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Inflation and the age of the universe predicts that the shape and fate of the universe is bound up with the average density of the matter it contains. If this average density exactly equals the critical density, the density required to arrest the infinite expansion of the universe but without bringing about its ultimate collapse, then Omega equals 1, and we live in a flat universe.
Astronomers have managed to see, at most, one-tenth of the matter necessary to close the universe. This, along with the relative motions of galaxies and galaxy clusters, even superclusters, points to the existence of dark matter--a lot of it, perhaps 90% of what's out there.
Perhaps the weakest part of our current theories of cosmological structure formation is that we don't know what dark matter is made of. Some would say, we don't even know that it exists, although there are powerful observational and theoretical clues that indicates that its there. Dark matter may take several forms, both baryonic and nonbaryonic--an idea that many purists find repugnant.
Baryonic dark matter simply fails to emit radiation detectable on earth, yet it is generally believed to exist to make up the deficit between the amount predicted from light element nucleosynthesis measurements and that visible in galaxies. Baryonic dark matter in the form of dim stars or more generally MACHOS will continue to be inventoried with every more sensitive telescopes, although it is likely much of it will remain too dim to be detected.
The apparent scarcity of such baryonic dark matter has led scientists to explore other, more exotic types of matter: exotic, non-baryonic particles. They come in two flavors: cold dark matter, or WIMPS (Weakly Interacting Massive Particles); or hot dark matter, the most likely candidate being the massive neutrino. But WIMPS are largely hypothetical and the evidence for neutrinos with mass is less than rock solid.
Neutrinos--the one kind of nonbaryonic dark matter we know exists--has a devilishly difficult mass to measure. Experiments with particle accelerators will continue to try to pin down the mass. If it is conclusively found to be non-zero, then neutrinos will make an important contribution to cosmic dark matter.
The nature of cold dark matter--the ingredient that seems to be required to make a successful cosmological model--therefore remains a mystery. If it is composed of WIMPS, then their detection in the laboratory would be a major step forward for the field, putting the entire class of dark matter-dominated models on a firm physical footing.
All of these uncertainties force numerical cosmologists to make some critical assumptions about the universe. How much baryonic, cold and dark matter should be included in the simulations, and what should they be? Is Omega in fact equal to 1? Some cosmologists think that the evidence points toward a less dense--and therefore open--universe.
Grand Challenge Cosmology Consortium members Renyue Cen and Jeremiah Ostriker believe that the only way to simulate the evolution of the universe is to decrease the percentage of dark matter. And, since the amount of ordinary baryonic matter is a known quantity, that means decreasing the total density of matter--perhaps to only one-third of that required for a closed universe. That, of course, would mean that the Standard Big Bang model (plus inflation) could be in serious trouble.
The search for dark matter will likely intensify in the coming decade. The most exciting prospect is that in the process of looking for dark matter, something completely unexpected will be found which will alter our view of our universe.
The age of the universe is another, equally vexing question. Astronomers have known for some time that only two pieces of information are needed to calculate the age of the universe: the velocity at which far-off galaxies are receding from us and their distance. From the ratio of these two numbers, the Hubble constant, cosmologists can determine how fast the universe is expanding and, by extension, the age of the universe.
Until recently, most estimates of the Hubble constant were around 50, putting the age of the universe at about about 20 billion years. But recent distance measurements using the Hubble Space Telescope and the Keck telescope atop Hawaii's Mauna Kea indicate that the universe might be considerably younger--about eight to twelve billion years old.
These new estimates present considerable problems to cosmologists. Astronomers who study the chemistry and life cycles of stars calculate, with considerable confidence, that the oldest stars in the Milky Way are about 14 billion years old. Clearly, something is missing from the picture.
The age problem is only confounded by the search for missing dark matter: if Omega is equal to 1, as inflation theory requires, then the gravitational force of all that mass would tend to slow down expansion, making the universe even younger than it now appears.
Einstein may yet save the day with his ill-fated cosmological constant. Because the prevailing view in 1916 was that the universe was static, he was dismayed to find that his theory of General Relativity predicted either an expanding or contracting universe, and introduced a fudge factor to his equations to make his calculations consistent with a static universe. This "cosmological constant" represented a repulsive force of energy equal to but opposite that of gravity. Einstein later dropped the cosmological constant when Hubble showed that the universe was indeed expanding, calling it "the greatest blunder of my life."
But now Einstein's blunder is gaining some cachet within cosmological circles, because the force, if it did (or does) exist, solves the age-missing mass crisis neatly. Inflation would still occur, but there would follow a long period of very leisurely expansion, giving stars and galaxies lots of time to form. At some point, the cosmological constant would take effect, increasing the rate of expansion with its anti-gravitational force. The current rate of expansion would no longer be a reliable indicator for the pre-cosmological constant rate of expansion. However, explaining the physical basis for the cosmological constant poses yet another challenge to the theorists.
Numerical cosmologists are confronted by yet another set of variables: How should they place their estimates on the age of the universe? Should they introduce a cosmological constant? If so, what should the value be? When would it kick in?
There is another approach to the question of large scale structure formation, however: the "top-down" theory, proposed by the Russian physicist Yakov Zel'dovich in the 1970s. This theory proposes that large-scale density fluctuations caused vast, pancake-like structures to form first. The pancakes eventually fragmented into galaxies and galaxy clusters.
Although the "top-down" theory has of late fallen out of favor, it may be enjoying something of a renaissance with a model, recently described by a cosmologist from the Institute for Astronomy at the University of Hawaii, which marries top-down with bottom-up theory. The marriage allows for hierarchical clustering at smaller scales within a Zel'dovich scenario operating at much larger scales.
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