Objective:

To learn about fleeing galaxies and the expanding universe.

New Concepts:

Hubble's Law, Hubble constant.

Term used in this unit:

Parsec

The distant galaxies are all speeding away from us. (We will explain how we know this in our last unit: The Doppler Effect.) Moreover, the further the galaxies are from us, the faster they recede, no matter which direction they are going in. This is known as Hubble's Law, named after Edwin Hubble, the American astronomer who made this milestone discovery in the 1920s. According to Hubble's Law, as long we can measure the speed at which a galaxy recedes, we can determine its distance from us, and vice versa. This rate of recession is called the Hubble constant.

Fig.1 A simple explanation of Hubble's Law is that the entire universe itself is expanding. This can best be understood by drawing a number of dots on the surface of a balloon and blowing it up. Notice how the distance between the dots increases as the balloon inflates. From the viewpoint of any dot on the balloon, it looks like all the other dots are receding away from it.

Our universe works in a similar way. You can imagine space-time as the balloon and the galaxies as the dots stuck on the balloon. As the universe expands, the galaxies go along with it; therefore, the distances between the galaxies increase. Every galaxy then sees other galaxies speeding away from it.

The Hubble Constant

The exact value of the Hubble constant still remains controversial among astronomers. Figures may fall anywhere between 40 km/second to 100 km/second per million parsecs. With the Hubble constant, we can calculate the receding speed of a galaxy by

receding speed = Hubble's constant * distance of the galaxy

For instance, if the Hubble constant = 75 km/s per million parsecs, then a galaxy at the distance of 50 million parsecs, or 150 million light years, recedes at the speed of 3,750 km per second. This is an astonishing speed, almost 5,000 times faster than our fastest commercial airliner -- the Concorde.

The discovery of Hubble’s Law is one of the greatest discoveries in history because it tells us that our universe as a whole is expanding. You might think that Hubble's Law places us at the very center of the universe, as all galaxies move away from us in every direction. But this is just an illusion, like the Sun appearing to revolve around us -- although it does not in reality.