The astonishing discovery last year that the universe's expansion is not slowing down, as assumed, but seems to be speeding up jolted theorists of the Big Bang whose concept for the explosive beginning and expanding evolution of the universe had, as Sir Martin Rees of Cambridge University in England put it, "lived dangerously for 30 years."
An indispensable corollary, introduced in 1981 and known as inflation, had breathed new vitality into the theory with a plausible explanation of how, in its first microseconds, the universe made the transition from initially featureless conditions to the ripples out of which mighty galaxies would grow. But scientists had been short on evidence to support the reality of inflation.
So, in a six-day meeting that ended here on Wednesday, the cosmologists were relieved to find their inflationary Big Bang theory more robust than ever.
"Two years ago, we couldn't have had this meeting -- we didn't have the data," said Dr. Michael S. Turner of the University of Chicago, who organized the Pritzker Symposium and Workshop on the Status of Inflationary Cosmology. "Now we do, and the data say we have a flat universe."
Flatness is the large-scale geometry for the universe predicted by the inflation concept. It means that the expanding universe will not collapse or thin out into infinity, but should maintain a gravitational balance between the two alternatives through a coasting expansion.
The cosmologists were also a little surprised by the outbreak of good feelings over issues that used to provoke intellectual donnybrooks. No one questioned the Big Bang itself, which would not have been the case earlier in the decade.
Only a few entered mild objections when Dr. Alan Guth of the Massachusetts Institute of Technology, originator of the first inflation concept, proclaimed, "Everything about inflation is incredibly solid," adding, "It's hard to imagine any alternative."
Yielding to this triumphal spirit, some of the bolder theorists seemed eager to crack even grander cosmic mysteries, like the possible existence of many more universes. Perhaps believing that cosmology abhors a vacuum of dispute, these bold ones stunned their colleagues by recommending the controversial anthropic principle be applied in determining the nature and properties of forces and matter in the universe.
In anthropic reasoning, simply put, the physical laws of the universe must be such that they allow for the existence of humans.
The primordial spurt of inflation would seem to meet the anthropic standard. Inflation was supposed to leave on space an imprint of ripples, temperature fluctuations as microwave signatures of slight density variations.
The Cosmic Background Explorer spacecraft detected these signatures in 1991, and more detailed observations recently have convinced scientists that these are the seedbeds of all the large structure that eventually appeared in the universe, the stars and galaxies and clusters of galaxies. Without them, there would be no intelligent life.
Cosmologists had thus traced the ultimate origin of galaxies back to the first instant after the Big Bang, and inflation, they said, had passed a critical test.
They also felt they were coming close, or close enough, to knowing the age of the universe: probably somewhere between 13 billion and 15 billion years old.
Two teams of astronomers still feud over differing calculations, with Dr. Wendy Freedman of the Carnegie Observatories in Pasadena, Calif., preparing refined data for a report in June.
But at least new research shows that, contrary to earlier reported estimates that were paradoxical and highly embarrassing, the oldest stars are not older than the universe.
All could agree that the evidence of a low-density universe is overwhelming.
There is not enough mass in ordinary matter, the stuff of stars, planets and people, or even the hypothesized exotic particles called cold dark matter to stop the universe from expanding forever.
If standard models of inflationary cosmology are correct, a universe that does not eventually collapse on itself (a closed universe) or expand forever into infinity (an open one) should have a critical density, expressed as omega equals one, and a flat geometry. The gravity of a critical density should slow expansion to a kind of coasting equilibrium between collapse and vanishing infinity.
By most measures reported at the meeting, based on studies of the light and motions of galaxy clusters, the density of mass in the universe may be no more than 20 percent to 30 percent or at most 40 percent of the theorized critical density. Only 5 percent is in the readily observed form of luminous stars, and perhaps another 10 percent is composed largely of the interstellar clouds of hydrogen gas. The rest is presumably mostly cold dark matter, unknown and so far undetectable.
Even the discovery, reported last year, that pervasive subatomic particles known as neutrinos have a slight mass does not add enough to the cosmic density to be significant, scientists said.
This shortfall in matter density has led theorists to revive a concept that Einstein had entertained and discarded, a kind of hypothesized vacuum energy known as the cosmological constant. No one has detected it or been able to explain its properties, except that the cosmological constant must be evenly distributed energy that exerts a negative pressure to counteract the restraining gravity of matter.
