#### Field theory

### 418th WE-Heraeus-Seminar

# Models of Gravity in Higher Dimensions

### - From Theory to Experimental Search -

### Bremen, 25.-29.08.2008

Leading experts in the field will give an overview of the whole subject. The seminar is devoted to diploma students, PhD students and researchers.

**Why higher dimensions?**

Starting with Kaluza and Klein, the unification of the fundamental interactions has ever since involved higher dimensions. In particular, string theory as a major candidate for such a unified theory and thus also for quantum gravity, needs higher dimensions for its consistency. The additional dimensions might be small and compact, but there might also be large extra dimensions. Detection of these would lead to exciting new physics to be discovered and lead to a new picture of the universe.

**Small extra dimensions**

Following Kaluza and Klein, for a long time the extra dimensions were thought to be small and compact, since otherwise the propagation of elementary particles through these extra dimensions would lead to observable consequences in contradiction with experiment. Compact small dimensions give rise to Kaluza-Klein modes, i.e., massive excitations of the particles, which are not observed either, and thus put experimental bounds on the allowed size of the extra dimensions. String theory, on the other hand, might suggest the Planck scale as the natural scale for the size of the compact extra dimensions.

**Large extra dimensions**

String theory also introduced the notion of D-branes, generalizations of membranes. Arkani-Hamed, Dimopoulos, and Dvali (ADD) then proposed that we might be living on such a brane in a higher-dimensional space, where matter and gauge fields would be localized on the brane, while gravity would not be restricted to the brane. Offering a solution to the hierarchy problem, which arises because gravity is so much weaker than the other fundamental interactions, this scenario also predicts observable consequences such as deviations of gravity from Newton's law. Whereas in the ADD scenario the extra dimensions are flat or only weakly curved, Randall and Sundrum suggested the possibility of strongly curved (warped) extra dimensions and thus provided an alternative way to make gravity much weaker

than the other fundamental interactions.

**Black Holes in higher dimensions**

Black holes in higher dimensions exhibit surprising features, since many cherished theorems of 4-dimensional black holes do not generalize to higher dimensions. For instance, black holes need not have horizons with spherical topology, and thus also black rings, black saturns, etc. can arise, together with the intriguing possibility of horizon topology changing transitions. These could be triggered by the Gregory-Laflamme instability, leading to a rich phase structure of such higher dimensional black objects, yet to be uncovered. The study of black holes in string/M-theory is of particular interest, because this allows to address the microscopic interpretation of black hole thermodynamics. Moreover, if large extra dimensions do exist, higher dimensional black holes might be produced in accelerators and thus provide us with a new experimental window to gravity.

**Experimental search for higher dimensions**

An important experimental test for the existence of extra dimensions is the measurement of the gravitational potential at short (sub-mm) distances. So far, no deviation from the Newtonian potential has been found down to distances of several μm. Since gravity would be much stronger on distances significantly smaller than the size of the extra dimensions, black holes could be produced more easily. The energy density needed for gravitational collapse might therefore be in the reach of high energy collisions at the LHC, which should become operational this year. On the astrophysical side, in a different scenario large extra dimensions might also lead to a large scale modification of the Newtonian potential, resulting in modifications of the perihelion shift, light bending etc. Such deviations might be detectable for future improved detection schemes.

**Note:**

We would like to call attention to another meeting with related topics:

**Black Holes: A Landscape of Theoretical Physics Problems.**

This meeting takes place at CERN, 25 August - 5 October 2008.

The organizers of this meeting will try to minimize the overlap of topics during the Heraeus seminar.

This seminar is supported by the

Wilhelm und Else Heraeus-Stiftung