In the second place, if the observed spectral shifts represent mere scattering, we can calculate the time since this scattering began; it proves to be many thousands of millions of years. Enormous though such a length of time is, it does not appear to be enough. We shall see later (Chapter III) how time leaves its mark, its wrinkles and its grey hairs, on the stars, so that we can guess their ages tolerably well, and the evidence is all in favour of stellar lives, not of thousands of millions, but of millions of millions, of years. If the nebulae owe their present motions to mere scattering, then the stars must have lived the greater parts of their lives before this scattering began. Such a hypothesis seems too artificial for acceptance, at any rate so long as any alternative is open. Of course we must frankly admit that our estimates of stellar ages may be found to need revision. Indeed they have been calculated on the supposition that no appreciable scattering of the type required by de Sitter’s cosmology has ever taken place. If not only the nebulae, but also the stars composing the galactic system, were huddled together at the beginning of time, our estimates of the lives of the stars would have to be substantially shortened, and it is conceivable, although I think very unlikely, that they could be reduced to lengths of the kind we have just considered; they certainly could not be so reduced if the original universe had a radius anywhere near to Eddington’s estimate of 1200 million light-years. Nevertheless the cosmologies of de Sitter and Lemaître undoubtedly require that the present-day universe must be expanding, that the nebulae must be retreating both from one another, and from us, and that their spectra must, as a consequence, shew displacements to the red. But there is nothing to compel us to identify these displacements with those actually observed. Many causes, other than motions of recession, are capable of reddening spectral lines, and only after we have deducted all the reddening due to these various other causes shall we be in a position to say that the residue represents a true motion of recession. Quite recently Dr Zwicky of Pasadena has suggested that gravitating matter diffused through space may redden all light which passes through the space. He gives dynamical reasons for his suggestion, calculates the amount of spectral shift to be anticipated in the light reaching us from the nebulae, and finds that it would account for practically the whole of the observed shift. It is possible that the greater part of the observed reddening of nebular light may be due to this or some similar cause, only a small fraction representing true motions of recession. If so, we can extend the age of the universe indefinitely, and are free to assign to the stars the very great ages which the evidence of general astronomy seems to demand. This we shall discuss fully in Chapter III.