Great Masses from Little Ripples Grew

By Tyson, Neil deGrasse | Natural History, February 2004 | Go to article overview

Great Masses from Little Ripples Grew


Tyson, Neil deGrasse, Natural History


The organization of matter into superclusters and voids began with subatomic variations in density during the earliest moments after the big bang.

Of the many unknowns that perturb the modern cosmologist, the absence of a theory that seamlessly blends quantum mechanics and general relativity nags the most. Those two streams of thought-the study of the very small and of the very large-remain immiscible in physics, even though they readily coexist in the same physical universe. Part of the reason they can coexist is that they apply at such different scales. A galaxy of 100 billion stars can pretty much ignore the physics of the atoms and molecules that make up its star systems and gas clouds; still larger agglomerations of matter-superclusters that typically comprise many thousands of galaxies-are even more indifferent to the molecules in their midst.

Yet galaxy superclusters owe their very existence to what physicists call "quantum fluctuations"-very slight non-uniformities that generally pertain only to things as small as atoms and molecules. Quantum fluctuations first appeared in the primeval cosmos soon after the big bang, when the entire universe was still immeasurably smaller than today's atoms and molecules. Within such a volume, it's not surprising that the laws of the very small would dominate. And recent satellite observations have given cosmologists hard evidence that the quantum fluctuations of matter, and energy in that early universe were just the right size to have given rise to the superclusters visible today.

Superclusters are probably the largest material structures in the universe-which makes whatever distribution they assume in space a central observational fact for cosmology to explain. For much of the twentieth century the distribution of matter in the universe was presumed to be both homogeneous (evenly sprinkled with galaxies in every location) and isotropic (appearing the same no matter which direction you look).

Homogeneity and isotropy may sound equivalent, but they're not. In a homogeneous universe, every position is similar to every other one, like the contents of a glass of homogenized milk. In an isotropic universe there is some vantage point from which the cosmos presents the same appearance in every direction.

To picture the difference, start with the way geographers mark longitude and latitude on Earth's surface. As you move away from the equator and toward the poles, the longitude lines get closer together, creating a nonhomogeneous globe. From the exact North Pole, however, where all the longitude lines converge, Santa Claus's view of the world is isotropic. Same holds for the South Pole. The grid of longitude and latitude looks the same from either the "top" or the "bottom" of the world.

Or imagine sitting atop a perfectly cone-shaped mountain, and suppose the mountain is the only topographical feature in the world. From your perch, every view would be identical to every other. The same thing holds if you happen to live at the center of an archery target, or if you're a spider sitting at the center of your web, waiting for lunch to drop in. In each case the scene is isotropic but decidedly not homogeneous. Only if space is isotropic everywhere-creating the same view for every possible observer-will it also be homogeneous. (Actually, mathematicians have demonstrated that if space is isotropic in just three places, it must be isotropic everywhere.)

It's also possible for a pattern to be homogeneous but nonisotropic. One example is a wall of identical rectangular bricks, laid according to the bricklayer's traditional craft. On the scale of several adjoining bricks and their mortar, the wall is the same everywhere-bricks, bricks, and more bricks, all framed in mortar. Yet if you're an observer turning your head as you look at the wall, the changing sight lines will intersect the bricks and mortar at varying angles, creating a different view every time you alter the angle of your head.

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