**Seminars for the year:**
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**Time: ** 17. December 2008, 13:30h

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

**Speaker: ** Rodrigo Olea

**Title: ** Lectures on Holographic Renormalization (Part III)

**Abstract:**

**Time: ** 15. December 2008, 13:30h

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

**Speaker: ** Rodrigo Olea

**Title: ** Lectures on Holographic Renormalization (Part II)

**Abstract:**

**Time: ** 10. December 2008, 13:30h

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

**Speaker: ** Rodrigo Olea

**Title: ** Lectures on Holographic Renormalization (Part I)

**Abstract:**

These lectures give an introduction to regularization techniques in anti-de Sitter gravity. We review holographic reconstruction of asymptotically AdS spacetimes in the context of AdS/CFT correspondence. We study how this procedure leads to standard (Dirichlet) counterterms method, which provides a background-independent cancellation of asymptotic divergences in the gravity action and in the Brown-York quasilocal stress-tensor. We also discuss on the existence of Weyl anomaly and vacuum (Casimir) energy as a criterion of identification of the corresponding boundary CFT.

**Time: ** 5. December 2008, 11:00h

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

**Speaker: ** Marko Vojinovic

**Title: ** LQG: spin networks and spin foams - a short review

**Abstract:**

We shall give a short review on the properties and geometric interpretation of spin networks and spin foams. The quantization of space and time is disscussed within the loop quantum gravity framework, and some frequently asked questions answered, such as Lorentz invariance, triangulation independence and similar. The calculation of the black hole entropy will also be sketched.

**Time: ** 28. November 2008, 11:00h

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

**Speaker: ** Rodrigo Olea

**Title: ** Topological Invariants, Kounterterms and Self-Duality in Four-Dimensional Anti-de Sitter Gravity

**Abstract:**

The AdS/CFT correspondence conjectures a profound relation between gravity with negative cosmological constant defined in a spacetime and a Conformal Field Theory living on its boundary. In order to realize this duality, it is necessary to extract the finite information contained in the action (and in the boundary stress tensor derived from it) by means of a procedure known as holographic renormalization. This results in the addition to the bulk action of intrinsic counterterms which cancel out the divergences in the asymptotic region.

As an alternative to the above method, counterterterms with dependence on the extrinsic curvature of the boundary (also known as Kounterterms) have been proposed. In any D=2n dimension, they are simply given by the boundary term which appears in the 2n-dimensional Euler theorem and, therefore, they are connected to the existence of topological invariants in that dimension.

In this seminar, we show how this "topological" regularization prescription generates the standard counterterm series by a suitable expansion of the fields. In particular, in four dimensions, adding the Gauss-Bonnet term to the action with a suitable coupling constant is equivalent to Balasubramanian-Kraus construction. This also opens the possibility of adding another topological invariant (Pontryagin term) to the action and fixing its coupling constant by taking a self-duality condition on the Weyl tensor. This argument provides a simple derivation of the holographic stress tensor/Cotton tensor duality recently noticed in the literature for soliton solutions and hydrodynamic models in AdS_4.

**Time: ** 21. November 2008, 11:00h

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

**Speaker: ** Marko Vojinovic

**Title: ** Interpretation of Kalb-Ramond Field in Riemann-Cartan Geometries

**Abstract:**

We give two different geometric interpretation of the Kalb-Ramond field of bosonic string theory. Given the generalized Papapetrou method for derivation of equations of motion for a p-brane in Riemann-Cartan spacetimes, we derive appropriate equations of motion. Comparing them with the equations postulated in string theory, we read off the dependence of the Kalb-Ramond field on the spacetime torsion and the spin of the string. The first interpretation relates the Kalb-Ramond field to the appropriate component of the contorsion tensor in a special geometry where contorsion tensor is factored into two parts. The second interpretation relates it to one of the extra components of contorsion coming from the compact extra dimension in a simple Kaluca-Klein model. We also demonstrate that it is not possible to interpret the Kalb-Ramond field as the potential for torsion in Riemann-Cartan geometries.

