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Week
of April 7, 2001; Vol. 159, No.
14

A Dark Force in the Universe

Scientists try to determine what's revving up the
cosmos

Ron Cowen

Three years ago, observations of distant, exploding
stars blew to smithereens some of astronomers' most
cherished ideas about the universe. To piece together an
updated theory, they're now thinking dark thoughts about
what sort of mystery force may be contorting the cosmos.

Observations of distant
supernova, including 1997ff, suggest that over the
past few billion years, a mysterious substance
called dark energy has caused gravity, at its
largest scale, to become repulsive. When the
universe was smaller and the density of matter
therefore higher, dark energy would have had a
negligible effect. Gravity would have exerted its
familiar universal attraction, slowing cosmic
expansion.
Z.
Levay/Space Telescope Science
Institute

According to the standard view of cosmology, the once
infinitesimal universe has ballooned in volume ever
since its fiery birth in the Big Bang, but the mutual
gravitational tug of all the matter in the cosmos has
gradually slowed that expansion.

In 1998, however, scientists reported that a group of
distant supernovas were dimmer, and therefore farther
from Earth, than the standard theory indicated. It was
as if, in the billion or so years it took for the light
from these exploded stars to arrive at Earth, the space
between the stars and our planet had stretched out more
than expected. That would mean that cosmic expansion has
somehow sped up, not slowed down. Recent evidence has
only firmed up that bizarre result (SN: 3/31/01, p.
196).

In 1929, Edwin P. Hubble discovered that distant
galaxies are fleeing from one another as if the entire
universe is swelling in size. Ever since, astronomers
have been hoping to answer a key question: Will the
expansion of the universe, slowed by gravity, go on
forever, or will the cosmos eventually collapse into a
Big Crunch?

Despite decades of effort and countless studies
devoted to the ballooning of the universe, the recent
findings stunned astronomers. Few suspected that all
along they were asking the wrong question.

"For 70 years, we've been trying to measure the rate
at which the universe slows down. We finally do it, and
we find out it's speeding up," says Michael S. Turner of
the University of Chicago.

An accelerated expansion would seem to contradict all
common sense, says Andreas J. Albrecht of the University
of California, Davis. Throw a ball into the sky, and
after it reaches a certain height, it will come back
down, he notes. Now imagine throwing another ball upward
and finding that instead of it falling back down, it
somehow keeps moving up faster and faster. For that to
happen, there would have to be some force pushing upward
on the ball strongly enough to overcome gravity's
downward tug.

Astronomers have come to believe that just such a
force is stretching the very fabric of space.

What is this mystery force?

Cosmologists have proposed that it derives from dark
energy—a substance whose properties and origin
scientists have only begun to explore. At stake is more
than just a better understanding of the fate of the
universe: The very presence of dark energy may enable
scientists to explain the fundamental forces of the
universe and tease out the hidden connections among
them.

Says Albrecht: "This is the most exciting endeavor
going on in physics right now."

Dark matter

Astronomers have dark imaginations. They're already
obsessed with another phenomenon that they call dark
matter, which is entirely separate from dark energy.
Dark matter is the invisible material that theorists say
makes up 95 percent of the mass of the universe. It
gathers into vast clumps and exerts an ordinary
gravitational tug on its surroundings. The proposed dark
energy, in contrast, would inhabit empty space and would
be evenly distributed throughout the universe.

Moreover, dark energy would exert a repulsive
force—what might be called antigravity. More accurately,
dark energy would be the flip side of ordinary gravity
because it would possess a strange property called
negative pressure. Something with negative pressure
resists being stretched, as a coiled spring does: Pull
on the spring and it pulls back.

To understand what pressure—negative or positive—has
to do with gravity, take a look at Einstein's general
theory of relativity. According to that theory, matter
isn't the only source of gravity. There are two other
sources: energy, which is interchangeable with mass
according to Einstein's famous equation E =
mc2, and pressure.

A familiar example of pressure is an inflated
balloon. In this everyday experience, pressure within
the balloon has a negligible effect on its gravity. At
physical extremes, however, pressure can dominate. When
that occurs, some strange things can happen, such as the
formation of black holes.

Pressure prevents a star as massive as the sun from
imploding under its own gravity. That's because the
radiation emitted by the star exerts a gaslike pressure
outward.

Stars more massive than the sun must exert an even
stronger pressure to counterbalance their gravity. For a
star greater than about four times the sun's mass, the
counterbalancing pressure becomes as strong as the
density of the star. When this happens, pressure
contributes as much as mass does to the gravitational
force, Einstein's theory says. In effect, the
gravitational pull inward drastically increases.

The more the star contracts, the greater its pressure
and density, and thus the stronger the gravity. Unable
to resist, the star undergoes a runaway collapse, and
its gravity becoming so strong that not even light can
escape its grasp. A black hole is born.

