Black Holes in AdS, the Universe, and Analog Systems

Look around you. How many dimensions do you perceive? One time, apparently, and three spatial dimensions, obviously. Obviously? If the holographic principle is correct then one of the dimensions you perceive might be considered as an illusion and gravity as well.The START project Black Holes in AdS, the Universe, and Analog Systems focused around the questions: how general is the holographic principle? does it apply to our actual Universe? what novel type of applications of black holes in other areas of physics does it provide? what can we learn about quantum gravity through black holes and the holographic principle?We addressed these questions in around 100 publications and theses in the newly established black holes research group at the Institute for Theoretical Physics at TU Wien. The START project provided the basis for building this group, which in the past six years hosted 8 postdocs, 12 PhD, 10 master and 19 bachelor students, as well as numerous international visitors and collaborators.Black holes were the key topic of this START project. They are probably the most fascinating macroscopic objects in our Universe. Most of them are generated through gravitational collapse of old stars similar to but at least three times heavier than our sun. Their defining property is that not even light can escape their gravitational field, so that it is possible to see them only through their gravitational interaction with visible matter.A surprising property of black holes is that the amount of information stored in them does not scale like their volume, as it would be usually the case, but like their area. This observation by Bekenstein and Hawking from the 1970ies inspired t Hooft and Susskind to formulate the holographic principle in the 1990ies. It states that one can describe the same physical situation using two apparently different theories in different dimensions. The higher-dimensional theory is a quantum theory of gravity and the lower-dimensional one a quantum theory without gravity.Holography has become a cornerstone in research on the elusive theory of quantum gravity. It has a precise implementation within string theory through the so-called Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence by Maldacena.We discovered that holography is far more general than AdS/CFT. We established a novel type of holographic correspondence, dubbed AdS/log CFT, discussed numerous implementations of holography beyond AdS/CFT, applied holography to non-abelian plasma physics, and provided the first concrete proposal for flat space holography, which indicates that holography applies to our actual Universe. Thus, one of the dimensions that you see around you indeed might be considered as an illusion.