Physics: Dark Matter, Dark Energy and Theory

Spiral Galaxy NGC 4414 - observations of stars in our galaxy and other galaxies demonstrate that our current theories of astrophysics are unsatisfactory in key areas.  Dark matter and dark energy are mysterious and controversial explanations for improving our theories.

Physicists don't often acknowledge that they are basically ignorant of some key aspects of our universe.  However, in the field of astronomy, we do hear this a lot!  David Cline is a professor of Physics and Astrophysics at UCLA.  Here is a quote from an article he wrote for the March 2003 Scientific American:
The universe around us is not what it appears to be. The stars make up less than 1 percent of its mass; all the loose gas and other forms of ordinary matter, less than 5 percent. The motions of this visible material reveal that it is mere flotsam on an unseen sea of unknown material. We know little about that sea. The terms we use to describe its components, "dark matter" and "dark energy," serve mainly as expressions of our ignorance.

Professor Kline's use of the word "flotsam" seems particularly apt since this is defined as: '... cargo that remains afloat after a ship has sunk.'   If we take the "ship" as the source of the universe's observable "cargo", then this ship may be buried so deeply that we have not yet fathomed its meaning.

Astrophysics, like all forms of science, is strongly tied to what is measurable.  Theories are developed, and improved, based on these measurements.  The measurements of the motions of stars, galaxies, and galaxy clusters do not match the current theories using the amount of visible matter, so these measurements imply a key role for dark matter.  Similarly, measurements of distant supernova and of the expansion rate, as well as the geometry, of the universe itself, imply a dominant role for a "dark energy" which exerts negative pressure and therefore causes an anti-gravitational response on the very large scale.

There is much controversy over how to modify the theories to match the measured data.  Current theories, including the General Theory of Relativity and the Standard Model of Fundamental Particles, can explain the observed motions of the stars/galaxies using the gravitational properties of huge amounts of dark matter - dark matter composed of new unmeasured particles which are properly distributed.  Similarly, dark energy is needed with current theories to explain the observation that the expansion rate of the universe is increasing with time - see the references below for additional "anomalous" measurements and discussions of dark energy. 

But wait, some physicists are saying that we need to stand back and look at some of the basic assumptions in the theories themselves.  For example, another article in Scientific American is entitled, "Does Dark Matter Really Exist?".  This article describes an idea first proposed in 1983 by physicist Mordehai Milgrom now at the Weizmann Institute of Science in Rehovot, Israel.  The idea, called MOND (for Modified Newtonian Dynamics), proposes to modify Newton’s second law, Force = mass x acceleration (F = ma), which then modifies the gravitational effects of mass.  MOND predicts that Newton’s law is valid except when mass is accelerating very, very slowly.  And, galaxies and systems of galaxies indeed accelerate very, very slowly over very, very long times.

The MOND theory is as successful at explaining the astronomer’s observations as more standard models that assume the existence of dark matter.  Dr. Milgrom is quick to point out that the MOND theory has its own set of problems, which he and other scientists are working on.  Time will tell, with further measurements, which theory is best.  However, it is worth noting the following quote from the article: "If we accept a departure from the standard laws of physics, we might do away with dark matter."

There are indeed many mysteries concerning our observable universe and the theories used to describe them..