Author: Selby Cull
Institution: Hampshire College
Date: June 2002
How much does nothing weigh?
Up until a few years ago, any reasonable person would have replied: "Nothing weighs nothing." Recently though, astronomers have been toying with the remarkable idea that nothing actually weighs something . and that this weight determines the structure, evolution, and fate of our universe.
The idea goes something like this: Empty space is not empty at all, but is filled with tiny particles that randomly pop into existence with their corresponding anti-particles, and then self-annihilate. What remains is a vacuum energy - the energy of empty space, or "dark energy." Unaffected by the regular matter we are so familiar with, or even the mysterious "dark matter" that appears to dominate the motions of galaxies, dark energy is embedded with perfect smoothness directly in the fabric of space, and its repulsive force is causing space to repel itself.
The roots of dark energy lie in the expansion of the universe. Directly following the Big Bang, when the universe was only 10-35 seconds old, it underwent a brief period of massive expansion. Since then, the expansion has continued, albeit at a slower rate. Cosmologists predicted that, because of the attractive effects of gravity, this expansion would eventually slow, and, if enough matter existed in the universe, it might even reverse itself and collapse in a "Big Crunch." Whether the universe expanded forever or collapses in a Big Crunch, cosmologists agreed that the expansion rate would slow with time.
If the universe was slowing down, then objects farther away from us (and therefore farther back in time) would be moving away from us at enormous speeds. However, in 1998, the Supernova Cosmology Project and the High-z Supernova Search Team discovered that supernovae in the far reaches of the observable universe are moving away from us at sluggish rates. This means they existed at a time when the universe was expanding more slowly than it is today - exactly the opposite result than cosmologists' predictions. The only explanation for this is that the expansion of the universe must be accelerating, and the only agent capable of causing this acceleration is dark energy.
Physicists still do not fully understand the properties of dark energy, but theories abound. The version that resonates best with observations is "quintessence," an invisible, variable energy field embedded in space that has caused the universe to undergo brief periods of rapid acceleration followed by periods of little to no acceleration. Aptly named, quintessence was the so-called "fifth element" in Greek mythology, and, in Rabelais' Gargantua, "Quintessence" was the name of the Queen of the Land of Speculative Science. For cosmology, quintessence is an enigmatic combination: the fifth fundamental force (along with the gravitational, electromagnetic, strong, and weak forces), and the height of speculation, since little is known about it.
One thing we know for certain, though, is that quintessence is a fundamentally fickle force. Early in the universe's history, quintessence was all-powerful, frantically driving inflation in the first moments after the Big Bang. For ages afterward, it lay quietly in space, struggling half-heartedly against gravity and allowing the universe to slowly expand. Today, however, quintessence seems to have returned to full-force, slapping gravity out of the way as it accelerates the universal expansion. As long as the density of quintessence remains constant - as long as the weight of nothing does not change - our universe will continue to expand, faster and faster, forever. Within 100 billion years, we would be able to see only a few other galaxies, perched on the rim of the observable universe. If, though, for any reason, quintessence changes again, this time to a negative value, the dark energy will yield to gravity and the universe will collapse.
A more constant form of quintessence was proposed by Albert Einstein in 1917 (his "cosmological constant") to explain how a universe dominated by his equations could be neither expanding nor contracting. Following Edwin Hubble's discovery in the 1920s that the universe was actually expanding, Einstein dismissed the cosmological constant as his "biggest blunder." Ironically, it now appears that the inconstant cousin of Einstein's cosmological constant is the driving force behind the expansion that Hubble observed.
In addition to providing a plausible mechanism for the expansion of the universe, quintessence may be the answer to many other questions that have plagued astronomers for years. For example, measurements of the cosmic microwave background radiation (a fingerprint of the early universe that was left behind only 300,000 years after the Big Bang) have indicated that we are living in a flat universe. The universe, however, does not have enough mass to have a flat geometry; in fact, it has less than half of the mass needed to reach the "critical density" at which it would not curve (Figure 1). It is now thought that dark energy may make up the missing 65%, allowing space to have its observed flat geometry.
While quintessence has astronomers and cosmologists all atwitter about the geometry of space and fate of the universe, this mysterious dark energy has physicists intrigued for a completely different reason. With a direct link to quantum mechanics and string theory, quintessence may be the best hope for establishing the long-sought-after "Theory of Everything" - the Holy Grail of modern particle physics. Dr. Andreas Albrecht of the University of California at Davis has recently devised the idea of "potentials," or energy functions, describing quintessence in terms of the speed of light, Planck's constant, and the gravitational constant. These potentials match equations resulting from superstring theory, establishing the first link between the two distinct camps of the quantum world and the cosmological world.
A host of problems and questions still haunt quintessence - as would be expected with any theory that proposes to resolve the fate of the universe, the geometry of space, the addition of a fifth fundamental force of nature, the origin of inflation, the union of the quantum and cosmological worlds, the "missing mass," and the acceleration of the universe. First of all, no one knows why quintessence has emerged again to speed up the inflation of the universe after so long a hibernation. Quintessence suggests that now is a crucial time in the history of the universe; however, cosmologists have traditionally fought to place our world (including our Earth, our Sun, and our time) away from any sort of special place in the universe - lest it fall prey to our own anthropocentric tendencies. Second, physicists have been unable to determine the density of the dark energy - their best estimates range from 120 times to 1055 times the density that cosmologists insist is necessary for a flat universe. The latter estimation is enough energy to prevent matter (galaxies, stars, atoms, lizards) from ever forming. Obviously, this is not what we see.
Despite its unanswered questions, quintessence appears to be the best theory we have to explain the acceleration of the universe and the geometry of space. This "energy of nothing" has evolved from Einstein's "biggest blunder" to one of the most challenging and exciting ideas of modern cosmology. Our conception of the fundamental forces of nature, our ideas of the structure of space, the first possible link between string theory and cosmology, and the fate of our universe all hinge on this mysterious dark energy, on the properties of empty space, on the weight of nothing.
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