Free Ultimate Explanations of the Universe by Michael Heller Page B

continually created in order to keep the density constant. As ageing nebulae drift apart, due to the general motion of expansion, new nebulae are formed in the intergalactic spaces by condensation of newly created matter. Nebulae of all ages hence exist with a certain frequency distribution.
    The principal assumption in the model of the steady state is that “the universe viewed globally does not change.” The authors of this model call this the perfect cosmological principle. It differs from the (ordinary) cosmological principle applied in the Friedman-Lemaître cosmology in that it assumes that the picture of the universe is independent not only of the observer’s position in space (as in the ordinary principle), but also of the point in time of his observation. A large part of Bondi and Gold’s argument boils down to propaganda on behalf of the perfect cosmological principle.
    Copernicus taught us that the Earth does not occupy a special place in space. Why should it have a special place in time? Cosmology is based on the assumption that the same laws of physics are valid throughout the entire universe. If, in accordance with relativistic cosmology, we assume that at the beginning of its evolution the universe experienced a superdense phase, then we can hardly expect the same laws of physics that we have today to apply in densities of the order of 10 93 g/cm 3 . But “if the universe presents the same aspect to every fundamental observer, wherever he is and at all times, then none of these difficulties and doubts arises.” 6
    Of course a static-state universe, in which nothing changes, obeys the perfect cosmological principle. But it does not conform with what is observed, since in a world that is static there would have to be thermodynamic equilibrium, since there are no changes. That is not what we observe. There are large temperature differences in the universe, and we ourselves, living organisms, are systems in states which are far from equilibrium. In other words, according to Bondi, the perfect cosmological principle, together with observation and the laws of thermodynamics, shows that the universe is not in a static state. Therefore it must be either expanding or shrinking. But
In a contracting universe the Doppler shift leads to a disequilibrium in which radiation preponderates over matter, whereas the opposite is true in an expanding universe. Accordingly, the steady-state theory, alone amongst all theories, deduces the fact that the universe is expanding from the local observations of thermodynamic disequilibrium. 7
    For the steady-state model the observations of the red shift in galactic spectra merely confirm that the theory’s deductive reasoning is right.
    But the agreement of the perfect cosmological principle with the observed expansion of the universe can only be upheld at the cost of the assumption that matter is continually being created in space, so as to maintain a constant mean density throughout the universe, despite its expansion. Bondi stresses: “It should be clearly understood that the creation here discussed is the formation of matter not out of radiation but out of nothing.” 8
    Of course, the creation of matter understood in this sense is in breach of the principle of the conservation of energy. Bondi and Gold are well aware of the fundamental role this principle plays in physics, but they emphasise that what is really important in physics is agreement with what is observed, “there is, however, no observational evidence whatever contradicting continual creation at the rate demanded by the perfect cosmological principle,” 9 which requires that a mass equivalent to an atom of hydrogen be created in every litre of volume at a mean rate of once in 5.10 11 years – and there is no experiment sensitive enough to detect such an amount.
    The perfect cosmological principle turned out to be a powerful enough assumption to allow for a determination of the geometry of the universe without