2.3.3 Physical evolution
Physical evolution entered astronomy in the wake of Darwin's biological evolution. Since then it has become the essence of our understanding of stars, galaxies and the universe.
Stellar evolution (starting with Lockyer's ``Inorganic Evolution'', 1900) lets us expect changing parameters of galaxies. Galaxy evolution is partly due to the evolution of its stellar population (brightness, colour, chemical abundances, gas content), partly due to other internal or external causes (diameter, mass, momentum, angular momentum - see above). The observable properties of the universe are affected by the evolving stars (Olbers Paradox), traceable by our knowledge of the stars (chemical abundances, evolutionary computations), apparently affected by the evolving galaxies (changing galaxy parameters mask and mimic universal evolution) and by its own evolutionary history (which we want to derive).
For the first cosmological problem, the Kepler - Lois de Chésaux - Olbers Paradox or the Dark Night sky Paradox, a number of solutions have been provided throughout the times, the finite life time of stars, i.e. stellar evolution, is one of them (Harrison 1981).
Early thoughts about the evolution of galaxies appear in Charlier's (1922) discussion referred to in the previous paragraph: collisions of nebulae are thought to create spiral pattern. Spitzer and Baade (1951) invoke collisions to account for the absence of dust (and thus of the spiral pattern traced by young stars) in S0 galaxies, for merging see above.
Figure 22. Tinsley's (1968) evolutionary tracks of galaxies in the Hubble diagram.
Galaxy parameters of unknown time-dependence, making the comparison between near and distant objects illusory were the major concern of Baade, as quoted by Sandage (1987). Tinsley (since 1968, Fig. 22) made us see the Hubble diagram not only as a diagram relating universal parameters but in some aspects as a counterpart to the stellar Hertzsprung-Russell diagram: the locus where galaxy evolution can be traced. An early reference to possible evolutionary effects is made by Hubble and Tolman (1935):
``With regard to the assignment of constant properties to the nebulae over long periods of time, is to be remarked that in the case of the most distant survey, to m = 21.0, the `average' nebula involved must have emitted the light which is now observed at a time of the order of 3 x 108 years in the past. If the recessional explanation of red-shift were adopted, this period would be so nearly comparable with the time of cosmic expansion - possibly of the order of 109 to 1010 years - that the assignment of constant properties might not appear justified, and in that case we might try to explain excess counts by assuming greater luminosities for the nebulae at earlier times.''
The search for traces of evolution is one of the great topics of current observational cosmology, as is the strife for finding evolutionary links between the background radiations, quasars, radio galaxies, and normal galaxies at different redshifts. The visible signs of evolution of the universe are the 3 K background radiation, first computed by Alpher and Herman (1948, see also Lemaître 1934, Sect. 4.1) and first measured by Penzias and Wilson (1965), and the abundances of chemical elements, of which a certain percentage can be attributed to the same hot early phase as the background radiation.
In the context of evolution in the expanding universe Lemaître (1931b) opened the discussion on what he later called the primeval atom:
``Sir Arthur Eddington states that, philosophically, the notion of the beginning of the present order of Nature is repugnant to him. I would rather be inclined to think that the present state of quantum theory suggests a beginning of the world very different from the present order of Nature. Thermodynamical principles from the point of view of quantum theory may be stated as follows: (1) Energy of constant total amount is distributed in discrete quanta. (2) The number of distinct quanta is ever increasing. If we were to go back in the course of time we must find fewer and fewer quanta, until we find all the energy of the universe packed in a few or even in a unique quantum. Now, in atomic processes, the notions of space and time are no more than statistical notions; they fade out when applied to individual phenomena involving but a small number of quanta. If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time, I think that such a beginning of the world is far enough from the present order of Nature to be not at all repugnant.''
Concerns that not all elements can be made in stars were voiced by Weizsacker (1938):
``From theory we must ask a list of testable suggestions when and where in the history of the cosmos the required temperatures and densities could have been realized.''
In 1946 Gamow wrote:
``It is generally agreed at present that the relative abundances of various chemical elements were determined by physical conditions existing in the universe during the early stages of expansion, when the temperature and density were sufficiently high to secure appreciable reaction-rates for the light as well as for the heavy nuclei.''
Two years later, Gamow (1948) considered element formation and subsequent galaxy formation according to the Jeans criterion in the expanding universe at the time when radiation = matter. Because for adiabatic expansion T 1 / R, the temperature can be obtained at any given time from the integration of dR / dt. Gamow obtained T = 340 K for the suggested time of galaxy formation at an age of the universe of 1.3 x 108 years.
The two weeks after Gamow's publication, Alpher and Herman (1948) repeated the computation and carried the temperature determination to a present value of T = 5 K.