Todas Notícias

Cosmological observations indicate that the current universe is roughly 69% dark energy, 26% dark matter, and 5% ordinary matter. Dark matter is usually assumed to be some type of elementary particle, but the lack of direct evidence for such a particle has inspired some researchers to look for other possibilities. One prospect, which has received renewed interest due to the discoveries of black hole mergers with LIGO and Virgo, is that dark matter could mainly be primordial black holes. In this talk, I will discuss why black holes could theoretically operate as dark matter, and then examine the existing limits on primordial black holes of different masses. This is a topic that brings in an unusually rich variety of arguments from astronomy and physics, with prospects for continued improvement with more gravitational wave discoveries.

Local: Sala Jayme Tiomno

Recent viscous hydrodynamical studies [1,2] of the strongly-interacting medium created at the Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC), show that bulk viscosity plays an important role in their phenomenological description. A temperature-dependent bulk viscosity in the hydrodynamical evolution of the medium can modify the development of the hydrodynamic momentum anisotropy differently in the high- and low-temperature regions. Thus, anisotropic flow coefficients of various observables are affected differently depending where their surface of last scattering lies. For the case of hadronic observables, they are predominantly sensitive to low temperature regions, while electromagnetic radiation is emitted at all temperatures. Therefore, bulk viscosity should affect electromagnetic radiation differently than hadron emission. The effects of bulk viscosity on one of the electromagnetic probes, namely photons, has already been investigated [1]. The same statement holds true for hadrons [2]. The goal of this presentation is to study how thermal dilepton production, the other source of electromagnetic radiation, gets modified owing to the presence of bulk viscosity at RHIC and LHC energies. With calculations at different collision energies, comparisons in the dilepton signal can be made and more robust conclusions regarding the role of bulk viscosity in high energy heavy-ion collisions can be drawn. Dilepton radiation from the dilute hadronic phase of the medium will also be included to ascertain whether these modifications may be observable in experimental data.

[1] Jean-François Paquet et al., Phys. Rev. C 93 no. 4, 044906 (2016)
[2] S. Ryu et al., Phys. Rev. Lett. 115 no. 13, 132301 (2015)

(Local: Sala Jayme Tiomno)

This talk addresses a new formulation of quantum mechanics in which the wavefunction Ψ(t, x) is discarded entirely. Instead, the quantum state is represented as an ensemble of quantum trajectories x(t, C), or “worlds.” Each of these worlds has well-defined real- valued particle positions and momenta, and is thereby classical-like. Unlike a classical ensemble, however, nearby trajectories/worlds can interact with each other dynamically, giving rise to quantum effects. Moreover, x(t, C) satisfies a trajectory-based action principle, which allows quantum theory (via the Euler-Lagrange equation and Noether’s theorem) to be placed on the same footing as classical theories. In this manner, a straightforward relativistic generalization can be obtained. These and other developments, e.g. for many dimensions, multiple particles, and spin, may also be discussed.

(Local: Sala Jayme Tiomno)

In this colloquium, we want to present the first steps of the formulation of a scattering theory on a curved spacetime, in the Haag-Ruelle approach. This approach determines the S-matrix without specifying a particular interaction, it is rather based on the asymptotic behaviour of massive solutions of the wave equation, which can be estimated with great generality. We will show the construction of wavepackets on de Sitter spacetime, whose masses are consistently defined from the structure of the Lorentz algebra, and estimate its asymptotic behaviour. Furthermore, we show that, in the limit as the de Sitter radius tends to infinity, the wavepackets tend to the wavepackets of Minkowski spacetime and the plane waves arising after contraction have support sharply located on the mass shell. We will also show how to construct (scalar) field operators from these wavepackets. Besides, we will argue that the equilibrium state is not a thermal state, thus avoiding known issues of scattering theory at finite temperatures, and we will discuss possible interpretations of the in- and out-states.