Ludwig-Maximilians-Universität München

Faculty of Physics | Theoretical Particle Physics

Theresienstraße 37

80333 München

+49 89 2180 4549

georgi.dvali[at]physik.uni-muenchen.de

Group Webpage### Description

Members: Gia Dvali, Lukas Eisemann, Georgios Karananas, Marco Michel, Florian Kühnel, Sebastian Zell*

We are theoretical particle physicists working in Physics Beyond the Standard Model. Thus, our main goal is to understand the fundamental laws of Nature and to link them with experimental signatures in high energy particle accelerators, cosmology and astrophysics. Additionally, we have a very strong interest in many-body quantum systems and quantum information. We have established a link between these two subject areas through black holes and cosmology.

##### Black holes and cosmology

First, black holes are unique connectors of gravity and quantum information. The reason is that due to their Bekenstein-Hawking entropy, black holes possess a maximal capacity of information storage. Moreover, there exists increasing evidence that macroscopic black holes play the key role in the unitarization of collision processes of elementary particles at energies much higher than the Planck mass – a phenomenon that is sometimes called classicalization. It is therefore absolutely crucial for particle physics to understand how black holes work at the microscopic level.

Of course, the dream program would be to manufacture black holes in accelerator experiments with particle collisions. This prospect, however, is rather dim unless Nature was generous by lowering the scale where quantum gravity effects become strong down to Tera-eV energies [1]. Only in this situation, LHC is able to create black holes, as predicted in [2]. Whether this was the case, we will only learn after the LHC data analysis will be completed and/or after a next generation of accelerators with higher collision energy starts operating.

In this light, alternative routes for understanding microscopic black hole physics become especially important. Our key idea is that the mechanism responsible for the large black hole entropy could be much more general and also occur in many-body quantum systems that are not connected to gravity, such as certain Bose-Einstein condensates. Thus, those systems can provide an alternative laboratory for shedding new light on basic principles of information storage and processing by black holes. This approach allows both to simulate black hole quantum information features as well as to generalize them to other contexts. The great advantage of many-body quantum systems is that they are much easier to control both in experiment and in theory.

Moreover, those systems can also be connected to cosmology. According to the commonly accepted inflationary paradigm, the early Universe went through a quasi-de Sitter state. De Sitter spacetime exhibits some close similarities to a black hole. For example, similarly to the black hole entropy, a de Sitter Hubble patch carries a microstate entropy called Gibbons-Hawking entropy. Our idea is that due to this entropy, de Sitter represents a critical state of enhanced memory capacity and therefore is subject to analogous phenomena as black holes and other systems within this universality class. This view has important implications for early cosmology as it opens up some qualitatively new ways of accessing our Universe’s very distant past, which is not possible within the standard semi-classical treatment of inflation. In particular, we have argued [8] that our Universe must have carried a primordial quantum memory pattern encoded in the microstate of those degrees of freedom that are responsible for the Gibbons-Hawking entropy. This pattern – due to a so-called memory burden effect – must have been imprinted in higher order correlators of density perturbations, thereby providing future observations with an exceptional opportunity of decoding this primordial quantum information.

The key idea of our framework [3] that makes the connection of high energy physics and many-body quantum systems very concrete is that macroscopic black holes and de Sitter spaces actually possess a fundamental description as many-body systems. Namely, they represent states of soft gravitons with very high occupation number. Both gravitational systems are at a very special quantum critical point where the quantum interaction strength of soft gravitons (which is minuscule) is almost exactly compensated by their enormously-high occupation number [4]. In this way, the key to understanding the mysterious quantum properties of black holes and de Sitter lies in their multi-particle nature at criticality. Thus, our picture naturally links black holes or the Universe itself with other systems with high occupation number of bosons such as Bose-Einstein condensates or coherent states at criticality. We therefore can gain a valuable interdisciplinary knowledge by studying the behavior of such systems.

##### Exemplary lines of our research include

**1. General methods and prototype systems (see [5])**

One of our key objectives is to develop methods for the theoretical study of information storage in many-body quantum systems. In particular, we aim to find and analyze critical states of enhanced information storage capacity. Both analytical and numerical methods play a key role. We use those to study prototype models with the ultimate goal of constructing systems that can be observed experimentally.

**2. Quantum breaking (see [6])**

Although any fundamental description of a physical systems needs to include quantum mechanics, the classical approximation works well in many contexts. However, it is of crucial importance to understand when a classical formulation breaks down, i.e., when the true quantum evolution deviates from the classical description. This amounts to quantum breaking. Many-body (quantum) systems are the ideal testground for the theoretical and experimental study of this phenomenon, because a fundamental quantum description is available for them. Quantum breaking has particularly important implications for black hole physics. That their classical description is expected to break down after half evaporation is key to understanding how black holes process information. The phenomenon of quantum breaking in a sense has even more dramatic consequences for de Sitter and inflationary cosmologies as it shows that contrary to the standard semi-classical view such Universes cannot be eternal and literally are subjected to the process of aging due to quantum effects [7].

**3. De Sitter and inflation (see [8])**

As explained, de Sitter space exhibits great analogies to black holes, in particular because of its Gibbons-Hawking entropy. Therefore, our studies of information storage in analogue many-body quantum systems can equally be used to draw conclusions about this spacetime. For inflationary cosmology, this has important consequences both for fundamental understanding of Universe’s cosmological history as well as for observations. In particular, we predict the existence of primordial quantum memories that are carried through the entire inflationary history and are potentially observable.

**References **

[1] N. Arkani-Hamed, S. Dimopoulos and G. R. Dvali, “The Hierarchy problem and new dimensions at a millimeter”, Phys. Lett. B 429 (1998) 263, arXiv:hep-ph/9803315.

[2] I. Antoniadis, N. Arkani-Hamed, S. Dimopoulos and G. R. Dvali, “New dimensions at a millimeter to a Fermi and superstrings at a TeV,” Phys. Lett. B 436 (1998) 257, arXiv:hep-ph/9804398.

[3] G. Dvali, C. Gomez, “Black Hole’s Quantum N-Portrait”, Fortsch. Phys. 61 (2013) 742, arXiv:1112.3359 [hep-th].

[4] G. Dvali, C. Gomez, “Black Holes as Critical Point of Quantum Phase Transition”, Eur. Phys. J. C 74 (2014) 2752, arXiv:1207.4059 [hep-th].

[5] G. Dvali, M. Michel, S. Zell, “Finding Critical States of Enhanced Memory Capacity in Attractive Cold Bosons”, Eur. Phys. J. Quantum Technology 6 (2019) 1, arXiv:1805.10292 [quant-ph].

[6] G. Dvali, D. Flassig, C. Gomez, A. Pritzel, N. Wintergerst, “Scrambling in the Black Hole Portrait”, Phys. Rev. D 88 (2013) 124041, arXiv:1307.3458 [hep-th].

[7] G. Dvali, C. Gomez, “Quantum Compositeness of Gravity: Black Holes, AdS and Inflation”, JCAP 1401 (2014) 023, arXiv:1312.4795 [hep-th].

G. Dvali, C. Gomez and S. Zell, “Quantum Break-Time of de Sitter,” JCAP 1706 (2017) 028, arXiv:1701.08776 [hep-th].

[8] G. Dvali, L. Eisemann, M. Michel, S. Zell, “Universe’s Primordial Quantum Memories”, JCAP 1903 (2019) 010, arXiv:1812.08749 [hep-th].

*external collaborator | École polytechnique fédérale de Lausanne