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**Time: ** 22. July 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Voja Radovanovic

**Title: ** NC SO(2,3)_* gauge theory and NC gravity

**Abstract:**

The noncommutative gravity is constructed in framework of NC SO(2,3)_* gauge theory. Applying Seiberg-Witten map NC action is expanded in power series in the noncommutative parameter. We find the equations of motion varying the expanded action with respect to vielbeins and spin connections. We find a NC correction to the flat spacetime and discuss the emergance of the Fermi normal coordinates.

**Time: ** 15. July 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Mihailo Cubrovic

**Title: ** AdS/CFT correspondence and quantum phase transitions

**Abstract:**

In the framework of AdS/CFT correspondence (gauge/gravity duality) we study the quantum phase transitions in strongly interacting field theories. We first focus on fermions at finite density and study the question: are there any stable states of fermionic matter, not breaking any symmetries, which cannot be adiabiatically continued to a Fermi gas? We identify the gravity dual of a Fermi surface, and show that the appearance of the Fermi surface follows the same scenario as order/disorder phase transitions: it corresponds to a quasi-normal mode of a Dirac fermion in the bulk. Then we try to understand if these fermions in CFT are deconfined ("fractionalized") or confined ("coherent"). We also study the interplay of the confinement/deconfinement transition and the conventional symmetry-breaking phase transitions. Finally, if time permits, we will discuss the thermalization of strongly interacting systems in contact with a heat bath using the thermo-field dynamics (TFD) approach, and show that this process can again be understood as the breaking of a gauge symmetry in the AdS gravity dual.

**Time: ** 1. July 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Vitaly Vanchurin

**Title: ** Theory and applications of string fluids

**Abstract:**

Many physical systems contain extended one-dimensional objects whose collective behavior is not well understood. These could be cosmic strings after phase transition, topological defects in liquid crystals, magnetic field lines in magnetohydrodynamics, vortex lines in quantum turbulence, galactic filaments in the large scale structure, gluon flux-tubes in quantum chromodynamics, or even fundamental strings near Hagedorn temperature. In this talk I will describe how such systems can be modeled in terms of the so-called string fluids whose distinguishing feature is the presence of a conserved antisymmetric tensor current. The theory is based on a kinetic theory of strings and on an effective filed theory descriptions of string fluids which I will discuss in some details. I will also discuss some applications of string fluids to relativistic networks of cosmic strings, superfluids and magnetohydrodynamics. In particular, I will present two new solutions of magnetohydrodynamics (one on curved background and one with dynamical gravity) that were obtained using an effective filed theory of string fluids.

**Time: ** 29. June 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Vitaly Vanchurin

**Title: ** Dual field theories of quantum computation

**Abstract:**

Given two quantum states of N q-bits we are interested to find the shortest quantum circuit consisting of only one- and two-q-bit gates that would transfer one state into another. We call it the quantum maze problem (for the reasons that I will described in the talk) which is relevant to both: quantum gravity and quantum computing. We argue that in a large N limit the quantum maze problem is equivalent to the problem of finding a semiclassical trajectory of some lattice field theory (the dual theory) on an N+1 dimensional space-time with geometrically flat, but topologically compact spatial slices. The spatial fundamental domain is an N dimensional hyper-rhombohedron, and the temporal direction describes transitions from an arbitrary initial state to an arbitrary target state. We first consider a complex Klein-Gordon field theory and argue that it can only be used to study the shortest quantum circuits which do not involve generators composed of tensor products of multiple Pauli Z matrices. Since such situation is not generic we call it the Z-problem. On the dual field theory side the Z-problem corresponds to massless excitations of the phase (Goldstone modes) that we attempt to fix using Higgs mechanism. The simplest dual theory which does not suffer from the massless excitation (or from the Z-problem) is the Abelian-Higgs model which we argue can be used for finding the shortest quantum circuits. Since every trajectory of the field theory is mapped directly to a quantum circuit, the shortest quantum circuits are identified with semiclassical trajectories. We also discuss the complexity of an actual algorithm that uses a dual theory prospective for solving the quantum maze problem and compare it with a geometric approach. We argue that it might be possible to solve the problem in sub-exponential time in 2^N, but for that we must consider the Klein-Gordon theory on curved spatial geometry and/or more complicated (than N-torus) topology.

