No Arabic abstract
A huge discrepancy between the zero-point energy calculated from quantum theory and the observed quantity in the Universe has been one of the most illusive problems in physics. In order to examine the measurability of zero-point energy, we construct reference frames in a given measurement using observables. Careful and explicit construction of the reference frames surprisingly reveals that not only is the harmonic oscillator fluctuating at the ground level, but so is the reference frame when the measurement is realized. The argument is then extended to examine the measurability of vacuum energy for a quantized electromagnetic field, and it is shown that while zero-point energy calculated from quantum theory diverges to infinity, it is not measurable.
In this review we present a theory of cosmological constant and Dark Energy (DE), based on the topological structure of the vacuum. The Multiple Point Principle (MPP) is reviewed. It demonstrates the existence of the two vacua into the SM. The Froggatt-Nielsens prediction of the top-quark and Higgs masses is given in the assumption that there exist two degenerate vacua in the SM. This prediction was improved by the next order calculations. We also considered B.G. Sidharths theory of cosmological constant based on the non-commutative geometry of the Planck scale space-time, what gives an extremely small DE density providing the accelerating expansion of the Universe. Theory of two degenerate vacua - the Planck scale phase and Electroweak (EW) phase - also is reviewed, topological defects in these vacua are investigated, also the Compton wavelength phase suggested by B.G. Sidharth was discussed. A general theory of the phase transition and the problem of the vacuum stability in the SM is reviewed. Assuming that the recently discovered at the LHC new resonance with mass $m_S simeq 750$ GeV is a new scalar $S$ bound state $6t + 6bar t$, earlier predicted by C.D. Froggatt, H.B. Nielsen and L.V. Laperashvili, we try to provide the vacuum stability in the SM and exact accuracy of the MPP.
It is well known that string theories naturally compactify on anti-de Sitter spaces, and yet cosmological observations show no evidence of a negative cosmological constant in the early Universes evolution. In this letter we present two simple nonlocal modifications of the standard Friedmann cosmology that can lead to observationally viable cosmologies with an initial (negative) cosmological constant. The nonlocal operators we include are toy models for the quantum cosmological backreaction. In Model I an initial quasiperiodic oscillatory epoch is followed by inflation and a late time matter era, representing a dark matter candidate. The backreaction in Model II quickly compensates the negative cosmological term such that the Ricci curvature scalar rapidly approaches zero, and the Universe ends up in a late time radiation era.
The today estimated value of dark energy can be achieved by the vacuum condensate induced by neutrino mixing phenomenon. Such a tiny value is recovered for a cut-off of the order of Planck scale and it is linked to the sub eV neutrino mass scale. Contributions to dark energy from auxiliary fields or mechanisms are not necessary in this approach.
The method of decoherent histories allows probabilities to be assigned to sequences of quantum events in systems, such as the universe as a whole, where there is no external observer to make measurements. This paper applies the method of decoherent histories to address cosmological questions. Using a series of simple examples, beginning with the harmonic oscillator, we show that systems in a stationary state such as an energy eigenstate or thermal state can exhibit decoherent histories with non-trivial dynamics. We then examine decoherent histories in a universe that undergoes eternal inflation. Decoherent histories that assign probabilities to sequences of events in the vicinity of a timelike geodesic supply a natural cosmological measure. Under reasonable conditions, such sequences of events do not suffer from the presence of unlikely statistical fluctuations that mimic reality.
We study the cosmological constant ($Lambda$) in the standard $Lambda$CDM model by introducing the textit{graduated dark energy} (gDE) characterised by a minimal dynamical deviation from the null inertial mass density of the $Lambda$ in the form $rho_{rm inert}propto rho^{lambda}<0$ with $lambda<1$ being a ratio of two odd integers, for which its energy density $rho$ dynamically takes negative values in the finite past. For large negative values of $lambda$, it creates a phenomenological model described by a smooth function that approximately describes the $Lambda$ spontaneously switching sign in the late universe to become positive today. We confront the model with the latest combined observational data sets of PLK+BAO+SN+$H$. It is striking that the data predict bimodal posterior probability distributions for the parameters of the model along with large negative $lambda$ values; the new maximum significantly excludes the $Lambda$, and the old maximum contains the $Lambda$. The improvement in the goodness of fit for the $Lambda$ reaches highly significant levels, $Deltachi_{rm min}^2=6.4$ for the new maxima, while it remains at insignificant levels, $Deltachi_{rm min}^2lesssim0.02$, for the old maxima. We show that, in contrast to the old maxima, which do not distinguish from the $Lambda$, the new maxima agree with the model-independent $H_0$ measurements, high-precision Ly-$alpha$ data, and model-independent $Omh^2$ diagnostic estimates. Our results provide strong hints of a spontaneous sign switch in the cosmological constant and lead us to conjecture that the universe has transitioned from AdS vacua to dS vacua, at a redshift $zapprox 2.32$ and triggered the late-time acceleration, and suggests looking for such mechanisms in string theory constructions.