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## Project

# Dirichlet-branes: links between elementary particle physics and cosmology. (FWOAL536)

Lacking experimental input, the exploration of extremely high energies over the past two decades has been based on mathematical and physical consistency of fundamental theories. String theory [1] manages to give a quantum mechanical description of the gravitational interaction which is needed to understand the physics of black holes and that of the Big Bang. An example of an important result is the microscopic explanation of the entropy of black holes [2]. The discovery of the gauge/gravity duality (AdS/CFT or more generally holography) [3] gave rise to a true revolution. One talks about holography when a (gravitational) theory in a certain number of space-time dimensions is described at the fundamental level by a (non-gravitational) theory in a lower number of space-time dimensions. This led to a manifold of spin-offs going from new unexpected techniques and concepts in fundamental mathematics all the way to novel calculational methods in QCD, surprising insights in certain aspects of the quark gluon plasma (two items relevant to the LHC) and even new methods in condensed matter physics... In all these developments, D(irichlet)-branes (solitonic states within string theory) played an absolutely central role.

Over the next years, high energy physics and the study of the early universe will be provided with a wealth of new experimental and observational data. Already in 2009 we will witness the startup of physics at the Large Hadron Collider (LHC) and the launch of the Planck satellite by ESA. LHC will allow for the exploration of energy scales never accessed before in any laboratory which hopefully will provide us with important hints on the underlying fundamental theory. Planck will make high precision measurements of the anisotropies of the cosmic background radiation (CMB) which will test theories describing the early universe. In this way the early universe can be used as laboratory for high energy physics. In view of the experimental and observational input in high energy physics, the greatest challenge for string theory is now to make connections to the real world as we observe it.

This task is not simple as before the present decade almost all progress was made for static space-times, while standard cosmology teaches that the universe started by a Big Bang and expanded after that, presently the expansion is even accelerating. A static universe is a very good approximation for earthly particle physics experiments; however, for a fundamental theory the incorporation of the expansion of the universe and the Big Bang singularity might very well be crucial. Over the past years it became clear that there is a "landscape" of metastable solutions of string theory [4], which resulted in numerous discussions about the predictive power of string theory. Most of this research, however, was carried out in the framework of effective field theories. In order to really understand how many solutions can consistently be described by string theory and whether any of them corresponds to our universe, it is crucial to develop techniques to describe string theory in time dependent backgrounds.

The interest in the physics of cosmological singularities has further been sharpened by the discovery of cosmological models where a contracting universe preceded our expanding universe. In those models [5] the density fluctuations which give rise to the observed anisotropies in the CMB arose in the contracting phase of the universe. Subsequently one assumes that these density fluctuation evolve unscratched through the big crunch/big bang transition to the expanding phase of our universe. Whether this assumption is valid is - at this moment - an unanswered question for the fundamental theory. Even for more conventional theories of the early universe - such as inflation - we do need a fundamental theory in order to understand why our universe was in an "appropriate" initial state. Finally, the fact that the expansion of our universe is presently accelerating screams for an explanation: is it caused by a(n unnaturally) small cosmological constant or by dynamical "dark energy"?

Because of the above, the study of cosmological singularities took center stage starting in 2002. The chosen strategy is to construct idealized models for space-times with cosmological singularities. In those models, one explores ways to extrapolate the laws of string theory - as we know them for the situation without cosmological singularities - in a consistent way. Our group has played a leading role in the development of such models in the context of perturbative string theory (time dependent orbifolds [6] and singular gravitational waves [7]), matrix theory [8] and the AdS/CFT correspondence [9]. On top of that, our group has obtained interesting results concerning the late-time evolution of cosmological models inspired by string field theory [10]. The further development of these various approaches is an important part of our project. In addition, we will focus on the classical and quantum geometry of D-brane models in the presence of fluxes. In the past our group obtained remarkable results concerning the effective description of D-branes in Kähler backgrounds (B-branes) [11]. Later we obtained concrete and systematic results on the geometry and characterization of D-branes in the presence of NSNS-fluxes [12]. Motivated by the discovery of the landscape of string solutions we aim in the present project to further develop the study of D-branes in the presence of fluxes. The first target will the full characterization and understanding of D-branes in arbitrary backgrounds with non-trivial NSNS-fluxes.

Over the next years, high energy physics and the study of the early universe will be provided with a wealth of new experimental and observational data. Already in 2009 we will witness the startup of physics at the Large Hadron Collider (LHC) and the launch of the Planck satellite by ESA. LHC will allow for the exploration of energy scales never accessed before in any laboratory which hopefully will provide us with important hints on the underlying fundamental theory. Planck will make high precision measurements of the anisotropies of the cosmic background radiation (CMB) which will test theories describing the early universe. In this way the early universe can be used as laboratory for high energy physics. In view of the experimental and observational input in high energy physics, the greatest challenge for string theory is now to make connections to the real world as we observe it.

This task is not simple as before the present decade almost all progress was made for static space-times, while standard cosmology teaches that the universe started by a Big Bang and expanded after that, presently the expansion is even accelerating. A static universe is a very good approximation for earthly particle physics experiments; however, for a fundamental theory the incorporation of the expansion of the universe and the Big Bang singularity might very well be crucial. Over the past years it became clear that there is a "landscape" of metastable solutions of string theory [4], which resulted in numerous discussions about the predictive power of string theory. Most of this research, however, was carried out in the framework of effective field theories. In order to really understand how many solutions can consistently be described by string theory and whether any of them corresponds to our universe, it is crucial to develop techniques to describe string theory in time dependent backgrounds.

The interest in the physics of cosmological singularities has further been sharpened by the discovery of cosmological models where a contracting universe preceded our expanding universe. In those models [5] the density fluctuations which give rise to the observed anisotropies in the CMB arose in the contracting phase of the universe. Subsequently one assumes that these density fluctuation evolve unscratched through the big crunch/big bang transition to the expanding phase of our universe. Whether this assumption is valid is - at this moment - an unanswered question for the fundamental theory. Even for more conventional theories of the early universe - such as inflation - we do need a fundamental theory in order to understand why our universe was in an "appropriate" initial state. Finally, the fact that the expansion of our universe is presently accelerating screams for an explanation: is it caused by a(n unnaturally) small cosmological constant or by dynamical "dark energy"?

Because of the above, the study of cosmological singularities took center stage starting in 2002. The chosen strategy is to construct idealized models for space-times with cosmological singularities. In those models, one explores ways to extrapolate the laws of string theory - as we know them for the situation without cosmological singularities - in a consistent way. Our group has played a leading role in the development of such models in the context of perturbative string theory (time dependent orbifolds [6] and singular gravitational waves [7]), matrix theory [8] and the AdS/CFT correspondence [9]. On top of that, our group has obtained interesting results concerning the late-time evolution of cosmological models inspired by string field theory [10]. The further development of these various approaches is an important part of our project. In addition, we will focus on the classical and quantum geometry of D-brane models in the presence of fluxes. In the past our group obtained remarkable results concerning the effective description of D-branes in Kähler backgrounds (B-branes) [11]. Later we obtained concrete and systematic results on the geometry and characterization of D-branes in the presence of NSNS-fluxes [12]. Motivated by the discovery of the landscape of string solutions we aim in the present project to further develop the study of D-branes in the presence of fluxes. The first target will the full characterization and understanding of D-branes in arbitrary backgrounds with non-trivial NSNS-fluxes.

Date:1 Jan 2010
→
31 Dec 2013

Keywords:Physics

Disciplines:Physical sciences