n the world of science, every idea starts with an observation. Or, more likely, thousands of observations--say, the exact position of a star or planet in the sky--piled up over time by a horde of patient researchers. To come up with a theory, the researchers also need some assumptions about how the universe works. Sometimes these are simply older theories which the scientific community has agreed are useful; Newton's Three Laws of Motion are a good example. Other times, the assumptions are "postulates," meaning that there's no particular proof or evidence for them, but they make sense and point to consequences that also make sense. An example of this is the Greek geometer Euclid's assertion that parallel lines never meet--a rule still in daily use some 2300 years later.
Anyway, given reasonable data and reasonable assumptions, scientists can then derive a theory (or, in science-speak, a "model")--such as the shape of a planet's suspected orbit--which can then be used to predict future observations, a process known as "interpolation." In fact, this is every theory's primary function: to help us estimate (and hopefully understand) what's happening around us, especially outside our immediate sphere of attention. And each time a prediction is later matched by actual measurements (e.g., the planet shows up right where the astronomers say it ought to), the correctness of the theory is demonstrated. Not "proved," because in science things can only be disproved. But with enough validating observations, the distinction becomes largely moot, and the model finds its way into textbooks and computer simulations as something very close to fact.
However, a model can also be used to extrapolate, i.e., to make predictions well outside the range of known--or even possible--observations. This is where we get our knowledge about things like the insides of black holes. Since light (and therefore information) can't escape from there, we have no way to measure what's going on. But armed with our strongest theories, we can at least make educated guesses about it. There's a very big difference, of course, between educated guesses and facts, so don't ever let anyone tell you they "know" what goes on in places they can't actually measure. They can strongly suspect, but that's all.
The first instants of time
Interestingly, there are several fields of science which rely almost exclusively on extrapolation, and cosmology is first and foremost among these. The confluence of particle physics, astronomy and--at least to some extent--religion, cosmology is the branch of science concerned with the first instants of time itself. Why are we here? How did we get this way? Could things have happened differently, or is the universe we know an inevitable consequence of inevitable starting conditions? Wrapped in a cloak of mathematics, cosmology hunts for the answers to these questions, and sometimes even deeper ones, like whether the universe shows signs of being a created rather than a natural phenomenon.
Cosmology, as a distinct scientific discipline, first came about in 1929, when astronomer Edwin Hubble announced the discovery that the light from distant galaxies was shifted toward the red--or lower-frequency--end of the spectrum, and that the amount of this "redshift" seemed to be at least approximately proportional to the distance of the galaxy being observed. Since Austrian physicist Christian Johann Doppler had shown, 87 years earlier, that the frequency of a sound or light wave emitted by an object decreased if the object was moving away from the observer (the familiar "train whistle" effect), Hubble's bizarre finding quickly led to the suggestion that all the galaxies in the universe were, for some reason, rushing away from one another. In other words, that the universe was expanding.
A science was born. Soon, cosmologists (now distinct from astronomers) were talking about the Big Bang, a titanic, 15-billion-year-long explosion of some "cosmic egg" whose nature and origins were not known. But problems were soon found with that theory--shouldn't the cosmic egg have collapsed into a black hole instead? Wouldn't the universe have had to expand faster than the speed of light to reach its current size? Et cetera. Einstein, in the early part of the 20th century, had better luck modeling the expansion in four dimensions, and successive generations have arrived at a multidimensional "Inflationary Universe" theory that attempts to unify all the particles and forces known to us today as "cooled-off" versions of much simpler phenomena in the compact, superhot early universe.
Gravity, interestingly enough, is actually one of the weakest forces in nature--much weaker than the nuclear binding forces which hold atoms together, or even the much wimpier attractions and repulsions of electromagnetism. Try it yourself: use a refrigerator magnet to pick up a paper clip. The force of that small strip of iron and plastic easily overcomes the gravitational pull of the entire Earth! Still, gravity is the only force capable of attracting all matter and all energy together, over any time or distance. What goes up--even 15 billion light-years up--must eventually come down, so an important question all along has been whether there's enough gravity in the universe to pull the Big Bang back together again in a "Big Crunch."
