The road to reality

The discussion around what we know about the universe is in continuous development. If we take the infographic below, for example, we are faced with three possible scenarios: an accelerated expanding universe that will conclude is run in a big rip, in which the universe eventually turns completely black; a universe in which expansion is in balance with gravity, but nevertheless destined to make the skies of planets black and starless; a universe where expansion is blocked and reversed to a big crunch.

Basically these three scenarios are all compatible with the standard cosmological model, this because of the unknown ingredients of matter and especially dark energy. If we remain within the standard cosmological model, yet another recent measure of the Hubble constant seems to confirm the big rip scenario: not only the University of California team in Davis led by Geoff Chen found yet another value of the constant different from that obtained from the cosmic background radiation, but this result would suggest an even faster expansion than would suggest the standard cosmological model, based on the theory of cosmic inflation(4).
This new result gives me the opportunity to rediscover some notes that I had written and not yet published relatively to a couple of models competing with standard inflation-based cosmology. Let's start with the cyclic universe. To want to be picky, there are two distinct cyclic models, the first developed by Paul Steinhardt and Neil Turok which has string theory(1) as its basis. The model, in fact, assumes that our universe is actually generated by the interaction between two branes - the equivalent in dimensions greater than 4 of the usual membranes (such as air filters) - which interact approaching and moving away with an elastic type force (a sort of hyper-dimensional spring). In this way, it is not only possible to explain the results expected by cosmic inflation, but also providing some forecasts on the cosmic background radiation that are not yet verifiable and inflation does not provide, but also give an account of the accelerated expansion and prove a limit, beyond which reverses itself until the destruction of the current universe and the creation of a new one.
More or less of the same philosophy is the big bounce model.
Developed by Martin Bojowald, the big bounce theory casts Big Bang with big crunch(2). Bojwald started from loop gravity. This model - I hope to write something more detailed about loop gravity in the coming months - sees the universe as constituted by little rings (10-35 m) with certain amount of energy. During gravitational collapse, this energy increases, but doesn't reach infinity, so when equations back in time, universe bounce in a new Big Bang!
In the theory there is a little detail: every universe forgets the previous one. In this way probably it could be possible recover the conservation of the second law of thermodynamics (entropy's law), but in 2011 B.J. Carr and A.A. Coley proposed a paper about some black holes lived in the previous universe(3)!
Carr and Coley propose an idea to verify big bounce, and here there is the logic paradox: if bouncing black hole exist, then universe has (little) memory of his past life, contraddicting big bounce; if bouncing black holes don't exist, big bounce could be correct, but this hasn't importance to us: Gravity Probe B (produced) new evidences in support of Einstein's model.
  1. P. J. Steinhardt, N. Turok (2005). The Cyclic Model Simplified. New Astronomy Reviews. 49 (2–6): 43–57. doi:10.1016/j.newar.2005.01.003
  2. Bojowald, Martin (2007). What happened before the Big Bang?. Nature Physics. 3 (8): 523–525. doi:10.1038/nphys654
  3. Carr, B. J., & Coley, A. A. (2011). Persistence of black holes through a cosmological bounce. International Journal of Modern Physics D, 20(14), 2733-2738. doi:10.1142/S0218271811020640 (arXiv
  4. Chen, G. C., Fassnacht, C. D., Suyu, S. H., Rusu, C. E., Chan, J. H., Wong, K. C., ... & Millon, M. (2019). A SHARP view of H0LiCOW: H 0 from three time-delay gravitational lens systems with adaptive optics imaging. Monthly Notices of the Royal Astronomical Society, 490(2), 1743-1773. doi:10.1093/mnras/stz2547 

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