More Than Meets the Eye

Anyone who follows science even a little knows that our universe is incredibly vast. In addition to Earth and other planets circling our star, the sun, there are countless billions of other stars. These form great, rotating islands of stars called galaxies that also number in the billions. In recent years, astronomers have found that what we see in the universe may be only a small fraction of the "stuff" that exists out there. Measurements of the rotation speed of galaxies show that they are rotating faster than they should for the amount of material they contain, and should be flinging their stars out into space. In other words, the combined gravity of the galaxy's constituent stars do not have enough mass to hold the galaxy together. Since galaxies are not coming apart, astronomers were forced to theorize that there must be more stuff in the galaxies than what they see or detect with instruments. They have called this stuff Dark Matter. No one, currently, has any idea of what it is.

In addition to dark matter, astronomers have discovered another cosmic anomaly. They have known for some time that galaxies are all moving away from each other as space and the universe expands. This expansion has been going on ever since the universe began with the big bang some 14 billion years ago. However, the gravitational attraction of all the galaxies for each other should be slowing down the universe's expansion. Over the past several years, astronomers have decided to measure what this rate of slowdown is. They did this by measuring two things in distant galaxies. First, the galaxy's speed of recession as indicated by their red shift--how their light is reddened due to the stretching of their light waves by the expansion of space. Second, the galaxy's distance. This is done by finding in the galaxy a particular type of star whose intrinsic brightness is known and measuring how its light is dimmed by its distance. Astronomers refer to such known brightness stars as a standard candles. The particular kind of star used in this study and occasionally found in galaxies, is called a Type Ia Supernova . When you make a graph of red shift (speed of recession) versus distance you would normally expect it to be a sloping straight line if the universe was expanding at a constant rate. If the plot curved slightly downward, the universe's expansion was slowing down as was expected. What they were surprised to find was that the plot curved upward indicating that the supernovae were dimmer than expected. That meant that the galaxies were farther away than expected, and that the universe's expansion was not slowing down but accelerating.

Apparently the gravity created by all the galaxies and dark matter must be counteracted by some repulsive or anti-gravity force. What this force, termed Dark Energy, is no one has a clue. Under current thinking, the visible stuff (planets, stars, galaxies, etc.) makes up only about 4% of the mass of the universe. Dark Matter represents about 23% and Dark Energy (Einstein showed that energy and matter are equivalent) about 73%. Cosmologists have some big mysteries to solve, so stay tuned!