As a result of evidence for an accelerating universe, Rees said, "Stock in the cosmological constant has gone up enormously."
Early last year, two rival astronomy teams reported observations of supernovas, or exploding stars, both near and far. The measure of their velocities provided the first direct, if tentative, evidence of an accelerating expansion of the cosmos.
Turner called this the "smoking gun" for something else in the universe besides matter, ordinary or exotic, and that something must be the "missing energy." This could be the unvarying cosmological constant or some other form of repulsive energy in empty space, like the hypothesized quintessence, which supposedly varies over time.
In this case, the standard model for an all-matter flat universe is dead -- but lives on among cosmologists as a matter-plus-exotic-energy flat universe. Still, some scientists cautioned against writing off the possibility of an open universe -- what if there is not enough vacuum energy to reach critical density? Though the original inflation models hinged on a flat universe, theorists have now suggested ways to have an open universe within the framework of inflation.
With so many important questions still unanswered, Dr. P. James E. Peebles of Princeton University worried about premature celebration of a theory triumphant. "This is a wildly healthy field, but still in its early days," he said. "We've got a lot to do yet."
Comforted by the congruence of data and theory, nonetheless, the cosmologists took a break one evening for a festive dinner. Breathtaking manifestations of the current epoch of cosmic evolution spread about them.
The full moon, an infrequent blue moon, cast a path of shimmering light on the waters of Lake Michigan, and the sparkling towers of the city skyline loomed in the distance, perhaps evoking anthropic speculation.
The after-dinner speaker was an astronomer, one of the searchers of the sky whose observations inspire or confound the theorists. This time he brought them to laughter, and a moment of humility. Even Dr. Stephen W. Hawking of Cambridge University, mute and paralyzed in a wheelchair nearby, managed a broad grin.
The astronomer, Dr. Allan R. Sandage of the Carnegie Observatories , was a protege of Edwin P. Hubble, whose 1929 discovery that distant galaxies were moving apart as the entire universe expanded led eventually to the Big Bang theory. Sandage reached deeper into history than Hubble in reminding the cosmologists that at least in broad outline, their scenario of cosmic history was a lot like St. Augustine's 1,600 years ago.
"The universe was brought into being in a less than fully formed state," Augustine wrote, "but was gifted with the capacity to transform itself from unformed matter into a truly marvelous array of structures and life forms."
Though they laughed at hearing how Augustine had anticipated their work, the cosmologists were not appreciably humbled. They knew they had at their command instruments of exploration, telescopes and spacecraft and particle accelerators, to fill in many of the details of cosmic history unimagined by Augustine. And they expected to make more progress in the next few years.
As in the past, different but complementary styles of practicing science will be brought to bear in approaching the mysteries of the universe: the grand pursuit of the big picture and the plodding collection and evaluation of data.
In one example, Rees of Cambridge divided today's community of cosmologists and astrophysicists into chess players and mud wrestlers.
The chess players, many of them particle physicists, practice fundamental cosmology, trying to define initial conditions and the origin and nature of the early density perturbations that seeded all subsequent structure in the universe. They also strive to characterize the amount and properties of matter and energy shaping the universe.
The mud wrestlers have what Rees called the messier job of environmental cosmology. They seek to explain the dynamics of how and when stars and galaxies and galactic clusters came to be a billion years or less after the Big Bang.
Rees predicted that in two years the chess players may pin down the densities of dark matter and ordinary matter in the universe. Then scientists should be able to concentrate on understanding the composition of both matter and energy in the universe: what exotic particles constitute dark matter and whether the vacuum energy component is in the form of a cosmological constant or something even more mysterious.
For the mud wrestlers of environmental cosmology, some of their breakthrough insights may have to await the development of even more powerful telescopes, like the Hubble Space Telescope's successor being planned for 2007. So far, telescopes peering deeper into the observable universe have yet to glimpse the dawn of star and galaxy formation.
Much is yet to be learned, Rees said in an interview, about the complex gas dynamics over large scales and feedback processes that are responsible for matter coalescing into stars and galaxies, without which there would be no universe as it is known today. Understanding this is "like weather prediction," he observed, "and we know how hard that is."
If Sandage introduced the element of humility, Peebles of Princeton ended the symposium on a note of introspection about how successful science, particularly physics, is done. His division of the community was between classicists and romantics, defining the two with a musical analogy: Bach was a classicist, inventive but following a sharply defined paradigm, and Wagner a romantic, unbounded, soaring from conventional forms.
