We Call Them Galaxies

We Call Them Galaxies

The early universe was filled with radiation and a plenum of matter, originally hydrogen and helium, formed from elementary particles in the dense primeval fireball. There was very little to see, if there had been anyone around to do the seeing. Then little pockets of gas, small nouniformities, began to grow. Tendrils of vast gossamer gas clouds formed, colonies of great lumbering, slowly spinning things, steadily brightening, each a kind of beast eventually structures in the universe had formed. We see them today. We ourselves inhabit some lost corner of one. We call them galaxies.

About a million years after the Big Bang, the distribution of matter in the universe had become a little lumpy, perhaps because the Big Bang itself had not been perfectly uniform. Matter was more densely compacted in these lumps than elsewhere. Their gravity drew to them substantial quantities of nearby gas, growing clouds of hydrogen and helium that were destined to become clusters of galaxies. A very small initial non-uniformity suffices to produce substantial condensations of matter later on.

Galaxy. The primordial galaxies spun increasingly faster, because of the conservation of angular momentum. Image: © Elena

As the gravitational collapse continued, the primordial galaxies spun increasingly faster, because of the conservation of angular momentum Some flattened, squashing themselves along the axis of rotation where gravity is not balanced by centrifugal force. These became the first spiral galaxies, great rotating pinwheels of matter in open space. Other protogalaxies with weaker gravity or less initial rotation flattened very little and became the first elliptical galaxies. There are similar galaxies, as if stamped from the same mold, all over the Cosmos because these simple laws of nature – gravity and the conservation of angular momentum – are the same all over the universe. The physics that works for falling bodies and pirouetting ice skaters down here in the microcosm of the Earth makes galaxies up there in the macrocosm of the universe.