434 SECTIONAL TRANSACTIONS.—A. 
Tuesday, September 12, 
Discussion on The expanding universe (10.0) :— 
Prof. Sir A. S. EppincTon, F.R.S.—The expanding universe. 
The observational facts which led to the idea of an ‘ expanding universe ’ 
are, I think, pretty well known. Outside our own galaxy of stars there are 
millions of other galaxies which appear to us as spiral nebule ; and these (so 
far as they have been observed) are found to be running away from us. 
The greater the distance, the faster they move ; and approximately, at least, 
the speed is proportional to the distance. This progression of speed with 
distance has been traced up to a distance of 150 million light years, where 
the speed amounts to 15,000 miles a second. For the moment we stop 
there because observation is becoming too difficult, but no doubt in due 
time still more remote nebulz with still higher speeds will be found. 
If we plot this distribution of motion it is easy to see that the spiral 
nebule are running away from each other just as much as they are running 
away from us. It is, therefore, not a case of scattering away from one point, 
but a general uniform expansion or dispersal in which all mutual distances 
are increasing in the same proportion. We have therefore an expanding 
system of galaxies—or, since the system of galaxies is all the universe we 
know, an expanding universe. We may therefore say it is a simple, direct 
fact of observation that our material universe is expanding. Whether it has 
always been expanding and always will expand is another question. 
I do not think the speakers who are to follow me will challenge this— 
I am judging by what I already know of their views—and it will probably 
be accepted as the common basis of discussion. It is therefore, perhaps, 
all the more incumbent on me to say that it is not universally accepted as 
proved ; there is scepticism in some quarters, and it must not be assumed 
that either I or the other speakers regard this scepticism entirely with con- 
tempt. We have generalised from data which are not so extensive and not 
so accurate as we could wish. It is often pointed out that what we actually 
observe is a red shift of the spectrum of the nebulz ; and although a red 
shift of the spectrum usually means that the object is running away from us, 
it is possible that there might be other causes. We can only reply that if 
some other cause is operating in the case of the spiral nebula, it is some 
entirely unknown circumstance of which we have as yet no hint, either in 
theory or observation. I suppose that entirely unknown circumstances 
might upset all our scientific inferences, and there seems no need specially 
to introduce this bogy in connection with nebular velocities. 
Turning to theory, it is a necessary consequence of the theory of 
relativity that there should exist in addition to the ordinary gravitation of 
bodies a repulsive force which we call ‘ cosmical repulsion.’ This is too small 
to affect small-scale systems such as the solar system, and we can only 
expect to detect it in a system on the largest possible scale—if it is detectable 
at all. Cosmical repulsion is of such a nature that if it acted on a system 
of galaxies at rest it would make it expand uniformly—just the type of 
expansion or dispersal that we observe. We must not forget that the 
galaxies are attracting one another in the ordinary way, and that this attrac- 
tion might, in certain circumstances, hold the dispersing force in check. 
However, if the dispersing force once gets the upper hand it will keep it ; 
because as it drives the galaxies further apart their mutual attraction will 
weaken and offer less opposition to the dispersal. ; 
