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Graphic image: Measuring the mystery
Scientists are seeing the big picture  
Supernovae and gas clouds become yardsticks for universe  
By Alan Boyle
    Dec. 24, 1998 —  Is the universe coming apart at the seams or precisely in balance? There’s astronomical evidence to back up either side of that argument, based on observations of exploding stars and distant gas clouds. But scientists say both claims could be correct. What is incorrect — or at least incomplete — is our understanding of how the universe works.  


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       COSMOLOGY’S BIGGEST BOMBSHELL came from researchers who studied the characteristics and brightness of a particular kind of exploding star. As detailed last week, their findings indicated that the universe was expanding at an ever-increasing rate, due to a strange characteristic of empty space variously known as vacuum energy, dark energy, the effect of the cosmological constant, lambda, propulsive gravity or even “antigravity.”
       “Clear all the matter and radiation that you can out of a piece of space, and there’s still energy there,” said University of Washington researcher Craig Hogan, a member of the international High-Z Supernova Search Team. “That is a property that is not predictable one way or another by the current theories.”
       This property, however, can only be detected at unusually large scales of distance — by close observation of exploding stars known as Type 1A supernovae. The High-Z team and another group of astronomers, the Supernova Cosmology Project, independently measured the brightness of Type 1A supernovae as well as the shift in the spectrum of its light (also known as redshift or “z”). For these particular supernovae, the brightness can tell you how far away each exploding star is, and thus how long ago the light waves started their journey toward Earth. The redshift, meanwhile, tells you how much the universe has expanded since the light left the star.
       The scientists cleverly matched up those two readings, expecting to find out how much the force of gravity has slowed down the initial Big Bang. But to their surprise, they found that the expansion rate has sped up in the past 5 billion years or so.
       Such an outcome set the scientific world abuzz: Science magazine declared “the accelerating universe” the top breakthrough of 1998. And in its January 1999 issue, Scientific American trumpets the “Revolution in Cosmology.”
       “Either the universe is dominated by a bizarre form of energy ... or our universe is just one strangely curved bubble of space-time in an infinite continuum,” the magazine said on its Web site.
       Now there’s even more evidence that the former is the case — that scientists are indeed detecting a bizarre form of energy in an otherwise straightforward space-time continuum. That’s the second part of the big picture.
‘As the universe becomes more sparse the vacuum energy makes a bigger and bigger difference.’
Carnegie Mellon University
       Even as Science was celebrating the “accelerating universe,” other researchers were announcing seemingly contradictory results: These scientists used a microwave telescope in Antarctica to observe the faint structure of glowing gas clouds from a time 300,000 years after the Big Bang, the cataclysm thought to have given rise to the universe as we know it.
       There are tiny variations in this structure — variations that were first observed by a satellite called the Cosmic Background Explorer, launched in 1989. By mapping the variations precisely, astronomers can calculate how much of a “kick” the material in the expanding universe had during its infancy.
       “We’re seeing for the first time with our telescope the size of distant clouds of gas that were really in the ancient past, and from that size we can infer some things about the history of the expansion of the universe,” Jeffrey Peterson, leader of the Viper Telescope project and an astrophysicist at Carnegie Mellon University, told MSNBC.
       In an announcement from the National Science Foundation, which funds the Viper project, Peterson said the readings indicated that the universe was given “just the right kick by the Big Bang to expand forever, never collapsing but also never becoming so dilute that gravity can be ignored.”
       Normally, this would imply that the expansion rate should be gradually slowing, not speeding up. Indeed, the National Science Foundation said the Viper results indicated that “the expansion of the universe is slowing at just the rate predicted by inflation theory” — which would contradict the growing consensus on the accelerating universe. But Peterson said that the situation was more complicated, due to the effect of vacuum energy.
       “Early in the evolution of the universe, you don’t see it, but as the universe becomes more sparse the vacuum energy makes a bigger and bigger difference. ... If the matter density is not sufficient to turn the universe around in time, then the vacuum energy would eventually take over. The results indicate that the vacuum energy is winning,” he said.
‘It’s possible to have a cosmological model which is flat but has enough vacuum energy to accelerate the universe in spite of that.’
University of Washington
       Hogan agreed that the results from the High-Z search and from Viper were complementary, not contradictory.
       “Vacuum energy is only just now becoming important for the expansion,” he said.
       The Viper results say more about the geometry of the universe than its ultimate fate, Hogan and Peterson said.
       “It’s not fair to say these results contradict each other,” Hogan said. “It’s possible to have a cosmological model which is flat (that is, governed by traditional Euclidean geometry) but has enough vacuum energy to accelerate the universe in spite of that.”
 More ‘Mysteries of the Universe’
       In the past, astronomers linked the two questions together: If the universe had just the right density, then its geometry was “flat” and its expansion would gradually slow down but never stop completely. If the universe were more dense, then it had a “closed” geometry, and gravity would eventually bring all the galaxies of the universe back together in a “Big Crunch.” If the universe were less dense, it would follow an “open” geometry, flying apart into a virtual nothingness known as the “Big Chill.”
       That linkage doesn’t necessarily apply anymore. Most cosmologists now believe we’re headed for the Big Chill, based on the supernova observations. But the Viper results indicate that a “flat” model of the cosmos will work just fine, if scientists can account for the vacuum energy that makes empty space expand.
       The results, taken together, tend to confirm scientists’ suspicions that this vacuum energy accounts for about 70 percent of the total matter-energy content of the universe. Another 20 percent or more of the universe is made up of “dark matter,” material that does not radiate light in any form.
       If we accept the speculation, this means that everything we can detect in the universe, from terra firma to those distant glowing clouds of gas, makes up only a tiny fraction of the cosmos.
       Vacuum energy could involve an exotic type of matter known as quintessence, or perhaps supersymmetric particles that play a role in the “Theory of Everything.” But that’s another story altogether. ...
       For now, the observations from distant stars and clouds of gas are posing theoreticians with a challenge similar to that posed by Shakespeare’s Hamlet: “There are more things in heaven and earth ... than are dreamt of in your philosophy.”
       Do you have further questions about the structure of the universe — or perhaps about less weighty topics, such as the cause of an ice cream freeze? Trade your questions, answers and observations on the “Mysteries of the Universe” Bulletin Board.
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Internet Sites NSF Telescope Sheds Light on Fate of Universe
Internet Sites The High-Z Supernova Search
Internet Sites Supernova Cosmology Project
Internet Sites Scientific American: Inflation Is Dead; Long Live Inflation
Internet Sites Introduction to Cosmology
Internet Sites Cosmology FAQ
Internet Sites Cosmos in a Computer

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