Daniel Grumiller

Assoc.Prof. Dr. Daniel Grumiller

Institut für Theoretische Physik der Technischen Universität Wien
Wiedner Haupstrasse 8-10
1040 Wien

Tel.: +43-1-58801-13634

e-mail: grumil@hep.itp.tuwien.ac.at

Web: http://quark.itp.tuwien.ac.at/~grumil/

Colliding gravitational shockwaves as holographic model of isotropization, with violation of null energy condition (but not of its quantum version) in forward lightcone. Plot shows energy density as function of time t and relevant spatial coordinate y.

The holographic principle relates unitary theories of quantum gravity, like string theory, with unitary quantum field theories in one dimension lower. The best studied realization of holography is the AdS/CFT correspondence, which has found a plethora of applications, in particular in the context of strongly coupled N=4 super-Yang-Mills theory, with potential spin-offs for heavy ion physics at RHIC, LHC and FAIR. One example is the modelling of isotropization in strongly coupled non-Abelian plasmas through the gravity-dual of shock waves in AdS5, for which seminal work has been carried out by Grumiller and Romatschke. Recent numerical studies with Ecker, Stanzer, Stricker and van der Schee have unravelled novel features of holographic entanglement entropy and (saturation of) the so-called quantum null energy condition, particularly in the interesting situation of colliding gravitational shock waves (see figure). Existing numerical methods can be further developed and applied to numerous interesting scenarios in (and beyond) holography, which provides a plethora of topics for PhD theses.

If true, the holographic principle must also work in flat spacetime. A couple of years ago, Grumiller and collaborators provided the first evidence for a holographic correspondence between a gravitational theory in flat space and a specific unitary field theory in one dimension lower, which opened up a new avenue to understand flat space holography. In recent years this proposal was checked in numerous ways, for instance by calculating stress tensor correlators and entanglement entropy both on the gravity side and the conjectured dual field theory (an ultra-relativistic limit of conformal field theory). While the checks so far all worked, there are numerous open issues in flat space holography, some of which can be addressed in future PhD work.

Inspired by the proposal by Hawking, Perry and Strominger, who introduced the notion of non-trivial zero energy excitations of black holes dubbed “soft hair” and by new near horizon boundary conditions by Donnay et al and ourselves, in 2016 we proposed a new set of boundary conditions that lead to astonishingly simple near horizon symmetries, namely infinite copies of the Heisenberg algebra. Another surprising outcome is a very simple entropy formula that is more universal than the Bekenstein-Hawking or the Wald entropy, as it applies also to theories with higher spins, gravitational Chern-Simons terms, both in anti-de Sitter and flat space. Moreover, our work triggered a new proposal for an explicit set of semi-classical black hole microstates. Despite of these exciting developments it seems we still have only scratched at the surface of soft Heisenberg hair. Hopefully, future PhD students can contribute to unravel the remaining mysteries of soft Heisenberg hair, near horizon symmetries and black hole microstates.