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Joseph Silk

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Joseph Silk

Joseph Silk

Astrophysicist Joseph Silk is without question one of the pioneers of modern cosmology.  His research has made its mark on all fields of cosmology, from the origin of density fluctuations which gave rise to the major structures of the universe, to galaxy formation and evolution.  Holder of an honorary doctorate from the École normale supérieure de Lyon and the University of Rome (2005), he has been awarded numerous prizes, including the Gold Medal of the Royal Astronomical Society (UK) in 2008. 



This honorary doctorate comes in recognition of an exceptional body of work, and many of those attending the ceremony will, at the very least, be familiar with silk damping. They will be less familiar with your current research interests. Can you tell us about what you have been working on recently?

I am studying a novel form of particle accelerator that promises to outdo any conceivable terrestrial machine. Two ingredients are required. One is a massive black hole. These are believed to be ubiquitous in our galaxy, and are the relic building blocks of the supermassive black hole at the centre of our galaxy. The second is the existence of dark matter in the form of weakly interacting elementary particles. Very near the black hole, these particles accumulate and collide with each other. In the case of a spinning black hole, the collisions occur at huge energy and the debris is able to escape. Perhaps we may use such systems one day to study the particle interactions at strengths that approach the ultimate scale allowed by theory, the Planck scale.


What makes this form of particle accelerator so promising? Which difficulties must first be overcome?

If one could ever detect debris from particle collisions near the horizon of a black hole the results could be of profound importance for our knowledge of the fundamental nature of the universe and its matter and energy content.  For example, one reach  collision energies that are vastly in excess of anything that the largest conceivable terrestrial particle collider could ever produce. We could, at least in principle, probe the nature of the unification of the fundamental interactions of nature. Of course one first has to find candidate black holes and their surrounding cusps of dark matter. The best prospect for locating these comes from gamma ray astronomy or, even better, in the more distant future from neutrino astronomy.


How would the study of such particle interactions further our understanding of galaxy formation?

Dark matter provides the infrastructure within which galaxies form. If the dark matter particles occasionally suffer mutual annihilations, as theory suggests, the energy released can play a role in ionizing and heating the primordial gas clouds from which the galaxies developed. The nature of the first stars in the universe might be influenced indirectly by the dark matter. Detecting the first light in the universe is one of the primary goals of the new generation of telescopes, such as the 42 metre European Extremely Large Telescope which is under design Study. Precisely when these first stars formed and in what numbers is influenced by the dark matter, its density fluctuations, and its interactions.



In addition to your cutting edge research activities, you have also penned a number of popular science publications. Do you see making cosmology accessible to a broader public as a fundamental part of your work?

It is crucial for scientists to communicate their work to a broader public. I am especially fortunate in that cosmology has such a broad appeal. It provides an invariably popular subject of dinner conversation. It is important to take advantage of this gateway and explain our subject, our goals, and our passions, in terms that are not just accessible but are interesting and appealing to the general reader.



As a Laureate of the Blaise Pascal International Research Chair, you were able to spend a year at the Institut d’Astrophysique de Paris in 1996. What attracted you to Paris, and what aspects of this stay stand out in particular, both in terms of research conducted here and the general environment?

The Institut d’Astrophysique de Paris hosts the greatest concentration in France of astrophysicists and cosmologists. My stay, spread over two years, enabled me to initiate many collaborations which continue to this day.


Can you give details of these collaborations? What links exist between the Oxford Astrophysics department and UPMC through the IAP?

I developed new research themes and benefited from interactions with a broad spectrum of scientists in the greater Paris area. One successful initiative that came out of my stay and after my move to Oxford was to set up a Marie Curie Training Site for Doctoral Training in Particle Astrophysics and Cosmology, in which my IAP contacts played an important role in attracting students.


Can you describe the research themes you began to explore while in Paris?

Two major new initiatives in my research came out of this period. One was the notion of the intimate and dynamic relation between supermassive black holes

and their host galaxy. Black hole and galaxy spheroid form coevally, in the early universe. As the black hole accretes gas and grows in mass it develops a violent outflow. When the black hole is sufficiently massive, essentially all remaining diffuse gas is dispersed, and the growth phase both of the galaxy and the black hole terminates. In this way, their masses are inextricably linked as is suggested by the  observational data.

Another project developed the implications of the accumulation of dark matter around the massive black hole in the centre of our own Milky Way galaxy. This led to predictions of enhanced fluxes of gamma rays and other products of the enhanced rate of collisions and annihilations of dark matter particles captured near the black hole. Searches for evidence of this phenomenon continue to be a theme of particle astrophysics experiments.


What does this honorary degree represent for you? Is there further scope for scientific collaboration with UPMC in the future?

I am delighted by this honour. I hope that it will lead to many further collaborations.