The contribution of pressure is "an aspect of gravity
that was there all along," notes Turner. He says that
anyone who accepts the reality of black holes has
implicitly accepted the notion that pressure can be a
key source of gravity.

According to Einstein's theory, pressure has another
mind-bending property: It can be negative. An object
having negative pressure resists being stretched. "Think
of negative pressure as silly putty or a rubber sheet.
The atoms don't want to be drawn apart; there's a force
that pulls them together," says Turner. Negative
pressure, he notes, would impart a springiness or
elasticity to space.

It's counterintuitive to think that a material such
as rubber, which draws itself inward when stretched,
could push objects outward. Yet if dark energy's
antigravity effect—it's ability to exert negative
pressure—were strong enough, it could swing the gravity
meter from the plus side to the minus side, Einstein's
theory dictates.

Gravity normally pulls matter together. Instead of
pulling, dark energy would cause gravity to push.
Instead of tugging and slowing the expansion of the
universe, dark energy would rev it up.

As bizarre as dark energy may seem, it's the only
theory to explain the accelerating cosmos that is
compatible with Einstein's general theory of relativity,
says Turner.

Dark energy

In its simplest version, dark energy would be a true
constant, equally distributed throughout the universe
and continuously exerting the same amount of force as
the universe expands. In 1917, Einstein posited a
version of this energy, which he called the cosmological
constant. Physicists have sporadically been returning to
that idea ever since. Because the cosmological constant
would exist even in the absence of matter or radiation,
its origins might lie within empty space itself.

This property could tie dark energy to one of the
stranger properties of quantum mechanics. Quantum theory
dictates that empty space—what physicists call the
vacuum—seethes with energy as pairs of particles and
antiparticles pop in and out of existence.

This vacuum energy has some subtle but measurable
effects. For example, it shifts the energy levels of
atoms slightly and exerts a force between closely spaced
metal plates (SN: 2/10/01, p. 86). In 1967, the Russian
astrophysicist Yakov B. Zeldovich showed that vacuum
energy has an intriguing property. The energy associated
with this nothingness has negative pressure.

That means vacuum energy could push galaxies apart at
ever-increasing speeds, making it an ideal candidate for
being the dark energy.

Alas, there appears to be a huge problem.
Calculations reveal that the energy stored in the vacuum
is 120 orders of magnitude larger than the dark energy
that cosmologists are positing.

"If the vacuum energy density really is so enormous,
it would cause an exponentially rapid expansion of the
universe that would rip apart all the electrostatic and
nuclear bonds that hold atoms and molecules together,"
note Paul J. Steinhardt of the University of
Pennsylvania in Philadelphia and Robert R. Caldwell of
Dartmouth College in Hanover, N.H., in a recent review
article. "There would be no galaxies, stars, or life."

It's likely, physicists admit, that they don't really
know how to calculate vacuum energy. That complication
may have to do with their limited knowledge about the
nature of gravity. Einstein's theory holds that gravity
curves empty space—the vacuum—but scientists don't yet
know how gravity does so on a quantum mechanical scale.

Thus, scientists have yet to unify quantum theory
with gravity. Some hold out the hope that when they do,
they'll miraculously find that the 120 orders of
magnitude drop to zero—almost. There might be just
enough vacuum energy left over to account for the amount
harbored by dark energy.

Many researchers think that's a forlorn hope,
however. They believe that a better understanding of the
vacuum energy will reveal it to be exactly zero.

In that case, dark energy would have to be something
else. Several theorists believe this something else
blankets the universe and varies with time and place.
Steinhardt, his University of Pennsylvania colleague
Rahul Dave, and their collaborators call this variable
form of dark energy "quintessence."

Quintessence takes on a different form and strength
depending on what time it is in the universe. Scientists
have established that just after the Big Bang,
high-energy radiation filled the universe and was the
dominant form of energy. Matter contributed very little
to the cosmic-energy budget. In that era, quintessence
would have mimicked the properties of radiation,
Steinhardt says. Like radiation, it would have exerted
positive pressure.

As the universe cooled and particles slowed, the
energy balance shifted in favor of matter. Material
started to clump together to form larger structures.
Steinhardt proposes that at the onset of that era, some
50,000 years after the Big Bang, quintessence changed.
As he and his colleagues see it, quintessence—dark
energy—settled down to a fixed value and began exerting
a negative pressure throughout the cosmos.

In this vision, the dark-energy density initially
paled in comparison with the density of matter. Gravity
thus acted in its familiar fashion, slowing the
expansion of the universe. But as the volume of the
universe continued to expand, its matter density
decreased. As matter density dwindled, the energy
density associated with quintessence remained
constant—or nearly so. Consequently, quintessence became
gravity's new boss. The expansion of the cosmos would
then have gone into overdrive.

