Gravitation and Cosmology Group







Roberto Emparan

Roberto Emparan

  • ICREA Research Professor
  • Fields of research: Classical and Quantum Gravity, String Theory, Black Holes
  • Publication list from SPIRES
  • Personal webpage: Myself at ICREA (Catalan Institution for Research and Advanced Studies)



In broad terms, my research aims at the elucidation of problems of quantum gravity, and in particular at developing the interface between String Theory and General Relativity.

Black holes are our best best guide towards understanding the properties of a quantum theory of gravity. A deep lesson that has been extracted from their study is that the high-energy behaviour in a theory of gravity is drastically different than in non-gravitational quantum field theories. In particular, quantum theories of gravity are holographic: they are formulated in terms of a non-gravitational theory that is defined at the boundary of the spacetime. These insights are explicitly realized in the AdS/CFT correspondence, which is arguably the deepest development in theoretical physics of the last twenty years.

Therefore gravity is not a fundamental interaction, but emerges holographically out of certain classes of strongly coupled non-gravitational theories. This idea can be seen as providing a means for constructing quantum theories of gravity. But it can also be turned around to the effect that General Relativity provides the appropriate theoretical tool to describe the dynamics of certain strongly-coupled systems, in a regime inaccessible to the conventional perturbative techniques. This is an astonishing example of how research into a subject as seemingly esoteric as quantum gravity can lead to conclusions applicable to more conventional and potentially accessible domains.

These ideas provide the background for my main research interest: understanding better the properties of gravity, both for its use as a theoretical tool, and for what one may learn about how to formulate quantum theories of gravity,  in other words, what is the structure of spacetime at its most fundamental level. This includes quite naturally the study of General Relativity and its black holes, in an arbitrary number of dimensions and in the theories that naturally incorporate the ideas of holography, namely, String and M-Theory. Over the years I have made several contributions to these subjects, as you can see from my publication list.

Black holes in Higher Dimensions and in String Theory

I like to stress the fact that the theoretical study of black hole solutions of Einstein's theory of General Relativity has proven to be, over the years, an extremely fruitful endeavor. Although this study has invariably appeared at first sight to be of arcane academic interest with no clear connection to observations, the history of the subject repeatedly shows that each new black hole solution was the right answer waiting for the appropriate question. This was certainly the case in astrophysics, where black holes play nowadays an indispensable role, and in recent years it is proving to be the case, too, with the study of quantum aspects of black holes and of black holes in String Theory. The AdS/CFT correspondence sprang directly from the study of rather sophisticated classes of black holes in String Theory. As I mentioned above, black holes have become nowadays our main source of insight into the properties of (non-gravitational) systems such as the quark-gluon plasma, and possibly also condensed matter systems close to quantum criticality. This is therefore a theoretical subject of study that can, rather unexpectedly, have an impact into disciplines that previously had been regarded as completely separate.

This past experience prompts us to investigate the fundamentals of black holes in General Relativity at large, and in String Theory, without necessarily narrowing down our focus onto immediate applications with observational consequences. Even if these are highly desirable, we believe that without a deep and strong understanding of the fundamentals of the field, the unexpected applications cannot develop. At the very least, our comprehension of several basic problems in String Theory and Quantum Gravity will improve.

A long-term line of research that I have initiated together with a worldwide group of collaborators aims at unravelling the structure of General Relativity in dimensions higher than four. Even in vacuum (i.e., pure gravity) the dynamics of gravity, and in particular about its black holes in higher dimensions exhibits a structure much richer than in four dimensions. One of our first results was striking: we found solutions for black holes with the shape of a donut, called black rings, that exhibit entirely new qualitative features.

Over the last years we have made great advances in this field, and our understanding of higher-dimensional black holes is now much more mature than just ten years ago. Recently we have developed a new viewpoint that provides both new concepts and new techniques to characterize and study higher-dimensional black holes, the so-called blackfold approach. Not only has it allowed to uncover large new classes of black holes, but it also provides a fascinating link with fluid dynamics, very much in the same spirit as the fluid/gravity correspondence in AdS/CFT.

As in every new developing field, unexpected results are being turned up at every corner. Stay tuned for more.

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