Changes

Early in this century, it was learned that the average mass density of the universe (the mass per volume) was the critical parameter determining the ultimate fate of the universe. If the average density is smaller than a defined critical value, found by a combination of theory and experiment, then the universe expands forever; all galaxies endlessly recede from each other, eventually cooling into the dead cinders left from stellar evolution. On the other hand, if the density is greater than the critical value, the recession stops and reverses. The galaxies then fall back together at ever-increasing velocities into a cosmic inferno-the "big crunch." Thus the average mass density of the universe determines whether it ends with a whimper or a bang, or more properly a diffuse, deathly cold universe or a fiery, cataclysmic big crunch.

Early in this century, it was learned that the average mass density of the universe (the mass per volume) was the critical parameter determining the ultimate fate of the universe. If the average density is smaller than a defined critical value, found by a combination of theory and experiment, then the universe expands forever; all galaxies endlessly recede from each other, eventually cooling into the dead cinders left from stellar evolution. On the other hand, if the density is greater than the critical value, the recession stops and reverses. The galaxies then fall back together at ever-increasing velocities into a cosmic inferno-the "big crunch." Thus the average mass density of the universe determines whether it ends with a whimper or a bang, or more properly a diffuse, deathly cold universe or a fiery, cataclysmic big crunch.

==Hubble Space Telescope image of M100==

===Hubble Space Telescope image of M100===

In the last couple of decades it has become firmly established that the visible universe-everything seen by employing the entire electromagnetic spectrum, from radio wavelengths to gamma rays-represents less than one-tenth of the total mass of the universe. For example, the spiral galaxy, shown in the photograph, is overwhelmingly composed of unseen or nonradiating matter. The visible universe, of consuming interest to us since our Neanderthal ancestors gazed heavenward, is truly like the tip of an iceberg. There are typically one hundred billion stars in a galaxy and about ten billion galaxies in the visible universe, yet all these galaxies, stars, and gas clouds seen in any part of the electromagnetic spectrum make up only from one-hundredth to one-tenth of the total mass of the universe.

In the last couple of decades it has become firmly established that the visible universe-everything seen by employing the entire electromagnetic spectrum, from radio wavelengths to gamma rays-represents less than one-tenth of the total mass of the universe. For example, the spiral galaxy, shown in the photograph, is overwhelmingly composed of unseen or nonradiating matter. The visible universe, of consuming interest to us since our Neanderthal ancestors gazed heavenward, is truly like the tip of an iceberg. There are typically one hundred billion stars in a galaxy and about ten billion galaxies in the visible universe, yet all these galaxies, stars, and gas clouds seen in any part of the electromagnetic spectrum make up only from one-hundredth to one-tenth of the total mass of the universe.