**Time: ** 16. October 2008, 12:00h

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

**Speaker: ** Nemanja Kaloper

**Title: ** Geometric Precipices in String Cosmology

**Abstract:**

We consider the effects of graviton multiplet fields on transitions between string gas phases. Focusing on the dilaton field, we show that it may obstruct transitions between different thermodynamic phases of the string gas, because the sign of its dimensionally reduced, T-duality invariant, part is conserved when the energy density of the universe is positive. Thus, many interesting solutions for which this sign is positive end up in a future curvature singularity. Because of this, some of the thermodynamic phases of the usual gravitating string gases behave like superselection sectors. For example, a past-regular Hagedorn phase and an expanding FRW phase dominated by string momentum modes cannot be smoothly connected in the framework of string cosmology with positive sources. The singularity separates them like a geometric precipice in the moduli space, preventing the dynamics of the theory from bridging across. Sources which simultaneously violate the positivity of energy and NEC could modify these conclusions. We provide a quantitative measure of positivity of energy and NEC violations that would be necessary for such transitions. These effects must dominate the universe at the moment of transition, altering the standard gas pictures. At present, it is not known how to construct such sources from first principles in string theory.

**Time: ** 3. October 2008, 11:00h

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

**Speaker: ** Marko Vojinovic

**Title: ** Does time exist?

**Abstract:**

Loop Quantum Gravity represents an example of a theory with no time defined. Using elementary considerations in classical mechanics, we introduce a notion of "intuitive" (Newtonian) time and discuss if and when it can be defined in a particular theory. We shall analyze in more detail an example of timeless double pendulum. The goal of the seminar is to point to the problem of time in LQG using this elementary example.

**Time: ** 26. September 2008, 11:00h

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

**Speaker: ** Maja Buric

**Title: ** Noncommutative spaces with spherical symmetry (part II)

**Abstract:**

In the first part of the seminar, some properties of noncommutative spaces and their differential structure are discussed. Also, some possibilities to define a noncommutative generalization of gravity are reviewed.
In the second part important details of the noncommutative frame formalism are introduced. It is applied then to the construction of the four dimensional noncommutative space with spherical symmetry.

**Time: ** 19. September 2008, 11:00h

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

**Speaker: ** Maja Buric

**Title: ** Noncommutative spaces with spherical symmetry (part I)

**Abstract:**

In the first part of the seminar, some properties of noncommutative spaces and their differential structure are discussed. Also, some possibilities to define a noncommutative generalization of gravity are reviewed.
In the second part important details of the noncommutative frame formalism are introduced. It is applied then to the construction of the four dimensional noncommutative space with spherical symmetry.

**Time: ** 5. September 2008, 11:00h

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

**Speaker: ** Aleksandar Mikovic

**Title: ** Multiverse

**Abstract:**

(to be announced)

**Time: ** 17. July 2008, 15:00h

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

**Speaker: ** Marko Vojinovic

**Title: ** Motion of extended objects in gravitational field with torsion

**Abstract:**

We consider the motion of extended objects in gravitational field with torsion. Using multipole formalism, from the assumptions of Poincar\' e invariance and localization of matter on some hypersurface, we derive equations of motion for a $p$-brane, along with appropriate boundary conditions. Then we analyze the most important cases of particle and string, thereby obtaining some interesting consequences. Namely, it turns out that the spin of Dirac particle does not couple to spacetime curvature, and furthermore can be neglected in single-pole approximation. Also, we considered one gross model of a meson which consists of a Nambu-Goto string with two particles attached to its ends. The model allows for the derivation of Regge trajecories with a correction due to internal angular momenta of the particles at the string ends.

**Time: ** 12. June 2008, 11:00h

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

**Speaker: ** Djordje Minic

**Title: ** The spectrum of pure QCD in 3D and 4D

**Abstract:**

I will review recent advances in analytic pure QCD (i.e. Yang-Mills theory) in 2+1 and 3+1 dimensions. These new analytic results are based on the novel vacuum wave-functionals written in terms of so-called corner variables. The analytic results are in excellent comparison with lattice simulation in 2+1 dimensions. For example, the masses of ground states given in terms of the square root of the string tension are indistinguishable from their lattice counterparts. The comparison between theory and lattice simulations in 3+1 is very good, i.e. up to a few to around ten percent.

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