• Tuesday 3 August 2010 at 10.15 in A315: Alexander Dolgov (University of Ferrara, Italy)
  Condensation of electrically charged bosons in solid state and cosmology
  Abstract: Screening of impurities in plasma in presence of Bose condensed electrically charged fields is considered. It is shown that the condensate drastically changes the asymptotic behavior of the screened potential. In particular, the exponential Debye screening is transformed into a power law one. Moreover, the screened potential oscillates similar to the Friedel oscillations found for fermions in the middle of the previous century. The Bose condensate of charged vector bosons is also considered. The condensate may be in two possible states: ferromagnetic or antiferromagnetic. It is shown that in the minimal electroweak model W-bosons condense in ferromagnetic state. This could lead to spontaneous magnetization of the primeval plasma and generate seed fields necessary for creation of large scale cosmological magnetic fields.
  
  
• Thursday 5 August 2010 at 10.15 in A315: Eliezer Rabinovici (Hebrew University, Jerusalem)
  Holography of AdS Bubbles
  Abstract: I will discuss work with J. Barbon on the holographic duals of the bubbles causing vacuum decay in AdS spaces. Lessons on the fate of big crunch singularities and their resolution will be drawn.
  
  
• Tuesday 17 August 2010 at 10.15 in A315: Francois Englert (Universite Libre de Bruxelles, Belgium)
  The hidden horizon and black hole unitarity
  Abstract: Since the theoretical discovery of black hole evaporation through Hawking radiation the question of the unitarity of its evolution has been a major challenge for our understanding of black hole physics and of quantum physics in general. Conventionally, one seems to be faced with an alternative: either there is, as in the original Hawking derivation, no information in the thermal state and unitarity is violated, or the information is contained in the radiation and unitarity is preserved, but then the Hawking derivation appears to be essentially incorrect. Violating unitarity is difficult to believe, in particular in view of some theoretical achievements of string theories and of the AdS-CFT correspondence. On the other hand, the simplicity of the Hawking derivation and the consistency of its conclusions with the Bekenstein entropy makes one reluctant to disregard it.
  Our basic ingredient to cope with the unitarity issue and transcend the alternative is the following. In conventional relativistic field theory, the S-matrix describing elementary particle interactions is evaluated in a flat background. If coupling to gravity is included, distinct S-matrix elements may, in principle, require distinct backgrounds. We shall argue that this effect indeed dramatically affects the black hole S-matrix.
  More precisely we propose a scheme in which the semi-classical approximation of a unitary black hole S-matrix in a space-time without horizon leads to the conventional approach in a space-time endowed with a classical event horizon. The latter follows from saddle point contribution to inclusive S-matrix amplitudes, while unitarity is borne out by the more detailed exclusive S-matrix amplitudes. Although the computations of unitary amplitudes would require a detailed theory of quantum gravity, the proposed scheme itself, which appeals to the metric description of gravity only in the vicinity of stationary points, does not.
  
  
• Wednesday 18 August 2010 at 14.15 in A315: Francois Englert (Universite Libre de Bruxelles, Belgium)
  Broken symmetries
  Abstract: - Physics, as we know it, is an attempt to interpret the diverse phenomena as particular manifestations of general testable laws. This vision of a world ruled by general testable laws is relatively recent. Essentially it started at the Renaissance and experienced a rapid development. The crucial ingredient was the inertial principle, initiated by Galileo (1564-1642), which essentially states that the uniform motion of a system does not affect the physics within the system and hence cannot be detected by an experiment performed within the system. This is a profound idea: the very fact that we do not feel such a motion confirms the universality of the Galilean physics approach to the understanding of nature in the sense that we ourselves may be viewed as a physical system.
  - Starting from the inertial principle, Newton formulated at the end of the 17th Century the celebrated universal law of gravitation. He envisaged the world as composed of small interacting entities, which we now call elementary particles. In the 19th century, Maxwell established the general laws of electromagnetism explaining electric and magnetic phenomena as well as the propagation of light. These laws were expressed in terms of a field, that is an object filling an extended region of space, propagating like a wave with the velocity of light and transmitting electric and magnetic interactions. The notions of particles and waves were unified in a subtle manner during the first decades of the 20th Century in Quantum Mechanics and the inertial principle was extended by Einstein to electromagnetism in the theory of Relativity. On the other hand the Newtonian law of gravitation was generalized by Einstein in 1915. The new theory of gravity, called General Relativity, opened to scientific investigation the cosmological expansion of the universe. These impressive developments in the first half of the 20th Century made it conceivable that all phenomena, from the atomic level to the edge of the visible universe, be governed solely by the known laws of classical general relativity and quantum electrodynamics, the quantum version of Maxwell.s electromagnetic theory.
  - Gravitational and electromagnetic interactions are long range interactions, meaning they are felt by objects, no matter how far they are separated from each other. But the discovery of subatomic structures revealed the existence of other fundamental interactions that are short range, being negligible at larger distance scales. In the beginning of the 60s, the theoretical interpretation of short range fundamental interactions seemed to pose insuperable obstacles.
  - The breakthrough came from the notion of spontaneous symmetry breaking. In 1964 Brout and Englert, and then independently Higgs, discovered a mechanism based on spontaneous symmetry breaking by which short range interactions are generated from long range ones. The mechanism of Brout, Englert and Higgs, allowed the theoretical analysis of short range forces by unifying in the same theoretical framework the two type of forces. This discovery, which will be explained in detail in this talk, permitted to extend laws known at the macroscopic level to the nuclear and the subnuclear levels and opened the way to a modern view of unified laws of nature

Seminars in 2007 , in 2008 , in 2009 . For talks earlier in 2010, see page source.

Hopefully the up to one hour long seminar/colloquium will be understandable to a wide audience.
Contacts: Keijo Kajantie (keijo kajantie at helsinki fi) [department / HIP seminar],
Vappu Reijonen (vappu reijonen at helsinki fi) [cosmo seminar]