## Final Activity Report Summary - SAG@SNS (Strings and gravity at the scuola normale superiore in Pisa)

String theory is a promising candidate for a unified understanding of all elementary interactions, including gravity. Aim of this project was to promote research in this field at the Scuola Normale Superiore, Pisa, Italy. This has been done by the Marie Curie Chair, Prof. M. Porrati in several ways.

First, by offering yearly courses in string theory, both at the introductory and advanced level, to students within and without the Scuola Normale. Second, by hosting for various lengths of time many distinguished scientists, who are among the leaders in string theory research worldwide. During their visits, they gave lectures on their current research and they participated in seminars and other teaching activities. Third, by organising workshops and by delivering lectures in Europe and the United States, to disseminate the results of his research. Fourth, by supervising several students during their thesis work at the level of BSc, MSc and PhD. Finally, by engaging in active research in string theory.

Prof. Porrati's research covered several aspects of string theory and its consequences for cosmology and particle physics. Prof. Porrati borrowed techniques developed in string theory to describe extended solitonic excitations (Dbranes) to give an effective description of the initial state of the Universe, irrespective of the underlying fundamental description. He used those techniques to constrain possible signal of new highenergy physics in the Cosmic microwave background (CMB). The constraints he found limit severely cyclic / preBig Bang cosmologies. Porrati and collaborators also proposed a solution to an apparent puzzle in cosmology, related to the famous 'selfenergy' divergence of gravitational interactions of point-like sources. They showed that even in a classical description of Einstein's gravity, one must perform a mass renormalisation, lest physics at short distances dramatically change cosmology at the very largest distance (the Hubble scale).

Effective theories cannot tell us what happened AT the Big Bang. One would need what is called an ultraviolet (i.e. high energy) completion of Einstein's General Relativity. An interesting model of cosmic Big Bang singularity was studied by Prof. Porrati and collaborators in the context of the so-called holographic AdS / CFT duality. It posits that quantum gravity in AdS space, that is a space with negative cosmological constant, can be completely described in terms of a field theory (CFT) living on its boundary (AdS, unlike flat space, has a well-defined boundary, though it can be reached only by massless particles). Using AdS / CFT Porrati and collaborators studied possible string resolutions of an interesting class of bigbang singularities, in particular showing that an apparent problem of the boundary theory is not as serious as previously conjectured in the scientific literature.

Finally, Porrati recently began a longterm study of modelling dependent features of interacting high spin, massive particles. These are necessarily present in string theory, and their eventual detection in future particle accelerators depends crucially on their properties. Porrati's goal is to show that all these particles become strongly interacting 'resonances' at a finite energy, parametrically close to their mass.

The aim of project SAG@SNS was to create a lively centre for research in string theory at the Scuola Normale. This has been achieved. Indeed, nowadays the Scuola Normale has an intense research activity, pursued among others by the recently hired Prof. A. Sagnotti, both in string theory and in the strictly related field of high spin physics. Students interested in string theory can find now at the Scuola Normale vastly more resources and activities than at the beginning of the project. SAG@SNS successfully seeded the today's research blossoming.

First, by offering yearly courses in string theory, both at the introductory and advanced level, to students within and without the Scuola Normale. Second, by hosting for various lengths of time many distinguished scientists, who are among the leaders in string theory research worldwide. During their visits, they gave lectures on their current research and they participated in seminars and other teaching activities. Third, by organising workshops and by delivering lectures in Europe and the United States, to disseminate the results of his research. Fourth, by supervising several students during their thesis work at the level of BSc, MSc and PhD. Finally, by engaging in active research in string theory.

Prof. Porrati's research covered several aspects of string theory and its consequences for cosmology and particle physics. Prof. Porrati borrowed techniques developed in string theory to describe extended solitonic excitations (Dbranes) to give an effective description of the initial state of the Universe, irrespective of the underlying fundamental description. He used those techniques to constrain possible signal of new highenergy physics in the Cosmic microwave background (CMB). The constraints he found limit severely cyclic / preBig Bang cosmologies. Porrati and collaborators also proposed a solution to an apparent puzzle in cosmology, related to the famous 'selfenergy' divergence of gravitational interactions of point-like sources. They showed that even in a classical description of Einstein's gravity, one must perform a mass renormalisation, lest physics at short distances dramatically change cosmology at the very largest distance (the Hubble scale).

Effective theories cannot tell us what happened AT the Big Bang. One would need what is called an ultraviolet (i.e. high energy) completion of Einstein's General Relativity. An interesting model of cosmic Big Bang singularity was studied by Prof. Porrati and collaborators in the context of the so-called holographic AdS / CFT duality. It posits that quantum gravity in AdS space, that is a space with negative cosmological constant, can be completely described in terms of a field theory (CFT) living on its boundary (AdS, unlike flat space, has a well-defined boundary, though it can be reached only by massless particles). Using AdS / CFT Porrati and collaborators studied possible string resolutions of an interesting class of bigbang singularities, in particular showing that an apparent problem of the boundary theory is not as serious as previously conjectured in the scientific literature.

Finally, Porrati recently began a longterm study of modelling dependent features of interacting high spin, massive particles. These are necessarily present in string theory, and their eventual detection in future particle accelerators depends crucially on their properties. Porrati's goal is to show that all these particles become strongly interacting 'resonances' at a finite energy, parametrically close to their mass.

The aim of project SAG@SNS was to create a lively centre for research in string theory at the Scuola Normale. This has been achieved. Indeed, nowadays the Scuola Normale has an intense research activity, pursued among others by the recently hired Prof. A. Sagnotti, both in string theory and in the strictly related field of high spin physics. Students interested in string theory can find now at the Scuola Normale vastly more resources and activities than at the beginning of the project. SAG@SNS successfully seeded the today's research blossoming.