**Time: ** 9. June 2016, 13:15h

**Place: ** Faculty of Physics, room 661

**Speaker: ** Vuk Mandic

**Title: ** First direct detection of gravitational waves

**Abstract:**

On September 14, 2015 the Advanced LIGO gravitational-wave detectors recorded the first signature of a merger of two black holes. This event marks the beginning of gravitational-wave astrophysics, enabling a new approach to studying various astrophysical phenomena. I will describe the LIGO detectors and the event they recorded, and I will discuss the implications of this observation as well as our expectations for future observations.

**Time: ** 6. May 2016, 12:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Vedad Pasic

**Title: ** Purely axial torsion waves as new solutions of affine metric gravity and analytic representation of the Dirac equation

**Abstract:**

In the affine metric gravity the spacetime is understood as a connected four-dimensional real manifold, equipped with the Lorentz metric and affine connection, where the use of independent affine connection gives a theory with 10+64 variables. Previously we have found a class of solutions for the quadratic affine metric gravity in the form of purely tensorial torsion waves, with the pp-metric and explicitly specified torsion. In a similar way we have found a new class of solutions in the form of purely axial torsion waves. Comparison with the known solutions of Einstein-Weyl theory gives us hope that these classes of solutions can be treated as afine metric models of some massless particle.

The Dirac equation without the mass term, as well as the spectral analysis of the corresponding operator is therefore of special interest. However, as we know, classical application of differential geometry in theories of gravity requires many complex constructions, so we wish to study formal self-conjugate first order differential operator which acts on pairs of complex scalar fields on a connected four-dimensional manifold. We look at geometrical properties of such an operator, which automatically provides all previously separate and independent constructions --- Lorentz metric, Pauli matrices, spin connection and the potential of the electromagnetic field. All this enables us to give a simple analytic representation of the Dirac equation.

**Time: ** 6. April 2016, 11:00h

**Place: ** Faculty of Physics, room 661

**Speaker: ** Maja Buric

**Title: ** Gravitational waves - from the idea to the direct detection

**Abstract:**

We will give a short historical and theoretical introduction to gravitational waves, with special attention to the binary systems as the sources of gravitational waves.

**Time: ** 25. March 2016, 13:00h

**Place: ** Institute of Physics, lecture room "dr Dragan Popovic"

**Speaker: ** Marko Vojinovic

**Title: ** Direct detection of gravitational waves in the LIGO experiment

**Abstract:**

(tba)

**Time: ** 11. March 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Marko Vojinovic

**Title: ** The Wick rotation and the effective action in field theory and gravity

**Abstract:**

Using the results in quantum field theory, we will discuss a reformulation of the Wick rotation which admits a generalization to diffeomorphism-invariant theories, in particular to gravitation. This generalization provides a concrete way to establish a relationship between theories with Euclidean and Lorentzian signatures in the framework of the effective action, both in field theory and quantum gravity.

**Time: ** 11. February 2016, 11:00h

**Place: ** Institute of Physics, room 300

**Speaker: ** Jorge Ovalle

**Title: ** Black holes from the Geometric Deformation: a possible sign of conformal symmetry

**Abstract:**

By using the Minimal Geometric Deformation (MGD) approach, originally developed in the context of the Randall-Sundrum Braneworld, we probe the incompatibility between this approach and f(R) theories. In consequence a generalization of the MGD is developed and used in the extra-dimensional context, generating thus new black hole solutions. We show that the MGD could be a direct consequence of a conformal symmetry associated with the new gravitational sector non described by General Relativity in the effective theory.

**Seminars for the year:**
2017
2016
2015
2014
2013
2012
2011
2010
2009
2008
2007