Dark Energy

First there was Hubble's law: the distant galaxies were moving away from us, and the more distant, the faster. That had a straightforward explanation: the universe was expanding. Like a rising loaf of raisin bread, each raisin - representing a galaxy - was moving away from every other raisin. A raisin twice as far away would be moving away twice as fast. Then astronomers noticed that the stars in each galaxy were moving too fast. Stars near the outer edge of the galaxies moved just as fast as those further in.

To see why this is strange, imagine an outer planet, like Neptune, were moving just as fast as an inner planet, like Venus. The gravitational force of the Sun on distant Neptune simply would not be strong enough to hold Neptune in the solar system, and it would fly off into space. Planets near the outer edge of the solar system travel less quickly than those further in.

This is simply not the case for the stars. Some attractive force, caused by matter we donšt see, has to supply the force tethering these stars.. This astronomers call "dark matter."

It was thought that there was enough of this dark matter to slow the expansion of the universe. Imagine the surprise of astronomers who, in 1998, discovered the most distant galaxies were, in fact, speeding up!

All matter, visible or not, attracts all other matter. This is Newton's law of universal gravity. The repulsive force responsible for this acceleration could not be a form of matter. Astronomers have dubbed it "dark energy."

Dark matter, dark energy - so much of the universe remains a mystery.

(11/28/07)