A Dark Force in the Universe . Scientists try to determine what is revving up the cosmos.

Science News Online    Week of April 7, 2001; Vol. 159, No. 14
  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
  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
  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
  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
  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
  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
  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.
  [IMAGE]  “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.”    
  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
  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
  Weiss, P. 2001. Force from empty space drives a machine. Science News

  159(Feb. 10):86.
  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.
  Copyright (c) 2001 Science Service.  All rights reserved.
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