148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1946 
atoms, since the universe apparently is still largely composed of simple 
hydrogen, is of the order of 10”. 
We have measured the velocities of a good and representative sam- 
ple of galaxies; but we should not extrapolate back too confidently in 
our search for the zero point of time. Throughout the past the rate of 
expansion may not have been constant at the presently observed value. 
It may have been increasing with time. That assumption would 
Jengthen the interval between now and the zero time of the expansion. 
But, however we try to modify the situation, within reason, we come 
to the conclusion that a few thousand million years ago our galaxy 
was in a region characterized by an exceedingly high density of 
galaxies, stars, and matter. 
We should now point out some important, almost dramatic, coinci- 
dences. It is remarkable enough that the age of the observed expan- 
sion of the metagalaxy is of the same order as the age of the earth’s 
crust, measured by radioactivity, and therefore of the earth itself. 
But this interval of a few thousand million years also measures the 
somewhat preliminarily determined ages of the oldest meteorites so far 
‘examined. It is also the interval through which star clusters like the 
Pleiades must have existed; they cannot be more than a few thousand 
million years old according to dynamical researches by Bok and others. 
And finally, this interval, according to the recent work of Chandra- 
sekhar, is consistent with the ages of wide binary stars. 
Although it has not been worked out quantitatively, there is a 
suggestion that in proving the existence of extended clouds of dust in 
our Milky Way, W. 8S. Adams at Mount Wilson has provided another 
indicator that our galaxy cannot have existed for very many thousands 
of millions of years. 
We come to the conclusion, important in cosmogony, that about 
half a dozen lines of evidence point to a time some 8,000 million years 
ago as an epoch when something momentous happened. One could 
call that moment “creation,” if he made his definitions carefully. He 
might simply call it the beginning of the epoch of the Expansion of 
the Universe, with its numerous consequent byproducts. Or he might 
more simply and safely call it To, and leave to the investigators of the 
future both the further testing of its reality and the speculations as 
to its meaning. 
It should promptly be put on record that all the astronomical evi- 
dence and speculation is not definitely on the side of this “short” time 
scale of a few thousand million years. To many of us that interval 
seems to cramp sidereal processes too severely. I have in mind the 
sequence of types of galaxies; some indications of the ages of the 
individual stars; the evolution of globular star clusters. It may be 
that these phenomena can be so interpreted that the short time scale 
is sufficient for everything we observe and reason about. But we 
