Dark Energy

Hubble's law: the distant galaxies are moving away from us, and the more distant, the faster. That has 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.

The Milky Way Galaxy, and many other galaxies, look like giant pinwheels. And like pinwheels, the arms rotate. But the stars in these arms move around the center too fast. Stars near the outer edge move 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." " Dark" because it cannnot be detected in any telescopes: visual, radio, infra-red, X-ray. No sign. But it must be there.

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.

(03/25/09)