1,721,802 research outputs found
Bounds on dark matter annihilations from 21cm data
No full textThe observation of an absorption feature in the 21 cm spectrum at redshift z ≈ 17 implies bounds on Dark Matter annihilations for a broad range of masses, given that significant heating of the intergalactic medium would have erased such feature. The resulting bounds on the DM annihilation cross sections are comparable to the strongest ones from all other observables
A micro-to-macro approach to returns, volumes and waiting times
Full text linkModelling stock prices has been a research topic for many decades and it is still an open question. Different approaches have been used in the literature, the majority of which can be classified within the so-called econometric framework and sometimes also referred to as the macro-to-micro approach. Another strand of literature relies on the modelling of directly observable quantities, the so-called micro-to-macro approach. Based on this second line of research, we propose a new multivariate stochastic process to model simultaneously price returns, trading volumes and the time interval between changes in trades, price and volume. The proposed model is based on a generalization of semi-Markov chain models and copulas and is motivated by empirical evidence that the three mentioned variables are correlated and long-range autocorrelated. Utilizing Monte Carlo simulations, we compared our model with real data from the Italian stock market and show that it can reproduce many empirical pieces of evidence. The proposed model can be used in the field of portfolio optimization, development of risk measure and volatility forecasting
Introduzione, in [Meridiana : rivista di storia e scienze sociali : 92, 2, 2018].
No full textThe period between the two World Wars marks the end of mass emigration for Italy. In the course of thirty years, the migratory flows were transformed: the number of starters was reduced and the trajectories changed. The new restrictions established by immigration countries – (e.g. the Quota Acts in the Twenties) – and the advent of the fascist regime redesigned the paths of Italian emigrants. At first, Mussolini gathered the legacy of liberal governments; then, since the end of the Twenties, he inaugurated a new migration policy, in line with the demographic policy of the regime and strengthening the link between emigration and foreign policy. As the historian Bertonha wrote, emigration became, «an evil to be preferred to the internal colonization and to that of the Empire». How, therefore, did the international conjuncture and the immigration restrictions that ensued, together with the new migration policies of the regime, limit the possibility of expatriation of people from Southern Italy? The closure of outlets, in fact, imposed new migration routes, which were mainly oriented towards Europe, as in the case of France and Germany, or colonies in Africa. It seems, however, that the new regime’s migratory policies – strongly connected from the 1930s to an aggressive, bellicose and imperialist foreign policy – have not always been able to provide an adequate response to millions of southerners who had traditionally travelled the migratory routes
Black hole echoes
No full textWe consider a very simple model for gravitational wave echoes from black hole merger ringdowns which may arise from local Lorentz symmetry violations that modify graviton dispersion relations. If the corrections are sufficiently soft so they do not remove the horizon, the reflection of the infalling waves which trigger the echoes is very weak. As an example, we look at the dispersion relation of a test scalar field corrected by rotonlike operators depending only on spatial momenta, in Gullstrand-Painlevé coordinates. The near-horizon regions of a black hole do become reflective, but only very weakly. The resulting "bounces"of infalling waves can yield repetitive gravity wave emissions but their power is very small. This implies that to see any echoes from black holes we really need an egregious departure from either standard GR or effective field theory, or both. One possibility to realize such strong echoes is the recently proposed classical firewalls which replace black hole horizons with material shells surrounding timelike singularities
Reviving QFT in (2+1)-dimensional de Sitter spacetime
No full textWe consider a conformally coupled scalar quantum field theory (QFT) on (2 + 1)-dimensional static Einstein universe R x S2 and write down the free theory Hilbert space. We explain that this theory is secretly a QFT in (2 + 1)-dimensional de Sitter space because all the quantum observables experience "quantum revivals," which naturally restricts the timelike R to the appropriate de Sitter time range. Our construction circumvents the causal obstruction to formulating QFT in de Sitter due to event horizons. There are not any in static Einstein. The "unitary gauge" description of the theory is realized by the zonal harmonics Pl(n n0). We verify that interactions with conformally invariant external sources are mediated only by these modes. Hence these modes comprise the complete basis of the "bulk" theory. When the theory is cut off in the UV, the basis dimension scales as the Bekenstein-Hawking formula
Statistical Inference for Markov Reward Processes and application to the Italian Bonus-Malus Systems.
No full textA multivariate Markov chain stock model
No full textWe propose a dividend stock valuation model where multiple dividend growth series and their dependencies are modelled using a multivariate Markov chain. Our model advances existing Markov chain stock models. First, we determine assumptions that guarantee the finiteness of the price and risk as well as the fulfilment of transversality conditions. Then, we compute the first- and second-order price-dividend ratios by solving corresponding linear systems of equations and show that a different price-dividend ratio is attached to each combination of states of the dividend growth process of each stock. Subsequently, we provide a formula for the computation of the variances and covariances between stocks in a portfolio. Finally, we apply the theoretical model to the dividend series of three US stocks and perform comparisons with existing models. The results could also be applied for actuarial purposes as a general stochastic investment model and for calculating the initial endowment to fund a portfolio of dependent perpetuities
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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