space. Cosmologists first invoked scalar
fields to explain cosmic inflation, a period shortly after the Big Bang
when the universe appears to have undergone an episode of
hyper-expansion, inflating billions upon billions of times in less than
a second.
Specifically, Scherrer uses a second-generation scalar field, known as a
k-essence,
in his model. K-essence fields have been advanced by Paul Steinhardt at
Princeton University and others as an explanation for dark energy, but
Scherrer is the first to point out that one simple type of k-essence
field can also produce the effects attributed to dark matter.
Scientists differentiate between dark matter and dark energy because
they seem to behave differently. Dark matter appears to have mass and
to form giant clumps. In fact, cosmologists calculate that the
gravitational attraction of these clumps played a key role in causing
ordinary matter to form galaxies. Dark energy, by contrast, appears to
be without mass and spreads uniformly throughout space where it acts as
a kind of anti-gravity, a repulsive force that is pushing the universe
apart.
K-essence fields can change their behavior over time. When
investigating a very simple type of k-essence field-one in which
potential energy is a constant-Scherrer discovered that as the field
evolves, it passes through a phase where it can clump and mimic the
effect of invisible particles followed by a phase when it spreads
uniformly throughout space and takes on the characteristics of dark
energy.
"The model naturally evolves into a state where it looks like
dark matter for a while and then it looks like dark energy," Scherrer
says. "When I realized this, I thought, ‘This is compelling, let’s see
what we can do with it.’"
When he examined the model in more detail, Scherrer found that it
avoids many of the problems that have plagued previous theories that
attempt to unify dark matter and dark energy.
The earliest model for dark energy was made by modifying the general
theory of relativity to include a term called the cosmological
constant. This was a term that Einstein originally included to balance
the force of gravity in order to form a static universe. But he
cheerfully dropped the constant when astronomical observations of the
day found it was not needed. Recent models reintroducing the
cosmological constant do a good job of reproducing the effects of dark
energy but do not explain dark matter.
One attempt to unify dark matter and dark energy, called the
Chaplygin gas model, is based on work by a Russian physicist in the
1930s. It produces an initial dark matter-like stage followed by a dark
energy-like evolution, but it has trouble explaining the process of
galaxy formation.
Scherrer’s formulation has some similarities to a unified theory
proposed earlier this year by Nima Arkani-Hamed at Harvard University
and his colleagues, who attempt to explain dark matter and dark energy
as arising from the behavior of an invisible and omnipresent fluid that
they call a "ghost condensate."
Although Scherrer’s model has a number of positive features, it also
has some drawbacks. For one thing, it requires some extreme
"fine-tuning" to work. The physicist also cautions that more study will
be required to determine if the model’s behavior is consistent with
other observations. In addition, it cannot answer the coincidence
problem: Why we live at the only time in the history of the universe
when the densities calculated for dark matter and dark energy are
comparable. Scientists are suspicious of this because it suggests that
there is something special about the present era.
For more news about Vanderbilt research, visit Exploration, Vanderbilt’s online research magazine, at www.exploration.vanderbilt.edu.
Media contact: David Salisbury, (615) 343-6803
David.Salisbury@vanderbilt.edu