Much of the expansion force in the universe is thought to be a remnant of that mysterious, initial cosmic happening that many call the Big Bang. Extremely distant structures, such as galactic clusters, have been moving away from each other since the beginning.

Universal expansion is not the whole story, however. In smaller scale structures, such as our solar system, gravitational attractive forces between structures dominate over the acceleration forces imparted by the Big Bang. On an even larger scale, blue Doppler Shifts indicate that the Andromeda galaxy is moving toward us, apparently because of the gravitational forces between it and our Milky Way galaxy.

What is this force called gravity? Where does it come from? Do we understand why gravity is always attractive, and never a repulsive force? The answer to these and related questions is, “not really.”

What do we know? Well, we know that “what goes up must come down.” We know that because when we toss an object upward, away from the Earth’s surface, it eventually comes down because of the Earth’s attractive force. Gravity is a major force not only on Earth, but also in the universe. It not only holds our feet to the ground; it also holds our atmosphere in place, holds the planets in their orbits around the sun, and is responsible for the ocean tides.

If we examine the history of gravity, we find that most the everyday ideas about gravity relate to the discoveries of Sir Isaac Newton. For example, Newton was the first scientist to associate the orbits of the planets with gravitation. An important conclusion from Newton’s work is that the gravitational force between two objects is directly proportional to the masses of the objects and inversely proportional to the square of the distance between them. This is called a universal law because it applies to any two masses, regardless of their size.

Clusters of galaxies, atomic particles, and human beings are all subject to gravitational forces. Also, gravity operates over all distances as far as we can tell. You are subject to the gravitational forces of distant celestial objects. But because of the differences in mass and distance, the gravitation force of the Earth have much more influence on you than do those of the giant planet Jupiter.

When you walk past people in the hallway you do not have to worry about being attracted to them gravitationally because that force is so small. Gravitational forces come into play in a tangible way only when at least one of the interacting masses is large. The forces of gravity usually are not discussed when you are studying atomic structure because the mass of tiny atomic particles makes their gravitational forces very small compared to the electromagnetic forces between them. However, when we consider the masses of galactic structures, the gravitational forces between them cannot be ignored.

In our dynamic universe, two fundamental forces are at work—expansion and gravitational forces. And Doppler Shifts can tell us when things are moving and in what direction they are moving—away from us or closer to us.