Challenged by recent unexpected, but compelling evidence that the universe is expanding at an accelerating rate, the theorists of cosmology are reacting as creative scientists should.
They are stretching their minds in search of explanations, even to the point of risking comparison with the queen in "Alice Through the Looking-Glass," who practiced believing "as many as six impossible things before breakfast."The immediate goal of their quest is an understanding of what appears to be an unknown form of energy associated with the vacuum of space. Cosmologists are calling it the "missing energy" of the universe. If both current theory and new observations are correct, this energy must be acting as a repulsive force to counteract gravity's restraining influence and thus speed up cosmic expansion.
Call the ideas of vacuum energy inventions of desperation or inspiration - as scientists have - but measurements of exploding stars, or supernovas, seem to leave cosmologists no alternative. Two rival teams of astronomers recently reported increasing evidence, based on distant supernova explosions, suggesting that the universe is not decelerating, as they had expected, but is flying apart faster than ever before.
The missing energy, scientists propose, exerts a negative pressure, which by the rules of Einstein's theory of general relativity, would exert a negative gravitational force, on the universe.
At a three-day workshop here at Fermi National Laboratory, a show of hands indicated that most scientists now agree, by a vote of two to one, that the supernova astronomers have made a strong case for an accelerating universe and thus for something like the missing energy.
William Press, a theorist at the Harvard-Smithsonian Center for Astrophysics, who called for the straw vote, said the supernova astronomers have provided the first observationally consistent evidence that there is some form of vacuum energy. He said that cosmologists were impressed that both teams had "moved from the hey-look-what-we've-seen stage to the right sort of self-criticism."
Scientists look to this energy to balance the books on cosmic mass, preserve their preferred model of the Big Bang theory of how the universe began, and predict how it all will end. In one scenario, if the missing energy is one of its significant, constant components, the universe will never slow to a halt, but continue accelerating forever.
In another, if all its mass was in the form of matter, the universe would slow its expansion to a halt, given infinite time. Recent observations of low matter densities all but rule out the eventual collapse of universe as a third possible fate.
The trouble is, no one knows the identity and nature of the missing energy. Scientists, with a shrug of shoulders, speak of "something strange" or "funny energy."
"We have an exciting puzzle to explain what the funny energy is," said Michael S. Turner, an astrophysicist at the University of Chicago. "But if you give theorists enough time, they can predict anything."
At the workshop, astronomers and cosmologists considered two emerging ideas for the missing energy: the cosmological constant and a class of models called quintessence, after Aristotle's fifth element of matter.
The cosmological constant, first invoked by Albert Einstein to correct what he took to be a flaw in his theory of gravity, would be relic energy from the creation of the universe, a product of high-energy physics. The amount of this "vacuum energy" in a given volume would remain constant through all time.
Thus as the universe expands, the total vacuum energy, and the repulsive force associated with it, would grow larger with time. But theorists resist the idea of the cosmological constant because of its history as a "fudge factor" and the fact that particle physicists are unable to explain the phenomenon. They point out that it is easier to explain a cosmological constant with an extremely large value or one that is zero than to understand how it could be close to zero, as in the models for the missing energy.
Turner said the cosmological constant was the simplest example of what the missing energy could be, and a good starting point in the search for a sound theory. "What was good enough for Einstein ought to be good enough for us," he said.
But Robert R. Caldwell of the University of Pennsylvania, calling the cosmological constant a "default solution," sought to make a case for quintessence, a newer idea that he advocates with Paul J. Steinhardt, another astrophysicist at Pennsylvania. The attraction of quintessence is that its energy densities vary with time.
Early in the universe, when matter appears to have been dominant, quintessence energy might have been weak, but now it might be catching up or surpassing matter, which becomes more and more dilute as the universe expands. This could account for the negative pressure causing the accelerating universe.
Also, as Turner said, "We want the missing energy to be there today and gone yesterday, in order to avoid interference with the growth of structure early in the universe."
Physicists at Fermilab tend to look with more favor on quintessence models because they seem to be more easily linked with realistic physical theories. Dr. Josh Frieman, head of Fermilab's theoretical astrophysics group, and his colleagues have developed a quintessence model that describes how the vacuum energy might be small today but would eventually evolve to a value of zero. The advantage of the model is that it involves light particles, called pions, that actually exist, but are invoked here in a different context.
Christopher T. Hill, a Fermilab particle physicist, complained that so far the hypotheses for vacuum energy "are very ad hoc, not tied into the principles of particle physics." The challenge, he said, is to find an entirely new family of low-mass particles in space.
In an interview, Steinhardt called this "a monumental issue not just for cosmology, but also for fundamental physics, whatever form the energy takes."
Neither the cosmological constant nor quintessence appeared to be well enough defined to engender enthusiastic support at the workshop, only questions and suggestions for ways to search for more direct evidence of the vacuum energy. Participants heard several proposals for investigating the problem with new X-ray astronomy spacecraft and ground-based and spacecraft surveys in the next decade of the cosmic microwave background, the radiation afterglow from the early universe.
Slight temperature variations in this otherwise uniform radiation are the textures out of which galaxies grew. They may help distinguish between missing-energy models and provide clues that the universe should consist of what is known as a critical density. That means its mass should be sufficient to keep it from collapsing or expanding forever.
On one thing scientists are in almost complete agreement: All recent observations show that the cosmic mass in the form of matter is low - no more than 20 or 30 percent of critical density. That figure includes ordinary matter found in stars and planets and people, as well as exotic and so far undetectable particles known as cold dark matter. In that case, cosmologists are faced with figuring out how to explain missing-energy values of as much as 70 or 80 percent of critical density.
This may prove difficult. Scientists reported that the lensing effects of intervening intergalactic matter on light from extremely distant celestial objects appear to reflect the amount of energy in the universe. So there is missing cosmic energy, they said, but not on the order of 80 percent of critical density.
Toward the end of the meeting, Steinhardt said: "There's remarkable agreement among very diverse measurements that negative pressure from energy makes all the measurements fit better. The case is getting stronger for a cosmological constant or a wide range of quintessence models."