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.
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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=97a 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=97what 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=97has
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=97it's ability to exert
negative
pressure=97were 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.
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"Dark energy" as
envisioned by
Pok mon.
(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
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