Universe, Closed or Open

The density parameter, a ratio of the real density of matter in the universe to a specified critical density, describes the shape of our universe. The shape of the universe will determine its ultimate fate. A closed universe would have a density value greater than the critical density value of 1, while an open universe, a value less than 1. Critical density, at 1, is the point where just enough matter exists to maintain a balance between collapse and infinite expansion. A closed universe is of finite size and matter cannot spread far enough apart to cease having a gravitational effect on neighboring matter during expansion. The result is eventual collapse, the big crunch. In an open, unbounded universe expansion is infinite and matter, spread ever thinner, has a decreasing gravitational effect on other matter.

The shape of the universe may also be described by the geometry of its curvature. Relativity describes gravity as the curvature of spacetime by an object's mass, the whole universe curved by the total content of its mass. There are three possible curvatures. All take into account an assumption about the way matter is distributed and how gravity works, along with the manner in which the universe first appeared in the big bang.

A positively curved universe, a sphere, is closed and finite. The angles of a triangle in this universe would add up to more than 180 degrees. The density of its mass would cause an eventual halt and reversal of expansion, followed by collapse. A negatively curved, hyperbolic universe, imagined as saddle-shaped, is infinite and unbounded. The angles of its triangles add up to less than 180 degrees, its negative critical density causing it to expand forever. A third possibility exists, that of [Page 1392]a flat universe, where the balance is just enough to enable the expansion to approach zero, yet not support collapse. A flat universe conforms to Euclidean geometry, where the angles add up to 180 degrees. Today, scientists generally regard our universe as flat, although the exact amount of mass present has not yet been determined. Most of the universe seems to be composed of little-understood invisible dark matter and dark energy.

Observations of temperature and other variables in the cosmic microwave background radiation, the afterglow of the big bang, have yielded more information about the likely geometry of space. These measures suggest a flat universe. A recent study, using information from the Sloan Digital Sky Survey, looked at the distribution of a great number of giant elliptical galaxies sufficiently distant to measure the evolution of these galaxies over the past 5 billion years. The resulting yardstick for measuring cosmic expansion again led to the conclusion that the universe is flat.