Abstract
The singularity theorems of the 1960s showed that Lemaître’s initial symmetry assumptions were not essential for deriving a big-bang origin for a vast multitude of relativistic universe models. Yet the actual universe accords remarkably closely with models of Lemaître’s type. This is a mystery closely related to the form taken by the 2nd law of thermodynamics and is not explained by currently conventional inflationary cosmology. Conformal cyclic cosmology (CCC) provides another perspective on these issues, one consequence being the necessary initial presence of a dominant scalar material that interacts only gravitationally, but which must ultimately slowly decay away in a novel but perhaps detectable way. According to CCC, our current universe picture provides but one aeon of an unending succession of expanding aeons each having an initial big bang which is the conformal continuation of the remote exponential expansion of its previous aeon. The observational status of CCC is briefly discussed.
Highlights
Georges Lemaître made a huge contribution to cosmology when he pointed out that Einstein’s equations, in a cosmological setting, in which the galaxies expand away from each other led one to believe that the universe could well have had a singular origin, a finite time ago
The actual universe is not exactly of this completely symmetrical nature, the mere presence of separate galaxies each containing myriads of separated stars implies that the mass distribution, and the Ricci-tensor distribution is, in detail very far from uniform. It is a natural expectation, as we evolve Einstein’s equations into the future, that the overall picture will remain well approximated by the exact Friedmann–Lemaître-type model. This is despite the fact that at the very local level, the development of black holes, in particular, would give us something greatly differing from the smoothed-out overall picture
Time reversing this picture, we find that the singular origin that we find in the FLRW models is something extremely special
Summary
Georges Lemaître made a huge contribution to cosmology when he pointed out that Einstein’s equations, in a cosmological setting, in which the galaxies expand away from each other (the observational evidence for which he was well aware of) led one to believe that the universe could well have had a singular origin, a finite time ago. The situation at even earlier times cannot have been any less imbalanced, in this regard, as follows from general thermodynamic principles This applies to the action of inflation, despite the fact that it is often argued that a very early exponential inflationary expansion would serve to iron out any spatial irregularities in the initial geometry of the universe. It may be seen, by considering a time-reversed collapsing universe that for a generic (non FLRW) Big Bang, the presence of an inflaton field is of no help whatever in smoothing out such an irregular initial singular expansion (see [14] for a more thorough explanation of this point).
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