"The parallels with our present situation are absolutely uncanny," Peebles said.
The Big Bang, to his way of thinking, "was discovered in a romantic way, a brilliant philosophical idea that has been remarkably successful because it is spectacularly simple."
Soaring chords of theory were followed by long interludes of experimentation and observation, the work of classicists. When this raised sticky questions, the romantics stepped in with new concepts like inflation and then dark matter.
Even if recent empirical evidence from classicists did seem to fortify standard inflationary cosmology, Peebles was troubled that inflation theory was being so readily accepted, without being fully tested, mainly in the absence of a viable alternative. "I find that distasteful," he said.
Peebles' skeptical tone did not seem to upset other cosmologists. "He's half enthusiast, half curmudgeon who wants to keep us honest," said Turner of Chicago. "He wants us to slow down the train before it goes over some cliff."
Soaring again above the playing fields of hard-won data and interpretation, the irrepressible romantics of cosmology gave their colleagues a start by proposing that serious thought be given to the anthropic principle and the possibility of countless parallel universes.
"The romantics see inflation as a done deal," Turner said in an interview. "Now they're dreaming about the most expansive implications of inflation."
Astrophysicists have generally dismissed the anthropic principle as unscientific, either obvious and proving nothing or hopelessly mystical. At the symposium, they greeted some references to it with mock hissing and hoots and cracked jokes about the A-word. But the credentials of the advocates guaranteed them a hearing.
Speaking through a computerized voice-synthesizer, Hawking acknowledged that inflation "does a good job" solving some of the problems with the Big Bang model, but noted that by itself inflation places no limits on matter, the energy vacuum or the amplitude of the density fluctuations out of which galactic structure grew.
"We might as well use the anthropic principle for fine-tuning problems of the hot Big Bang," he said. "I doubt very much we will find a nonanthropic explanation for the cosmological constant."
The British cosmologist Dr. Brandon Carter coined the term anthropic principle in 1974 as astrophysicists began contemplating the concept. Since life exists in at least one place in the universe, the physical laws governing the universe, the fundamental forces and particles and so forth have to be such to account for the evolution of stars and galaxies or anything else essential to the emergence of intelligent life.
Dr. Andrei Linde of Stanford University, a leading inflation theorist, cautioned that the principle should be invoked with care, but that it seemed sensible to assess all cosmic parameters, all elements brought to bear in support of theories, with one view in mind, namely, "what allows us to be here."
"Only the anthropic principle plus inflation will explain the universe as we see it," Linde concluded.
Both Linde and Guth of MIT have taken another romantic plunge with the idea of "eternal inflation." If an explosive event like the Big Bang, followed immediately by a brief phase of rapid cosmic expansion, happened once, they posit that it could happen an infinite number of times, and may well have.
Conceiving of an inflation that cannot reproduce bubbles of new universes many times, Guth said, seemed "as implausible as discovering a species of rabbits incapable of reproduction." He paused. "So universes reproduce like rabbits."
From the front row, Dr. Rocky Kolb of the University of Chicago piped up, "You mean, pull it out of the hat."
"OK, I'll look for a different analogy some other time," Guth responded.
If there are parallel universes elsewhere, each would have started with its own big bang, grown from a separate inflationary bubble and probably acquired entirely different laws of physics. Ours may not be a typical universe. Instead of the four dimensions of space-time in this universe, Linde suggested, other universes could have as many as 11 dimensions; some could be dimension-challenged, with only 3. Some universes could be stillborn or unstable and short-lived, thus lacking the time to evolve stars and planets where life might emerge.
It could be, invoking anthropic reasoning, that a universe must have, say, a cosmological constant of precisely the right value and properties to support intelligent life.
The romantics may have outdone themselves with their eternal inflation and a multiverse instead of a universe. Even if scientists established sound reasons for their existence, parallel universes would be discrete and widely separated entities, beyond communication with each other.
No one in this universe, cosmologists said, would be able to gain any direct knowledge of other universes, though in time the theorists might make a persuasive case for their existence.
Human beings would then have discovered the ultimate limit to their knowing what is out there. They might find solace, though, if there is anything to the anthropic principle, in thinking that theirs is a defining presence in the one universe they know and are trying to comprehend.
