Fine tuning of parameters of the universe

The mechanism of production of a large number of universes is considered. It is shown that universes with parameters suitable for creation of life are necessarily produced as a result of quantum fluctuations. Fractal structures are formed provided fluctuations take place near a maximum of the potential. Several ways of formation of similar fractal structures within our universe are discussed. Theoretical predictions are compared with observational data.Comment: 9 pages, 1 figur

Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach

Light is affected by local inhomogeneities in its propagation, which may alter distances and so cosmological parameter estimation. In the era of precision cosmology, the presence of inhomogeneities may induce systematic errors if not properly accounted. In this vein, a new interpretation of the conventional Dyer-Roeder (DR) approach by allowing light received from distant sources to travel in regions denser than average is proposed. It is argued that the existence of a distribution of small and moderate cosmic voids (or "black regions") implies that its matter content was redistributed to the homogeneous and clustered matter components with the former becoming denser than the cosmic average in the absence of voids. Phenomenologically, this means that the DR smoothness parameter (denoted here by $\alpha_E$) can be greater than unity, and, therefore, all previous analyses constraining it should be rediscussed with a free upper limit. Accordingly, by performing a statistical analysis involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a flat $\Lambda$CDM model we obtain for the extended parameter, $\alpha_E=1.26^{+0.68}_{-0.54}$ ($1\sigma$). The effects of $\alpha_E$ are also analyzed for generic $\Lambda$CDM models and flat XCDM cosmologies. For both models, we find that a value of $\alpha_E$ greater than unity is able to harmonize SNe Ia and cosmic microwave background observations thereby alleviating the well-known tension between low and high redshift data. Finally, a simple toy model based on the existence of cosmic voids is proposed in order to justify why $\alpha_E$ can be greater than unity as required by supernovae data.Comment: 5 pages, 2 figures. Title modified, results unchanged. It matches version published as a Brief Report in Phys. Rev.

Linear Datalog and Bounded Path Duality of Relational Structures

In this paper we systematically investigate the connections between logics with a finite number of variables, structures of bounded pathwidth, and linear Datalog Programs. We prove that, in the context of Constraint Satisfaction Problems, all these concepts correspond to different mathematical embodiments of a unique robust notion that we call bounded path duality. We also study the computational complexity implications of the notion of bounded path duality. We show that every constraint satisfaction problem \csp(\best) with bounded path duality is solvable in NL and that this notion explains in a uniform way all families of CSPs known to be in NL. Finally, we use the results developed in the paper to identify new problems in NL

Model Selection based on the Angular-Diameter Distance to the Compact Structure in Radio Quasars

Of all the distance and temporal measures in cosmology, the angular-diameter distance, d_A(z), uniquely reaches a maximum value at some finite redshift z_max and then decreases to zero towards the big bang. This effect has been difficult to observe due to a lack of reliable, standard rulers, though refinements to the identification of the compact structure in radio quasars may have overcome this deficiency. In this Letter, we assemble a catalog of 140 such sources with 0 < z < 3 for model selection and the measurement of z_max. In flat LCDM, we find that Omega_m= 0.24^{+0.1}_{-0.09}, fully consistent with Planck, with z_max=1.69. Both of these values are associated with a d_A(z) indistinguishable from that predicted by the zero active mass condition, rho+3p=0, in terms of the total pressure p and total energy density rho of the cosmic fluid. An expansion driven by this constraint, known as the R_h=ct universe, has z_max=1.718, which differs from the measured value by less than ~1.6%. Indeed, the Bayes Information Criterion favours R_h=ct over flat LCDM with a likelihood of ~81% versus 19%, suggesting that the optimized parameters in Planck LCDM mimic the constraint p=-rho/3.Comment: 6 pages, 3 figures, 1 table. Accepted for publication in EP