The answer? Maybe not.
Flat in four dimensions
In 1991, a satellite called Cosmic Background Explorer, or COBE, examined the "microwave background" of empty space--the leftover heat from the Big Bang--and found small variations in its intensity from one patch of sky to the next. "Like ripples in a pond," many astronomers said, although the actual structures--minute temperature variations of a thousandth of a degree or less--don't look nearly as regular as that. Still, these results matched the predictions of the inflationary universe theory well enough that Peter Jennings announced the theory's "confirmation" on the ABC Evening News. Of course, these weren't his own words--they were someone else's, scrolling by on a teleprompter, and Jennings' reaction as he spoke them was comically uncertain. As a friend of mine described it, "he looked like someone had asked him to say Elvis was still alive."
Then in 1999, a high-altitude Antarctic experiment called Balloon Observations of Millimetric Extragalactic Radiation and Geophysics--BOOMERANG--repeated COBE's survey with much finer resolution, and found the same result: that the microwave background is somewhat rippled, in a way that seems to agree with today's cosmological theories. And this month, scientists are further announcing that if gravity is actually bending space in the manner predicted by Einstein's Theory of General Relativity, these ripples should be magnified or reduced by a kind of "lensing" effect, which is in fact not observed. This doesn't make Einstein wrong--rather, it means the universe is "flat" in four dimensions, and will continue to expand forever.
So far so good, but from a theoretical standpoint this "flatness" requires a universe composed mostly of "dark matter" and "dark energy," bizarre substances which have never been observed and whose (literally) repulsive properties do not resemble those of the normal "baryonic" matter of which our bodies (and stars and planets) are made. Hmm.
Fertile assumptions; a green theory
It's helpful to remember that the only data to support this theory is a microwave background which, like any other temperature map of any other physical object, is not perfectly uniform. And the non-uniformity is neither completely random nor completely structured. Is that really so surprising? A load of assumptions are then trucked in to fertilize the ground, until a green theory finally pops its stem out.
Does the redshifted light of distant galaxies actually imply that they're moving away from us? There are still proponents of F. Zwicky's 1929 "tired light" hypothesis who would argue against it. And if those galaxies are receding, does that mean the entire universe is expanding, even the parts we can't see? Eric J. Lerner's crowd of magnetohydrodynamicists see evidence of the Big Bang as a localized phenomenon in a matter/antimatter universe much older and larger than we've previously imagined. And are gravity, electromagnetism and nuclear forces really distinct and inherent properties of matter? Respectable astrophysicists like Bernhard Haisch, Alphonso Rueda and H.E. Puthoff envision a universe composed of vibrating charges and nothing else, where forces and particles and even mass are all simple, straightforward manifestations of the laws of magnetism. I could go on and on about this, I really could.
Certainly, the BOOMERANG conclusions are rooted in quite reasonable physics and math. Nobody denies this. But the assumptions which underlie them aren't the only assumptions kicking around, and they may not even be the most obvious. Michael Turner, an astrophysicist from the BOOMERANG team, tells reporters unambiguously that "we live in a flat universe," and the boldness of this statement provides an interesting--and to my ear, somewhat foolish--counterpoint to Peter Jennings' quiet, confused skepticism.
Flat? Not flat? Big Crunch or Big Unknown? Whether either man is right we may never know, but on matters this distant from our firsthand experience, perhaps confidence should be the rarest observation of all.
Wil McCarthy is a rocket guidance engineer, robot designer, science fiction author and occasional aquanaut. He has contributed to three interplanetary spacecraft, five communication and weather satellites, a line of landmine-clearing robots, and some other "really cool stuff" he can't tell us about. His short fiction has graced the pages of Analog,
Asimov's, SF Age and other major publications, and his
novel-length works include Aggressor Six, the New York Times Notable
Bloom, and upcoming The Collapsium.
