30 MARCH 2006
a patient
that is 13.7 billion year old, the universe is in pretty good health.
Results of its latest checkup were reported in March, with no alarming
surprises. The cosmos seems to be behaving just the way that scientists
think it should -- in accord with their favorite theories. 
Of
course, it's really not the universe's health that's of concern, but rather
the health of those theories. And the latest readings seem to show that
those theories are healthier than ever.
Those readings come from a satellite known as WMAP,
the cosmic equivalent of an MRI scanner. Rather than scanning bones or
the brain, WMAP scans the leftover glow of light produced after the Big
Bang exploded the universe into existence. Once hotter than the sun, that
light has now cooled to a chilly 3 degrees above absolute zero, thanks
to the dilution caused by the expansion of the universe. At that temperature,
the cosmic light flows in the form of microwave radiation, a sort of radio
static present throughout all of space. 
Artist's conception of WMAP Spacecraft, created by NASA WMAP Science Team
These microwaves are no good for cooking, but they are great for revealing secrets of the universe's distant past. Imprinted on the microwaves are patterns impressed only 380 million years after the universe was born, reflecting the then-tiny clumps of matter that in the eons since have grown into galaxies.
Those matter clumps created small differences in the microwave temperatures recorded at different points in the sky. Even more subtle patterns can be discerned by the angles that the microwave radiation waves are oriented, a property called polarization. (You can change the orientation of ordinary light hitting your eyes by wearing polarized sunglasses; the microwave orientation was directed mainly by collisions with electrons.) Measuring temperature and polarization produces something like a fossil map of the sky, preserving a portrait of the universe as a young cosmos.
Even better, those patterns probably reflect tiny energy blips distributed throughout space when the universe was a mere fraction of a second old. If so, scientists ought to be able to deduce precisely what happened to bring the universe into being -- and to explain all the properties that the universe exhibits today.
A mere quarter of a century ago, scientists had only vague ideas about how old the universe is, how fast it is growing and the relative amounts of its ingredients. Best guesses pegged the age at between 10 billion and 20 billion years, the expansion rate at somewhere between 40 and 100 expansion units, and specified the recipe as simply some ordinary matter maybe mixed with some additional matter of an unknown type. Nobody knew how much.
Today, thanks to WMAP and other recent experiments, the universe's stats read more like the back of a baseball card: Age, 13.7 billion years; growth rate, 73 units; and a recipe containing three major ingredients: ordinary atoms, 4 percent; unknown form of matter, 22 percent; and a form of unusual energy, 74 percent. All of these numbers could still vary a bit one way or another, but are mostly precise enough to resolve many of the old questions about the universe's features. What more could you possibly want to know?
Well, as with any good science, answers generate questions. Plenty of mysteries remain. A synopsis of the cosmic doctor's report would probably identify five big ones:
1. What exactly happened in the Big Bang? Hardly anybody who knows what's going on doubts the basic idea of the big-bang birth of the universe -- a tiny, hot and dense seed bursting into a rapidly expanding bubble of space and time. But the bang idea doesn't explain everything, such as the precise patterns of galaxies in space. If, however, the initial bang was abruptly followed (within a trillionth of a trillionth of a second) by a huge burst of rapid expansion (called inflation), today's view of the universe makes sense.
Timeline of rapid expansion or "Inflation"
by NASA/WMAP Science Team, from NASA's WMAP
site.
WMAP results show hints that variations in the microwaves are slightly less exaggerated at smaller distances, as the inflationary idea would suggest. But the hints are not definitive, and, in any case, they don't yet distinguish various versions of the inflation idea. While the accumulating evidence strongly supports an inflation-enhanced Big Bang, inflation's truth is not yet certain and its precise details remain mysterious.
2. Is the universe infinite? So far, appearances favor an endless universe, without limit, expanding forever. But deception remains possible. The universe might look infinite while actually just repeating itself. In much the way you could travel around the Earth's equator forever, retracing your steps, an infinite voyage through the universe might bring you back to where you started. Efforts to find signs of such a repetitive universe have so far failed, but you never know.
3. Is there more than one? Infinite or not, the prospect remains that the visible universe isn't the only bubble of space and time that exists. If the inflationary idea is correct, then it is highly likely that countless other big bangs have burst universes into being. There's no chance of traveling to such universes, or even communicating with them, but it may be that their existence is a necessary consequence of the existence of our universe.
4 What is the dark stuff? While scientists
now think they know how much of each 
ingredient
goes into making the universe, they don't really know what some of those
ingredients are. One is "dark matter" -- certainly not ordinary atoms,
but otherwise of unknown identity. Even more mysterious is the "dark energy."
It exerts a repulsive force, causing the universe to expand ever more
rapidly. Scientists have lots of ideas about what it might be, but the
evidence so far is inconclusive.
5. Why are we here, now? The repulsive dark energy creates an additional mystery. How can life exist in a universe that expands forever, ever more rapidly? For almost the entire lifetime of such a universe, the cosmos would be cold and dark with all its stars long burned out. Life could survive only when such a universe was still basically a baby, yet only after the Big Bang cooled off enough for stars to form. With such a narrow window of opportunity for life, how could we be so lucky? Only, perhaps, because the universe is so mysterious, in ways that we still don't know about.
E-mail: tsiegfried@nasw.org
