Showing papers by "Luciano Pietronero published in 2018"

Fractal universe and cosmic acceleration in a Lema\^itre-Tolman-Bondi scenario

Abstract: In this paper we attempt to answer to the question: can cosmic acceleration of the Universe have a fractal solution? We give an exact solution of a Lemaitre-Tolman-Bondi (LTB) Universe based on the assumption that such a smooth metric is able to describe, on average, a fractal distribution of matter. While the LTB model has a center, we speculate that, when the fractal dimension is not very different from the space dimension, this metric applies to any point of the fractal structure when chosen as center so that, on average, there is not any special point or direction. We examine the observed magnitude-redshift relation of type Ia supernovae (SNe Ia), showing that the apparent acceleration of the cosmic expansion can be explained as a consequence of the fractal distribution of matter when the corresponding space-time metric is modeled as a smooth LTB one and if the fractal dimension on scales of a few hundreds Mpc is $D=2.9 \pm 0.02$.

Conventional/unconventional superconductivity in high-pressure hydrides and beyond: insights from theory and perspectives

Abstract: The observation of a superconducting critical temperature $$T_\mathrm{c}$$ exceeding 200K in ultra-dense hydrogen sulfide has demonstrated that high- $$T_\mathrm{c}$$ superconductivity can be achieved also in compounds where the superconducting pairing is mediated by phonons. This poses interesting challenges and opportunities. In particular, in this paper, we present a theoretical overview of the following points:

Band structure and Van Hove singularities in H $$_3$$ 3 S

Abstract: We study the hopping-resolved contribution of the tight-binding parameters to the Van Hove singularities of H $$_3$$ S in the $$Im-3m$$ cubic structure at 180 GPa close to the Fermi level. By means of a $$7\times 7$$ tight-binding model, we study the sensitivity of the DOS close to the Fermi level to a change in the tight-binding parameters. We show that the most relevant parameter affecting the DOS at the Fermi level is the direct $$W_{{\text {sp}}{\sigma }}$$ hopping between the 3s and the 3p orbitals of S. In this way, we can understand some peculiar features of H $$_3$$ S and also give some suggestion for enhancing $$T_\mathrm{c}$$ .

A New and Stable Algorithm for Economic Complexity

Abstract: We present a new metric estimating fitness of countries and complexity of products by exploiting a non-linear non-homogeneous map applied to the publicly available information on the goods exported by a country. The non homogeneous terms guarantee both convergence and stability. After a suitable rescaling of the relevant quantities, the non homogeneous terms are eventually set to zero so that this new metric is parameter free. This new map almost reproduces the results of the original homogeneous metrics already defined in literature and allows for an approximate analytic solution in case of actual binarized matrices based on the Revealed Comparative Advantage (RCA) indicator. This solution is connected with a new quantity describing the neighborhood of nodes in bipartite graphs, representing in this work the relations between countries and exported products. Moreover, we define the new indicator of country net-efficiency quantifying how a country efficiently invests in capabilities able to generate innovative complex high quality products. Eventually, we demonstrate analytically the local convergence of the algorithm involved.