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Since the publication of the first edition of this little book,

our knowledge about the structure of space in the large

("cosmological problem") has had an important development, which

ought to be mentioned even in a popular presentation of the

subject.

My original considerations on the subject were based on two

hypotheses:

(1) There exists an average density of matter in the whole of

space which is everywhere the same and different from zero.

(2) The magnitude ("radius") of space is independent of time.

Both these hypotheses proved to be consistent, according to the

general theory of relativity, but only after a hypothetical term

was added to the field equations, a term which was not required

by the theory as such nor did it seem natural from a theoretical

point of view ("cosmological term of the field equations").

Hypothesis (2) appeared unavoidable to me at the time, since I

thought that one would get into bottomless speculations if one

departed from it.