International School for Advanced Studies

About: International School for Advanced Studies is a education organization based out in Trieste, Friuli-Venezia Giulia, Italy. It is known for research contribution in the topics: Galaxy & Dark matter. The organization has 3751 authors who have published 13433 publications receiving 588454 citations. The organization is also known as: SISSA & Scuola Internazionale Superiore di Studi Avanzati.

Metadynamics with Adaptive Gaussians

Abstract: Metadynamics is an established sampling method aimed at reconstructing the free-energy surface relative to a set of appropriately chosen collective variables. In standard metadynamics, the free-energy surface is filled by the addition of Gaussian potentials of preassigned and typically diagonal covariance. Asymptotically the free-energy surface is proportional to the bias deposited. Here, we consider the possibility of using Gaussians whose variance is adjusted on the fly to the local properties of the free-energy surface. We suggest two different prescriptions: one is based on the local diffusivity and the other on the local geometrical properties. We further examine the problem of extracting the free-energy surface when using adaptive Gaussians. We show that the standard relation between the bias and the free energy does not hold. In the limit of narrow Gaussians an explicit correction can be evaluated. In the general case, we propose to use instead a relation between bias and free energy borrowed from umbrella sampling. This relation holds for all kinds of incrementally deposited bias. We illustrate on the case of alanine dipeptide the advantage of using adaptive Gaussians in conjunction with the new free-energy estimator both in terms of accuracy and speed of convergence.

A Prototype Model of Stock Exchange

Abstract: A prototype model of stock market is introduced and studied numerically. In this self-organized system, we consider only the interaction among traders without external influences. Agents trade according to their own strategy, to accumulate their assets by speculating on the price's fluctuations which are produced by themselves. The model reproduced rather realistic price histories whose statistical properties are also similar to those observed in real markets.

The imprint of the cosmic dark ages on the near-infrared background

Abstract: The redshifted light of the first (Population III) stars might contribute substantially to the near-infrared background (NIRB). By fitting recent data with models including up-to-date Population III stellar spectra, we find that such stars can indeed account for the whole NIRB residual (i.e. after 'normal' galaxy contribution subtraction) if the high-redshift star formation efficiency is f* = 10-50 per cent, depending on the initial mass function (the top-heaviest requiring lowest efficiency) and on the unknown galaxy contribution in the L band (our models, however, suggest it to be negligible). Such an epoch of Population III star formation ends in all models by z e n d 8.8, with a hard limit z e n d 260 M O ., locking their nucleosynthetic products in the compact remnant or by postulating an extremely inhomogeneous metal enrichment of the Lya forest. We discuss these possibilities in detail along with the uncertainties related to the adopted zodiacal light model.

Renormalization group flow in scalar-tensor theories: II

Abstract: We study the UV behaviour of actions including integer powers of scalar curvature and even powers of scalar fields with functional renormalization group techniques. We find UV fixed points where the gravitational couplings have nontrivial values while the matter ones are Gaussian. We prove several properties of the linearized flow at such a fixed point in arbitrary dimensions in the one-loop approximation and find recursive relations among the critical exponents. We illustrate these results in explicit calculations in d = 4 for actions including up to four powers of scalar curvature and two powers of the scalar field. In this setting we note that the same recursive properties among the critical exponents, which were proven at one-loop order, still hold, in such a way that the UV critical surface is found to be five dimensional. We then search for the same type of fixed point in a scalar theory with minimal coupling to gravity in d = 4 including up to eight powers of scalar curvature. Assuming that the recursive properties of the critical exponents still hold, one would conclude that the UV critical surface of these theories is five dimensional.