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Cross section of a black hole (© Johan Jarnestad/The Royal Swedish Academy of Sciences)

16/10/2020

The recently announced 2020 Nobel Prize in Physics celebrates three scientists who have dedicated their careers to the study of one of the universe's most exotic phenomena: black holes. Roger Penrose, Reinhard Genzel and Andrea Ghez share this year's prize for work that complements each other's discoveries. On the theoretical side is Penrose, whose award cites his "discovery that black hole formation is a robust prediction of the general theory of relativity”. Providing evidence to that theory is the observational work of Genzel and Ghez, cited by the Nobel Prize “for the discovery of a supermassive compact object at the centre of our galaxy”.

To put a perspective on the scientific impact of this year's Nobel winners, ICTP talked with Paolo Creminelli, head of the Centre's High Energy, Cosmology and Astroparticle Physics section. Creminelli is a cosmologist whose research focuses on inflation (the earliest phase of the evolution of the universe), dark energy and theories of gravity beyond general relativity. His comments are summarized below.

**Q: What has been the impact of Penrose's work on cosmology?**

It has been quite substantial. The reason why he got the prize was that he invented a new, mathematically sophisticated technique to prove that under some assumption, black holes form. In particular, when an astrophysical object reaches some condition, it is unavoidable that it collapses. Penrose's technique was not only used for black holes, but also in other contexts, such as the Big Bang. It opened a new way of approaching these kind of problems in physics, and from this point of view was really revolutionary.

**Q: Penrose's theorems prove the existence of singularities, regions of infinitely strong gravity. How are theorists approaching this problem?**

We don't have a solution. Penrose's theorems show that it is unavoidable for singularities to be created. Singularities are where our current understanding of physics breaks down. This has to do with putting together general relativity with quantum mechanics, and this is something that theories like string theory should be able to do. On the other hand, from a technical point of view many of these singularities are still a mystery. What is good (or bad) is that these singularities are shielded, something that Penrose studied at length. In the case of black holes, they occur inside the black hole horizon, so for all practical purposes the existence of this singularity does not affect the exterior of a black hole. But it remains a completely open problem in theoretical physics.

**Q: What are the hopes for what further observations like those of Genzel and Ghez can tell us about black holes?**

In the past few years many things have happened regarding the observation of black holes. Nowadays, we observe black holes in many ways. For instance, we observe black holes when they coalesce from a binary system to form a bigger black hole because they emit gravitational waves. We also observe direct images like the ones of the Event Horizon Telescope. Besides these, there is the technique by the two Nobel laureates (Genzel and Ghez), which has to do with studying the stars around super massive black holes to understand what is at the centre of the galaxy. I think that all of these techniques are going to improve substantially in the near future, and I think the general theme is that we are going to be more and more sure that black holes are what they are supposed to be, which is the original solution found in general relativity. We are going to see these in more detail.

Roger Penrose (front row, fifth from right, sitting next to ICTP founder Abdus Salam)

visited ICTP in 1975 for the First Marcel Grossmann Meeting on General Relativity.

What is nice about black holes is that they are very simple objects: they are only described by the mass and the angular momentum, which is how much it is rotating. Once you have these two numbers you have a complete description of the black hole. This is fascinating because these are the simplest objects in nature. As observations become better and better, we are going to check that black holes are compatible with the solutions described by these two numbers. At the moment, everything is compatible with general relativity, but we only have a rough image of a black hole.

**Q: How has your own work benefitted from Penrose's discovery?**

Something that I was working on recently: imagine that black holes are not as simple as what general relativity predicts. In the jargon, we say maybe the black holes have hair, in the sense that they are more complicated than what is predicted by General Relativity. One tries to modify general relativity to find solutions which are slightly different. Naturally, if I start modifying gravity, I will have some modification in the cosmological evolution and at the same time in some theories a modification in the predictions for black holes.

Coming back to Penrose, he opened a new way to study general relativity, he opened a toolbox of mathematical techniques which are useful for general relativity and they can be used in contexts that are quite far from black holes. Indeed, I was using techniques which are inspired by Penrose's work recently in my latest paper, where we studied the beginning of the universe and the period of inflation. It has nothing to do with black holes or singularities, but the techniques are similar.

Black holes are useful also as sources of gravitational waves. One topic I have been exploring in my group is to use gravitational waves as a probe of what is going between emission and observation. Gravitational waves propagate through the universe and tell us what the intervening medium is that they go through They can say something, for instance, about dark energy. From this point of view it is another reason why black holes are interesting, not only as the most compact massive object but also as the most efficient source of gravitational waves.

For further reading, ICTP's Marie Curie Library has a number of books by Roger Penrose in its collections, as well as a book by one of this year's co-winners, Reinhard Genzel.

*--Mary Ann Williams*