Objective:

To learn what will become of our universe.

New Concept:

Big Chill, Big Crunch, critical density.

New Term used in this unit:

general theory of relativity.

The universe is expanding, but expanding to what? According to the general theory of relativity, the space-time is affected by matter. The equation that describes the behavior of the entire universe is surprisingly simple; mathematically, it can be written in one line. Here is how it looks in plain English:

the growth rate of the universe = some combination of its matter density.

The future of the universe therefore depends on how much matter the universe contains. There are two possible ends waiting for us:

Fig.1 The Big Chill end of the universe. If the universe contains too little matter, it will expand forever. The stars will become dimmer and dimmer until their light is finally extinguished after exhausting their nuclear fuel. The universe will thus end up a dead state with no light and no heat.

Big Chill

If the universe does not have enough matter within it, then the gravitation generated by matter would be too weak to stop the universe from growing. This would cause the universe to expand forever. As the universe continued to grow, the galaxies would get dimmer and dimmer as they sped away. Moreover, the stars would inevitably exhaust their nuclear fuel and their light would eventually be extinguished. The universe would thus end up having no light and no heat -- which is the state known as the Big Chill (Fig.1).

Fig.2 The big crunch. If our universe has sufficient matter within it, the attractive force of the matter will slow the growing down, and some day the expanding will turn into shrinking. When the universe starts collapsing, the temperature and matter density will become higher and higher, like the Big Bang in reverse. Eventually the universe will fall back in a Big Crunch. Another Big Bang might restart itself after the crunch. Maybe there even was a Big Crunch, or many Big Crunches, before the Big Bang that shaped our universe.

Big Crunch

On the other hand, if the universe contains sufficient matter, the gravitational attraction of the matter would be strong enough to slow down the expansion until the universe began to shrink. When the universe started collapsing, it would eventually return to its childhood state. The temperature and matter density would get higher and higher, just like the Big Bang in reverse. The universe would eventually end in a Big Crunch (Fig.2). Afterwards the universe might restart itself with another Big Bang.

Then how much matter is sufficient for a Big Crunch? If, after averaging all matter throughout the entire space, the universe has a matter density larger than the critical density, (see below) then the universe will inevitably fall back in the Big Crunch; otherwise it will end up in the Big Chill.

Critical Density...

is the average mass density that determines whether the universe will end up in the Big Chill or in the Big Crunch. It it is equal to about 1x10^-29 gram per cubic centimeter. This number is based on the calculation of the general theory of relativity.