Big Bang Theory

Galaxies are moving away from the Earth in all directions because all galaxies are receding from each other, that is, every galaxy is getting further away from every other galaxy. There is a simple way to visualize this effect. Imagine a partially blown up balloon, on the surface of which a number of spots (representing galaxies) are drawn. As the balloon inflates, each spot gets farther away from its neighbors; furthermore, the distance from any one spot to any other on the same side of the balloon (as measured on the surface of the balloon) grows faster the farther away from each other the two spots become. This can be seen by imagining three spots in a row (A, B, and C), with the balloon's surface expanding evenly between them. The distance from A to B is growing at x inches per second, and so is the distance from B to C; the distance from A to C must therefore be growing at 2x in/s (x + x = 2x). Thus no matter where an observer is located on the balloon, they will observe that all the other spots on the balloon's surface obey Hubble's law: the farther away they are, the faster they recede. There is no preferred direction, no preferred position, and no "center" of the balloon's surface. The behavior of galaxies in our spatially three-dimensional universe is analogous to that of the spots on the two-dimensional surface of the balloon.

There is an important implication to this model. If all galaxies are moving away from each other with a velocity proportional to their separation, then at all earlier times the galaxies were closer together. If one goes back far enough, there must have been a time when all were at the same position—that is, there must have been a beginning to the Universe. In fact, if the expansion has been constant for all time, the age of the Universe is simply the inverse of Hubble's constant. The situation is not quite so simple, as scientists expect Hubble's constant to change over time (the gravitational attraction between the galaxies has a tendency to slow the rate of expansion, while recent observations show that the expansion of the Universe is actually accelerating under the influence of a still-mysterious force that acts opposite to gravity).

Hubble's original measurements gave an age of the Universe of about two billion years. This immediately caused dispute, since it was known from measurements of radioactive decay that the age of the solar system is more than twice this value. How could the solar system have been formed before the Universe itself? It is now known that Hubble's original measurements were in error. Current measurements put Hubble's constant in the range of 50–100, giving an age of 10–20 billion years.