Science can be thought of as a journey, one in which we only know the path, not the destination. In our explorations of the universe, our path to understanding has taken many unexpected twists and turns. At one time we believed the stars and planets were attached to revolving crystal spheres. Later we adopted a simple and elegant picture of gravitation between freely moving bodies that could be understood by virtually anyone. But because of recent improvements in our methods for gathering evidence, we're discovering the universe is more complex and fascinating that we knew, or could have known.

In 1998, because of new observational evidence, cosmologists reluctantly abandoned much of what we thought we understood about the large-scale physics of the universe. It turns out there is a repulsive force that has no explanation in present physics, a "dark energy" that is pushing galaxies apart at the largest scales, one that requires us to overhaul our picture of how the universe changes over time.

This article presents a brief history of cosmology, introduces some useful tools for understanding the physical issues, then uses those tools to explain this new discovery and its implications for our picture of the universe.

To navigate this article, press the arrow buttons or use the drop-down lists at the top and bottom of the page, or click a section title below.

Overview	This page.
Differential Equations	An explanation of some mathematical notation used in the article.
Classical Cosmology	The picture of the universe before Einstein.
Relativistic Cosmology	Changes in cosmology brought about by relativistic ideas.
Big Bang Cosmology	A universe with a beginning and possibly an end.
Dark Energy Observed	A discussion of the first dark energy observations and its implications, including a mathematical treatment.
A Dark Energy Model	An empirical equation that can be used to picture dark energy's effect on the universe, plus some graphs of its results.
Discussion and Conclusions	Implications of dark energy and related issues.
The Space Applet	An online, interactive Java graphic simulation of gravitation and dark energy.
Space Applet Java Listing	The Java program listing for the Space Applet.
Selected Replies	Reader responses to this article.