Luis Gonzalez-Mestres
Pre-Big Bang, vacuum and noncyclic cosmologies
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ABSTRACT. WMAP and Planck open the way to unprecedented Big Bang phenomenology, potentially allowing to test the standard Big Bang model as well as less conventional approaches including pre-Big Bang cosmologies. An illustration is provided by the recent claim (Gurzadyan et al.) that the cosmological sky would be a weakly random one with mostly regular signal. This work has been followed by an interesting and useful debate. Whatever the conclusion, it appears that a detailed study of WMAP and Planck data can have significant implications for pre-Big Bang theories. Not only for cyclic cosmologies following the analysis recently proposed by Gurzadyan and Penrose, but also for noncyclic approaches incorporating a new fundamental scale beyond the Planck scale and, possibly, new ultimate constituents of matter with unconventional basic properties as compared to standard particles. Alternatives to standard physics can be considered from a cosmological point of view concerning vacuum structure, the nature of space-time, the origin and evolution of our Universe, the validity of quantum field theory and conventional symmetries, solutions to the cosmological constant problem, inflationary scenarios, dark matter and dark energy, the interpretation of string-like theories... Lorentz-like symmetries for the properties of matter (standard or superbradyonic) can then be naturally stable space-time configurations resulting from general cosmological scenarios that incorporate physics beyond the Planck scale and describe the formation and evolution of the present vacuum. But an even more primordial question seems to be that of the origin of half-integer spins, that cannot be generated through orbital angular momentum in the usual real space-time. It turns out that the use of a spinorial space-time with two complex coordinates instead of the conventional four real ones presents several attractive features. Taking the cosmic time to be the modulus of a SU(2) spinor leads by purely geometric means to a naturally expanding universe, with a ratio between cosmic relative velocities and distances equal to the inverse of the age of the Universe. No reference to standard matter, hidden fields, gravitation or relativity is required to get such a result that looks quite reasonable from an observational point of view. We discuss basic ideas and phenomenological issues for noncyclic pre-Big Bang cosmologies in the present context.