It's no coincidence that humans are living at a time
when it's possible to observe cosmic acceleration, says
Steinhardt. The same shift in the mass-energy balance
that gave rise to stars, galaxies, planets, and life
also transformed quintessence into a cosmic accelerator.

Steinhardt admits he hasn't come up with any
fundamental explanation of why the quintessence field
would change in this way. The answer, he says, could lie
in new physics, perhaps in a new elementary particle
implied by quintessence. The explanation could also
provide a hint about how physicists might tackle one of
their thorniest and most intriguing
challenges—explaining the existence of the fundamental
forces and how they intertwine. Quintessence, or dark
energy, could be a linchpin that holds together both old
and new physics.

In a version of quintessence proposed by Albrecht and
his University of California, Davis colleague
Constantinos Skordis, the repulsive force may come from
other, unseen dimensions or even from other universes
beyond our own. That dovetails with a theory from
elementary particle physics, which posits that our three
dimensions plus time are but a tiny part of a much
broader, multidimensional canvas.

The extra dimensions wouldn't have a direct influence
on our own four-dimensional space-time. But because
gravity exerts itself by distorting space, the
gravitational field associated with the extra dimensions
might affect our own. Albrecht suggests that gravity's
ability to repel as well as attract could stem from the
existence of those other dimensions. Those dimensions in
turn could provide additional hints about another deep
puzzle of physics—the quantum nature of gravity, he
notes.

Albrecht says his theory offers another advantage. It
describes quintessence by using only simple constants of
nature, such as the speed of light, the gravitational
constant, and Planck's constant of quantum mechanics.
The quintessence field that he and Skordis construct
from these constants could indeed have become dominant
long after the Big Bang, prompting the current phase of
accelerated expansion.

Albrecht acknowledges the ad hoc nature of
quintessence theories, which are still in their infancy.
"We each have our own angles," he notes. "They all have
a lot of weaknesses."

Cosmic expansion

Several studies now in the works may enable
astronomers to confirm whether or not cosmic expansion
is accelerating. Moreover, the studies could also reveal
which of the two proposed forms of dark
energy—quintessence or vacuum energy—is driving that
acceleration. Astronomers think they can distinguish the
two types of dark energy because quintessence would give
the universe a smaller push.

"Dark energy" as envisioned by
Pokémon.
(c)
Wizards of America

If vacuum energy really is the dark energy, then the
universe will expand forever at an accelerating rate.

If quintessence proves correct, then the amount by
which space has stretched over the past few billion
years is less than if dark energy is the vacuum energy.
Because the volume of the cosmos is smaller in a
quintessential universe, supernovas up to a few billion
light-years from Earth would appear somewhat brighter
and fewer galaxies would exist within a given span of
cosmic time. Under the quintessence theory, the dark
energy varies in time and space, so determining the fate
of the cosmos isn't so straightforward.

Indeed, dark energy might even be a fleeting
phenomenon that gives the universe an extra kick for
several billion years and then disappears. In that case,
it could resemble an extended replay of inflation—the
brief, mysterious epoch of hyperexpansion that is
thought to have occurred during the earliest moments of
the universe (SN: 12/19 & 26/98, p. 392).

Dark energy "is involved in very fundamental issues,"
says Turner. "This could be a key to understanding the
forces of nature, including the quantum theory of
gravity."

Strange as dark energy seems, Turner notes, "I
guarantee you it's not going away."

References:

Albrecht, A., and C. Skordis. In
press. Phenomenology of a realistic accelerating
universe using only Planck-scale physics. Physical
Review Letters. Available at http://xxx.lanl.gov/abs/astro-ph/9908085.

Huterer, D., and M.S. Turner.
Preprint. Probing the dark energy: Methods and
strategies. Available at http://xxx.lanl.gov/abs/astro-ph/0012510.

Further Readings:

Cowen, R. 2001. Starry data support
revved-up cosmos. Science News 159(March 31):196.

______. 1998. The greatest story ever
told. Science News 154(Dec. 19&26):392.
Available at http://www.sciencenews.org/sn_arc98/12_19_98/Bob1.htm.

Weiss, P. 2001. Force from empty
space drives a machine. Science News 159(Feb.
10):86.

Sources:

Andreas Albrecht
Department of
Physics
University of California, Davis
One
Shields Avenue
Davis, CA 95616

Robert R. Caldwell
Department of
Physics and Astronomy
Dartmouth College
6127
Wilder Laboratory
Hanover, NH 03755-3528

Paul J. Steinhardt
Department of
Physics and Astronomy
University of
Pennsylvania
209 South 33rd Street
Philadelphia,
PA 19104-6396

Michael S. Turner
Department of
Astronomy and Astrophysics
University of
Chicago
Chicago, IL 60637-1433

From Science News, Vol.
159, No. 14, April 7, 2001, p.
218.

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