2. The Hubble Constant

To explain the birth of the universe, Hubble used a technique similar to the one currently used by radar guns and discovered that the galaxies were generally receding from earth. Since there is nothing astronomically special about earth's place in the universe, this must be taking place across the universe: All the galaxies are flying apart. The relationship between the speed at which a galaxy appears to be moving away and its distance from earth is given by Hubble's constant. From this we can figure out that the big bang occurred approximately 13.7 billion years ago.

1. Omega

The fate of the universe depends upon a calculation. If the big bang imparted enough velocity to the galaxies, they could fly apart forever. But if it didn't, then the galaxies would find themselves similar to rockets without escape velocity. They would be pulled back together in a big crunch—the reverse of the big bang. That tipping point is called Omega; it's the ratio of the total amount of matter in the universe divided by the minimum amount of matter needed to cause the big crunch. If Omega is less than one, the galaxies will fly apart forever. If it's more than one, then sometime in the far-distant future the big crunch will happen. The best estimate at the moment is that Omega lies somewhere between 0.98 and 1.1. So the fate of the universe is still unknown.