**Is the expansion of the Universe a natural effect due to the internal dynamical properties of the physical vacuum? Can one rule out the possibility that the vacuum is unstable (or metastable) and naturally expands emitting conventional matter and energy similarly to a form of latent heat? Such a scenario based on new physics would deeply transform Cosmology and, in particular, make useless the notion of dark energy as well as the standard cosmological constant.**

The instability (or metastability) considered here concerns the cosmic size of the physical vacuum. It is assumed that the vacuum "likes" to expand, expands permanently and releases matter and energy as it expands. How to check such a possible situation, or the contrary?

Cosmology is often presented as an exact science with a well-established standard theory similar to the standard model of Particle Physics. But does such a propaganda correspond to reality? And what do we really know, even concerning the parameters of the fashionable cosmological patterns? These models involve a basically unknown dark matter and a mysterious dark energy associated to a cosmological constant that generates serious problems when one tries to understand its origin from a Particle Physics point of wiew.

Actually, as I have already emphasized in several articles, we even do not know the internal structure of the physical vacuum. Is the vacuum really empty, except for the zero modes of harmonic oscillators and for the quantum fields we condense in it? Or does it have a pre-existing structure and dynamics of which standard particles are excitations? As early as 1995, and in subsequent papers, I suggested a pattern of the last kind: the vacuum can be made of new ultimate constituents at a deeper level than conventional particles and quantum field theory. The subject has more recently been dealt with in my contributions to the ICNFP conferences since 2012.

Then, the ultimate preonic constituents of the physical vacuum are expected to have a critical speed larger that that of light, just as the speed of light is much larger than the speed of sound. In condensed matter, the critical speed of phonons and solitons is much smaller (around a million times) than that of the particles forming the matter where such excitations are generated. Similarly, the preonic constituents of vacuum can obey a "super-relativity" where they would be the equivalent of the bradyons (non-tachyonic particles) of the standard Lorentz symmetry. For this reason, I called superbradyons such superluminal preons. In the presence of superbradyons with a critical speed much larger than that of light, cosmic inflation is no longer necessary and pre-Big Bang patterns naturally replace the standard Big Bang approach.

A preonic vacuum with a nontrivial internal structure and dynamics can produce even stronger effects. The expansion of our Universe may actually be due to a permanent instability (or metastability) of such a vacuum. The permanent expansion of the vacuum at the cosmic level can then be the natural source of the Universe evolution and be accompanied by the emission of standard matter and energy without any need for the usual dark energy and cosmological constant. Instead of the Big Bang model, an alternative scenario based on the role of vacuum would then describe the new cosmology thus emerging.

In my previous note, I already pointed out that in such case a matter energy density decreasing with time more slowly than in the usual cosmological model, due to the vacuum radiation, can by itself produce the observed acceleration of the expansion of the matter Universe. There can also be other (residual?) reasons for such an acceleration that will not necessarily last forever (see also my previous ICNFP papers). Clearly, the subject deserves further investigation. In particular, is it possible to find a clear signature of the superbradyonic vacuum?

Surprisingly, CERN experiments may have just found signatures of the preonic vacuum constituents. If the vacuum can also emit free superbradyons with a speed much larger than the speed of light, such objects are expected to emit "Cherenkov" radiation in the form of conventional particles. As the energy of the superbradyon will in all cases be much larger than its momentum times the speed of light, its decay modes involving conventional matter will be basically consistent with such a comparatively small total momentum. This is somehow close to the profile of events found at LHC and interpreted as black hole candidates. Do such CERN "black hole candidates" actually involve superbradyons emerging from the physical vacuum, or just standard matter emitted similarly? Can superbradyons be produced by LHC collisions?

The possible cosmological role of a metastable vacuum and the interpretation of the CERN "black hole candidates" are dealt with in the two posters that I have presented at the ICNFP 2016 Conference. See also the transparencies of my lecture of July 6 at this conference.

Luis Gonzalez-Mestres

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