150 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946: 
galaxies and the expansion of the universe. The average density 
of matter throughout our planetary system and even throughout our 
galaxy is so high that the dispersive tendency is overcome by the 
great gravitational forces involved. Even in a cluster of galaxies— 
like the one in our vicinity which includes our galaxy, the Andromeda 
Nebula and its companions, the Magellanic Clouds, and half a dozen 
others—gravitation still controls the situation, and our cluster of 
galaxies is not dissolving under cosmic repulsion—at least not with 
marked rapidity. The cohesion that maintains our cluster apparently 
operates in a number of other close associations of galaxies which 
have come into our records. The mean density in them is high 
enough for gravitational control. But throughout metagalactic space 
in general the mean density is perhaps but one-hundredth of the value 
within the cluster of galaxies. The density is, in fact, about 10-* 
grams per cubic centimeter and too low for gravitation any longer 
to maintain the situation. The expansion that has set in under the 
repulsive force will still further lower the mean density, and it there- 
fore now appears that we (the metagalaxy) are doomed to infinite 
dissipation. 
At the same time, through the operation of the laws of thermo- 
dynamics, the heat of the stars is going out into the coldness of space. 
The universe is steadily approaching the heat-death—a coldness near 
absolute zero in an empty world. 
Such destinies define the future of the metagalaxy only if the 
processes are forever one-directional. Perhaps they are not. Even 
now we cannot say that the reverse building-up processes are not 
going on in some parts of the universe. We see less than 1 percent 
of it. There is, however, no substantial evidence or argument for 
the cyclic restoration of heat and density. And some cosmogonists 
are bold enough to abstain from wishful thinking. 
A further section of this paper could be written to review the 
situation with regard to interstellar dust and its possible role in the 
forming of stars and in the development of galaxies. Also we 
might present the preliminary hypotheses that deal with the evolu- 
tionary passage of galaxies along the continuous sequence of forms that, 
includes systems that are globular, oblate, spiral, and irregular; and 
discuss the nature and the formation of spiral arms. The genetics 
of star clusters is an allied study of basic significance. But all these 
galaxy-sized problems are so remote from those dealing with the 
crust of the earth and its genesis that we can scarcely justify their 
discussion in a collection of geological papers. We close this astro- 
nomical contribution with the statement that the planet earth has, 
after all, a most perplexing and exciting role in the story of 
cosmogony. 
