### Last month the University of Cambridge’s Centre for Theoretical Cosmology honoured Stephen Hawking’s 75th birthday with an international conference and public symposium, attracting 180 participants and an audience of 450 people, respectively.

The meeting was particularly relevant in light of the recent LIGO collaboration’s discovery of gravitational waves from black hole mergers, and, consequently, the new window on the universe that this has opened.

Four very well-known science communicators – Professors Brian Cox, Gabriela González, Martin Rees and Stephen Hawking himself – gave popular lectures for the public symposium. The afternoon meeting had remarkable impact through livestreaming videos of the lectures to the Discovery Channel social media sites across Europe, the US, Africa and Asia.

Viewing figures indicated that, during the four hours the meeting was running, around 3.75 m people accessed the relevant Discovery Facebook and YouTube pages, far exceeding the impact of all previous Discovery livestreaming events and setting a new benchmark in science engagement with the public.

All the lectures were of exceptionally high quality. Cox eloquently described the development of modern concepts in physics that have defined the perception of our place in the universe. González described the exciting discovery of gravitational wave events by the LIGO team, while looking forward to a new era of gravitational-wave astronomy. Lord Rees, the Astronomer Royal, took the audience on a fascinating journey from small-scale exoplanets out beyond the observed universe to the multiverse.

Finally, Hawking recounted his own life in physics and his contributions to momentous developments in our understanding of black holes and cosmology, which remain at the heart of pivotal theoretical and experimental programmes.

The scientific conference that followed was roughly divided across days into topical themes. The talks on the Monday were mostly devoted to cosmology. Eiichiro Komatsu and Hiranya Peiris explained how observational data from the cosmic microwave background (CMB) and from galaxy surveys can be used to constrain theories of inflation and fundamental physics, while also describing future prospects for experimental programmes seeking to discover primordial gravitational waves in CMB polarisation data.

Slava Mukhanov argued that standard inflation makes definite predictions, in contrast to some criticisms of the theory, and that current data does not necessitate elaborate extensions to inflation. After a brief overview, Andrei Linde discussed “attractor” solutions of inflationary theories for which the kinetic term has a pole in field space.

After lunch Jim Hartle discussed multiverses in quantum cosmology and Thomas Hertog described ongoing work with Hawking on obtaining a smooth exit from eternal inflation. Raphael Bousso argued that theorems in classical general relativity can be used to motivate conjectures about quantum field theory, and explained how one such conjecture can be proved for a class of quantum field theories.

Finally, Renata Kallosh discussed amplitudes and finiteness of maximal supergravity, motivating models of inflation from fundamental theory. The day ended with a dinner at Trinity College at which Fay Dowker and Jim Hartle gave speeches recalling their time spent working with Hawking.

Tuesday morning’s talks were devoted to gravitational waves, following LIGO’s sensational discovery of gravitational waves from merging black holes. Pablo Laguna discussed black hole kicks and possible observational signatures. Harald Pfeiffer describing different approaches to the modeling of gravitational waves from black hole binaries. González then gave an overview of LIGO’s results so far, and Bruce Allen discussed how Hawking and Gary Gibbons made serious attempts to build a gravitational wave detector in Cambridge.

In the afternoon, Frans Pretorius argued that by combining the observations of multiple binary black hole mergers it might be possible to extract information about several quasinormal modes during the “ringdown” phase of the final black hole and thereby test the no-hair theorem. He also emphasized the challenge in modeling gravitational wave sources in alternative theories of gravity to further observational tests of general relativity. Mihalis Dafermos gave an overview of the cosmic censorship conjectures. Finally Ted Jacobson reviewed the subject of Hawking radiation from black hole analogues and Jeff Steinhauer described his experimental work aimed at detecting analogue Hawking radiation in a system involving a Bose-Einstein condensate.

On the Wednesday, Douglas Stanford discussed his work on traversable wormholes in the anti-de Sitter/conformal field theory correspondence. Gary Gibbons discussed the gravitational memory effect, and its relation to work he did with Hawking. Andy Strominger gave an overview on his work on infrared effects in QED and quantum gravity. Gary Horowitz argued that the AdS/CFT correspondence implies that it is impossible to pass through certain types of singularity in quantum gravity. He also discussed a new type of supersymmetric black hole solution, with topology outside the horizon, and explained why it presents a puzzle for microscopic calculations of black hole entropy.

The conference ended with some closing remarks by Bob Wald, who, after praising Hawking’s many contributions to gravitational physics, explained in some detail why his paper Particle Creation by Black Holes is such a monumental achievement. He did identify one flaw in the paper, however – the word “gauge” is misspelled four times!

- The Gravity and Black Holes Conference and Symposium was supported by the IOP High Energy Particle Physics, Gravitational Physics, Astroparticle Physics and Mathematical and Theoretical Physics Groups

### Harvey Reall

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- International conference celebrates Hawking’s 75th birthday - 9 August 2017