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Quintessence, accelerating the Universe?

When it doubt, go back to the basics. That's just what cosmologists have done to explain why our universe seems to be accelerating.

The new buzz word in cosmology these days is 'quintessence', borrowed from the ancient Greeks who used the term to describe a mysterious 'fifth element' - in addition to air, earth, fire and water - which held the moon and stars in place. Quintessence, some cosmologists say, is an exotic kind of energy field that pushes particles away from each other, overpowering gravity and the other fundamental forces.

If quintessence is real, it certainly wouldn't be rare. Two-thirds of the universe would be made of the stuff. At the Texas Symposium on Relativistic Astrophysics in Austin in 2000, Paul Steinhardt of Princeton University explained how quintessence became the dominating force in the universe a few billion years ago, relatively recently, he says. Steinhardt didn't exactly warm over the crowd with his new theory.

Cosmology used to be a tranquil occupation. As recent as the late '90s, most folks were in agreement that, yes, the universe is expanding. At question was simply whether the expansion would slowly come to a halt and bring the universe falling back in on itself, or whether the universe would continue to float apart but at a slower and slower rate. If there was enough matter in the universe, then gravity would halt the expansion and suck all that we know into the 'big crunch'. All cosmologists had to do was to add up the mass in the universe.

But in 1998, cosmologists were shaken off their seats by the discovery that the universe is expanding at an astonishing rate. New and improved observations of distant supernovae were rendering the 'big crunch' question null and void.

Supernovae are star explosions, and there are a couple of varieties. One, called a Type Ia supernova, explodes with a characteristic energy. With a decent idea of the explosion's absolute and apparent brightness, astronomers can determine the distance to these objects. Then, knowing the redshift, they can calculate how fast the supernovae are moving away from us. Turns out that the most distant Type 1a supernovae are moving away much faster than closer ones, suggesting that the universe's expansion is actually accelerating, not decelerating.

There are a few non-believers, with good cause. Some say these most distant supernovae may just look far (that is, dim) because intervening dust scatters their light. Also, we cannot be certain that the most distant supernovae explode in the same way as closer ones.

Most cosmologists, however, have hopped on the accelerating universe train. Their job now is to explain how this can be physically possible. Shouldn't the force of gravity, the great attractor, keep the universe from flying apart?

Einstein thought about this, but for the wrong reason. He developed a fudge factor called the cosmological constant. Einstein, and everyone else in the early 20th century, thought the universe was static and that everything was contained within the Milky Way galaxy. The cosmological constant was an anti-gravity 'vacuum' force that kept gravity from pulling the universe in on itself. By 1930, Edwin Hubble discovered that the Milky Way was but one of a multitude of galaxies and that the universe was expanding. So, there was no longer a need for a cosmological constant. Einstein dropped the number from his equations, calling it his 'greatest blunder'.

The problem with the cosmological constant, Steinhardt says, is that it is indeed constant. It yields the same force throughout time. Observational evidence indicates that whatever this force is that is accelerating the universe, it hasn't been constant over time. There had to be periods when the force was negligible, otherwise stars and planets and chipmunks never would have formed.

"The cosmological constant is a very specific form of energy, a vacuum energy," Steinhardt said. "Quintessence encompasses a wide class of possibilities. It is a dynamic, time-evolving and spatially dependent form of energy with negative pressure sufficient to drive the accelerating expansion."

Vacuum energy is the potential energy in an absolute vacuum, devoid of matter or radiation. Think of a chimney sucking air from the living room; that's the universe's matter expanding into the great unknown. Quintessence is a quantum field with both kinetic and potential energy. Depending on the ratio of the two energies and the pressure they exert, quintessence can either attract or repel.

Quintessence became a force to be reckoned with about 10 billion years ago, according to the theory. That may seem rather early on in a 15 billion-year-old universe, but cosmologists don't see it that way. The dark energy was created when the universe was 10-35 second old; it did not cause the universe to accelerate for another five billion years. That's a factor of more that 1050 -- and relatively recently in terms of redshift and the size of the universe.

Steinhardt suggests that quintessence turned on during the transition from a radiation to matter-dominated universe, when it was cool enough for atoms and eventually stars to form.

But what is quintessence made of? No one knows for sure. Radiation, ordinary matter and likely dark matter all have positive pressure. They therefore exert a gravitationally attractive force. Anything with negative pressure, the general theory of relativity dictates, would have a gravitationally repulsive force.

For quintessence, the quantum field would have a very long wavelength, about the size of the universe. Its kinetic energy depends on the rate of oscillations in the field strength; its potential energy depends on the interaction of the field with matter. The more kinetic energy, the more positive the pressure - which isn't so likely for a universe-long wavelength. So for now, potential energy and negative pressure dominates. Hence, quintessence is a repulsive force.

This can change, Steinhardt says. Quintessence interacts with matter and evolves over time. Quintessence can also decay into new forms of hot matter or radiation. So we are not necessarily doomed to a universe that expands forever, stretching every atom from here to infinity.

Sounds nice, but not everyone is sold.

"The theory of the accelerating universe is a work in progress," says James Peebles, professor emeritus at Princeton University. "I admire the architecture, but I would not want to move in just yet."

Indeed, at the Texas Symposium, polite arguments over quintessence stretched well into the next talk. Some suggested that the nature of dark energy would become clear with a better understanding of gravity and gravitational waves. Steinhardt was admittedly at a loss with some of the questions. Astronomers and cosmologists are intrigued by quintessence; they simply need more details.

We won't be able to hold quintessence in our hands; nor can we create probes to detect it directly. At best, we need instruments that can determine that effect of quintessence on the universe over time. Two space science missions are promising, Steinhardt said.

The Supernova Acceleration Project (SNAP) would systematically search for large numbers of distant supernovae, beyond the reach of most land-based telescopes. Saul Perlmutter of Lawrence Berkeley National Laboratory, who also spoke at the Texas meeting, is leading the effort and described a not-so-complicated satellite with a two-meter telescope dedicated to finding supernovae at high redshift. SNAP would find about 2,000 supernovae a year, enough to significantly close the error bars on calculations of the universe's rate of expansion. The mission is not yet funded. If selected it would be launched by 2020.

The other mission is the Wilkinson Microwave Anisotropy Probe (WMAP), which was launched in 2001. WMAP's data stream is completed and analysis is ongoing. WMAP has made many important discoveries, however, including setting constraints on dark energy and the geometry of the universe.

Steinhardt certainly has a good track record. He was one of the originators of the inflation theory and predicted an accelerating universe back in 1995. If quintessence does prove to be something that scientists can sink their teeth into, it would be yet another confirmation of Einstein's theories, as well as a fine nod to the ancient Greeks who sent us down this path.

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Author: Christopher Wanjek; updated by Kelly Whitt

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