138 research outputs found
A quantum Friedmann flat spacetime: Uncertainty Relations, Thermodynamics and some cosmological consequences
We present Friedmann flat spacetime uncertainty relations (STUR) together
with some cosmological implications. An interesting link between the Principle
of "gravitational stability against localization of events" (PGSL) and the
holographic Bekenstein entropy bound (HEB) is also investigated. The same
theorems leading to our STUR are used to calculate, thanks to the holographic
principle, the entropy of the universe at its apparent horizon. The generalized
entropy formula can be used to discuss interesting links with a quantum
spacetime.Comment: Contributed paper to the Fourteenth Marcel Grossmann Meeting on
General Relativity, University of Rome "La Sapienza", Italy, 12 - 18 July
2015; edited by Massimo Bianchi, Robert T Jantzen, Remo Ruffini. (World
Scientific, Singapore, 2017) p.3739 - p.3743. Contributions in parts present
on arXiv:1102.0894; arXiv:1308.2767; arXiv:1405.6816; arXiv:1506.0857
The fractal bubble model with a cosmological constant
We generalize the fractal bubble model (FB), recently proposed in the
literature as an alternative to the standard CDM cosmology, to include
a non-zero cosmological constant. We retain the same volume partition of voids
and walls as the original FB model, and the same matching conditions for null
geodesics, but do not include effects associated with a nonuniform time flow
arising from differences of quasilocal gravitational energy that may arise in
the coarse-graining process. The Buchert equations are written and partially
integrated and the asymptotic behaviour of the solutions is given. For a
universe with , as it is the case in the FB model, an initial void
fraction with hyperbolic curvature evolves in such a way that it asymptotically
fills completely our particle horizon. Conversely, in presence of a non
vanishing , we show that this does not happen and the voids fill a
finite fraction , where the value of is
expected to depend on and the initial fraction and also to
be small. For its determination, a numerical integration of the equations is
necessary. Finally, an interesting prediction of our model is a formula giving
a minimum allowed value of present day dark energy as a function of the age of
the universe and of the matter and curvature density parameters at our time.Comment: Published on Class. Quantum Gra
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