Hydrodynamics of ultra-relativistic bubble walls

In cosmological first-order phase transitions, gravitational waves are generated by the collisions of bubble walls and by the bulk motions caused in the fluid. A sizeable signal may result from fast-moving walls. In this work we study the hydrodynamics associated to the fastest propagation modes, namely, ultra-relativistic detonations and runaway solutions. We compute the energy injected by the phase transition into the fluid and the energy which accumulates in the bubble walls. We provide analytic approximations and fits as functions of the net force acting on the wall, which can be readily evaluated for specific models. We also study the back-reaction of hydrodynamics on the wall motion, and we discuss the extrapolation of the friction force away from the ultra-relativistic limit. We use these results to estimate the gravitational wave signal from detonations and runaway walls.Comment: 30 pages, 11 figures. v2: typos corrected, reference added, a new fit provided in the appendix. v3: a section on GWs added; matches version accepted in NP

Hydrodynamics of phase transition fronts and the speed of sound in the plasma

The growth of bubbles in cosmological first-order phase transitions involves nontrivial hydrodynamics. For that reason, the study of the propagation of phase transition fronts often requires several approximations. A frequently used approximation consists in describing the two phases as being composed only of radiation and vacuum energy (the so-called bag equation of state). We show that, in realistic models, the speed of sound in the low-temperature phase is generally smaller than that of radiation, and we study the hydrodynamics in such a situation. We find in particular that a new kind of hydrodynamical solution may be possible, which does not arise in the bag model. We obtain analytic results for the efficiency of the transfer of latent heat to bulk motions of the plasma, as a function of the speed of sound in each phase.Comment: 44 pages, 19 figures. v2: some comments and references added. v3: Some discussions and a figure added. Results unchanged. Matches published versio

Gravitational waves from a very strong electroweak phase transition

We investigate the production of a stochastic background of gravitational waves in the electroweak phase transition. We consider extensions of the Standard Model which can give very strongly first-order phase transitions, such that the transition fronts either propagate as detonations or run away. To compute the bubble wall velocity, we estimate the friction with the plasma and take into account the hydrodynamics. We track the development of the phase transition up to the percolation time, and we calculate the gravitational wave spectrum generated by bubble collisions, magnetohydrodynamic turbulence, and sound waves. For the kinds of models we consider, we find parameter regions for which the gravitational waves are potentially observable at the planned space-based interferometer eLISA. In such cases, the signal from sound waves is generally dominant, while that from bubble collisions is the least significant of them. Since the sound waves and turbulence mechanisms are diminished for runaway walls, the models with the best prospects of detection at eLISA are those which do not have such solutions. In particular, we find that heavy extra bosons provide stronger gravitational wave signals than tree-level terms.Comment: 30 pages, 12 figures. v2: typos corrected, explanations improved, a figure and a few references added. v3: comments added; matches JCAP versio

Optimal Divisionalization for Selling Networks of Cable Television Services

In this article, a condition for the optimal division’s number is calculated, for a market with two cable operators who offer a network service. The rationale for justifying the partial covering of the national market from the cable operators is presented. Furthermore, a problem of moral hazard is revealed, which is able to appear through the implementation of franchising schemes with independent divisions. This is particularly interesting because it can be applied to several industries such as Cable Television and Entertainment, and other activities including Internet and Computer Games Centres, which offer Internet broadband access and network games.Cable Television, Divisionalization, Franchising.

The Contagion Effect of the Terrorist Attacks of the 11th of September

In the context of interdependence of the financial markets, it becomes interesting to analyze the implications associated with the Terrorist Attacks of the 11th of September of 2001, in the USA, in terms of the development of contagion mechanisms between the main international stock exchanges. The sample is subdivided in two periods, in order to capture two different conjectures, that is, the pre-attack period, and the one that is concerned with the post-attack period. The results obtained through the estimation of a vector autoregressive model, incorporating an error correction mechanism, are presented. These results provide the detection of cointegrating relations among the economic variables, in study. A dynamic analysis is done, using exogeneity block tests, in order to check the existence of causality relations, which are defined in a Grangerian sense. For a forecasting purpose, the techniques of the variance decomposition of Cholesky, and of the impulse response functions are used. The occurrence of contagion is ratified by the results, starting from the terrorist attacks in the USA, which yielded a bigger volatility, with positive sign, in the Portuguese, and in the English Stock Exchanges.Contagion, Stock Exchange, Vector Autoregressive Model.

Competitive Advantages of the Beira Interior (Portugal): A TOWS Approach

The formulation of a competitive strategy implies an extended understanding, in terms of the industrial structures, of the mains fields where the nations compete and those structures evolve. The environmental conditions of a region, and of its industries, determine both the generic strategies, and the alternative strategies, which are already implemented. One of the instruments for strategic analysis, which combines external variables and internal variables, is the TOWS Matrix. This instrument allows the analysis of the present strategies, and the relationship between the variables, and also the presentation of proposals for alternative strategies, in order to identify or to reinforce the competitive advantages of the unit of analysis. Considering as unit of analysis the region of Beira Interior (Portugal), this article aims to provide a TOWS Matrix application. Several strategic alternatives for the region are, also, presented, taking into consideration the opportunities and the forces, previously, detected, in order to assure the transition for the ideal strategic quadrant. Finally, the conclusions and the guidelines for future research are presented.Strategy, TOWS Matrix, Entrepreneurship, Innovation.

Importance-sampling computation of statistical properties of coupled oscillators

We introduce and implement an importance-sampling Monte Carlo algorithm to study systems of globally-coupled oscillators. Our computational method efficiently obtains estimates of the tails of the distribution of various measures of dynamical trajectories corresponding to states occurring with (exponentially) small probabilities. We demonstrate the general validity of our results by applying the method to two contrasting cases: the driven-dissipative Kuramoto model, a paradigm in the study of spontaneous synchronization; and the conservative Hamiltonian mean-field model, a prototypical system of long-range interactions. We present results for the distribution of the finite-time Lyapunov exponent and a time-averaged order parameter. Among other features, our results show most notably that the distributions exhibit a vanishing standard deviation but a skewness that is increasing in magnitude with the number of oscillators, implying that non-trivial asymmetries and states yielding rare/atypical values of the observables persist even for a large number of oscillators.Comment: 11 pages, 4 figures; v2: minor changes, close to the published version, title changed to conform to PRE guideline