**Black holes leave their marks all over the observed universe. They do however also inspire new and exciting ideas about space and time itself, both in the micro-cosmos and on the large scales of the universe.**

This August around 150 researchers from all of the world gathered for a Nordita program and conference devoted to “Black Holes and Emergent Spacetime”, organised in part by fellow OKC-er Larus Thoilacius and myself. In this blog post I shall try to explain part of the excitement and also how some ideas that were discussed address the dark matter and dark energy puzzles directly, as well as the details of the fluctuations in the cosmic microwave background.

## Background

The background story starts with the realization by Bekenstein, Hawking and others that black hole behaviour is captured by thermodynamics, when temperature and entropy are identified correctly. Since modern physicists associate thermodynamics to an underlying statistical description of a quantum system, many vague ideas about the nature of such a system has since been proposed.

Finally, in 1997, Juan Maldacena, who was then a first-year postdoc, wrote a paper about black holes that has now gathered more than 12 000 citations. It listed a number of quantum mechanical systems that could precisely and completely describe spacetime and gravitational physics.

Maldacena’s discovery is as close to an experimental breakthrough as theorists get. It changed our perspective on the world in a completely unexpected way, and hordes of theorists have set out to apply it and understand it on a deeper level. Recently, a large community again focuses on black holes and the spacetime as an emergent rather than fundamental concept.

## Tasting menu

Before coming to potential applications to observable physics let me highlight some other of the ideas I personally found interesting and fun. They seem distant from observation at present, but can be of conceptual importance.

• Non-singular version of black holes: In higher-dimensional gravity there is a plethora of solutions without the physical singularities that plague the standard rotating Kerr black hole solution. Could it be that actual physical black holes are nonsingular when studied in detail?

• Andy Strominger of Harvard discussed the ideas developed by Hawking, Perry and himself on how information is transported out to arbitrary distances from black holes and could be recovered by detecting so called BMS charges (although in practise many orders of magnitude more work than at

LIGO would be required).

• Nobel laureate and world-renowned particle physicist Gerard ‘t Hooft explained his puzzling recent ideas about the non-classical geometry of black holes. He said that his picture would lead to (in principle) observable correlations between Hawking radiation in opposite directions from a black hole. ‘t Hooft was the first to realise some of the “holographic” properties of gravity that later was concretised in Maldacena’s conjecture.

• So called Higher Spin Black holes were clarified by Juan Jottar from ETH, Zürich. Higher spin symmetries are symmetries that may be an alternative to supersymmetry in regulating short-distance problems of field theory. Another merit of these models is that they permit precise calculations, although technically demanding. Fundamental black hole physics is such a murky subject that explicit constructions are in dire need. The connection gravity and quantum mechanical system promises to be simpler in this case than in most other.

• Jonathan Lindgren, from Brussels, had found exact solutions of particles colliding to form general black holes. Of course, this problem is beyond reach in 3 space dimensions, but his 2-dimensional solution is still interesting.

We had two talks a day in four weeks and 30 talks in the conference, so these examples of talks by a Nobel laureate, a professor, a postdoc and a PhD student cannot do justice to the scope of the program.

## Cosmology

There were two talks in the program that focused on cosmology. Both apply the idea of holography to a time evolving universe. These ideas are most naturally applied to universe with an almost constant acceleration of its expansion, i.e. to a quasi-de Sitter universe, although other cases can be described with more effort.

Erik Verlinde described an ambitious project that aims to derive both the effects of dark energy and dark matter as consequences of holography and a spacetime that is emergent rather than fundamental. This is a setting in which spacetime is subject to thermodynamic laws and relations. As I understood it dark energy is proposed to be an elastic medium, and what we interpret as dark matter is then merely the effects of the interactions of ordinary matter with the dark energy medium. I think it would be interesting to find out whether this is really possible. There have been other suggestions on how to avoid dark matter, for example the heavily criticized MOND proposal of a modification of Newton’s laws. Verlinde got the question about how he would explain the apparent separation of dark and luminous matter in the so-called Bullet Cluster, a system of two colliding clusters. His answer was that in contrast to MOND he proposes a general dynamical and relativistic framework that may well lead to such effects, whereas his initial estimates are necessarily crude and rely on thermodynamic equilibrium. Other calculations are needed for time dependent phenomena like the collision in the Bullet Cluster.

Kostas Skenderis presented holographic descriptions of an inflationary era, which permit a direct calculation of the fluctuations of the microwave background. A holographic perspective involves a quantum field theory as in-data to the computation, but in contrast to standard inflation it does not presuppose a geometric description of the inflationary spacetime. This is a definite advantage, since a geometric spacetime is likely to clash with quantum gravity in one form or another. I think it is also exciting to have an entirely new kind of model with a straightforwardly calculable effect on the CMB.

*– Bo Sundborg, professor at the Oskar Klein Centre (bo@fysik.su.se)*