**Jean-Pierre Luminet Interview**

The Jean-Pierre Luminet has published more than a dozen novels and poetry books. When he looks at the sky, he imagines a non-infinite universe that is shaped like a soccer-ball. But this estimation –even if it seems so- is not poetry. Luminet, born in Provence (France) in 1951, is also an **astrophysicist**. His theory of a finite universe in the shape of a dodecahedron made the __cover of Nature -the prestigious journal- in 2003__. As well as working as a researcher for the

**CNRS (France)**, Luminet is one of the main world specialists in “the most fascinating objects in astrophysics”:

**black holes.**

**People think that the Universe is infinite. Is the Universe infinite?**

Many people ask about the shape and the size of the Universe. The conception of the size of the Universe –if it is infinite or not- is very old. For instance, in Antiquity people and philosophers thought of the Universe as a closed Universe, finite, but with a border. Now we know that it is nonsense to consider that the Universe can have a real frontier. So the tendency is to believe that the Universe is infinite, but in fact the interesting point is that new geometries built in the XIX century –called non-Euclidian geometry- and also what we call topology –which is a branch of geometry that tries to understand the global shape of space- propose **a coherent mathematical equivalent model of space which is finite without an edge. **These models, purely geometrical, are completely compatible with all the observations, and compatible of course with the cosmological solution of General Relativity called the Big Bang models.

**You have the theory that the shape of the Universe is like a soccer ball. **

Among the various possibilities –until the beginning of the XX century- many cosmologists believed that the physical Universe was infinite. There is a real problem for physicists because it will never be proven; of course, infinity cannot be measured physically. So, other cosmologists –and I was one of them- proposed specific models of **a finite universe, but without an edge, with a special shape, a strange shape which can create an optical illusion**. And in 2003, after the release of data from the NASA telescope –which mapped what we call the fossil radiation, the cosmic microwave background, which in turn gave us very deep information about all the parameters of the Universe-, we proposed in the review Nature a model, called by journalists “the Soccer Ball Universe” because -in fact, essentially- the basic shape of **the physical Universe is a little bit like a soccer ball but without an edge**. Namely, it’s a little bit like in a three-dimensional video game: you are in this volume and each time you try to go out of the volume -it’s of course impossible because the Universe has no edge-, you re-enter on the other side. So this gives you a geometrical model for the shape of the Universe, which is completely compatible with many observations, and which creates **a little cosmic illusion** with the real Universe (the 12 sides of the soccer ball would act as mirrors), which could be a little bit smaller than the observable Universe.

**What happens if in the Universe you go far enough in one direction?**

Precisely in this kind of universe model -which is finite, without an edge-, if you travelled on a rocket –always straight on, without going back- one day would make a full turn of the Universe and you’d get back home, just by going straight on. So this is typically born from strange geometrical properties of this kind of spaces. And I repeat that this kind of geometrical model is completely compatible with the Big Bang model, so the shape of the real field called Universe could have this kind of geometry.

“Some universes could last only a few milliseconds, but others would be eternal”

**What do you think about parallel universes? Could there be many universes?**

An interesting idea, coming to us first from science fiction and now coming from a recent development of what we call **quantum gravity** -namely a new theory trying to put together Einstein’s general relativity and quantum mechanics- in a new conception of space and time. This is the interesting notion of what we call **the Multiverse** -namely the idea that in fact all the Universe that we know, understand and can also measure and observe with parameters, age and size-, which would be just one very particular example within a set of a huge number of universes, all coming from what we call the quantum fluctuation of energy of a vacuum. In some new theories -in **string theory** for instance- we have to assume that space has more than three dimensions -for instance, 10 space dimensions and 1 dimension of time- and also that the notion of quantum vacuum energy is fundamental. And the spontaneous fluctuation of the energy of the quantum vacuum can produce space-time matter, namely universes. Our universe would just be a very particular solution, but the other solution would produce other universes with completely different characteristics.

**Could you give us an example?**

For instance, in this parallel universe there would be no stars, no astronomers of course, and maybe no black holes. Maybe in another universe there would be only black holes or only photons, only light. Some universes could be very short-lived, lasting only a few milliseconds; others would be eternal. In fact, in this theory **there are approximately 10 ^{500 }solutions, so there is almost an infinity of solutions**. All the possible realizations of universes could be obtained in this kind of theory, but it is still very hypothetical. String theory does not yet have an exact equation that can be solved, just mathematical ideas that people try to put together with real physics. But success is still far away.

**Would these universes be somehow connected to ours?**

So where are these parallel universes if they exist? Well, it depends on the model, because there are many models and each model proposes a different kind of parallel universes. In some models, for instance in **the inflationary universe model** (the ultra-fast expansion of the universe in its initial instants), you have a single universe but within this universe you have very large regions which in the past have been subjected to inflation, namely a specific process that greatly extends the size of space, creating a sort of bubbles of space-time, but with specific physical properties. And if the model of what we call** chaotic inflation** did not occur in the same way in all the places in space and time, it would create many bubbles with different physical properties.

**How would these bubbles be?**

In some sense, each of these bubbles can be called a universe. So now the question is whether two adjacent bubbles can collide or interact. Some people who believe in this theory –I don’t, but that doesn’t matter- try to say that an adjacent bubble universe could have an influence on some special structures seen in the cosmic microwave background. This is only one example of the Multiverse. In other models, the Multiverses are just in other dimensions. For instance, in string theory -with a space of 10 dimensions- we can consider that the Universe is just a three-dimensional section of a ten-dimensional space. And if you change the section –the cut-, you change the Universe. It’s called **the brain theory**. Each three-dimensional brain could correspond to a different universe.

“We have measured black holes weighing more than 10 billion solar masses”

**In your book ***Black holes*, you try to answer the question “Are black holes actually monsters that devour light and energy?”. Well, are they?

*Black holes*, you try to answer the question “Are black holes actually monsters that devour light and energy?”. Well, are they?

I’ve spent a lot of my research time on black holes, the most fascinating objects in astrophysics. We know of the existence of at least two kinds of black holes. **Stellar-mass black holes**, which are formed by the gravitational collapse of single massive stars. These black holes are not very big, just a few kilometers in size. And maybe in our galaxy there are several millions of stellar-mass black holes. Some of them can be observed indirectly because they belong to binary systems. In a binary system, the black hole in fact attracts the gaseous envelope of the other star and the gas flows into the black hole before disappearing into it. It is heated at very high temperatures, emitting a lot of x-rays. And this is actually detected.

**And the other kind of black holes? **

Now –maybe most interestingly and spectacularly- we believe, and are almost sure –due to observations-, that in the center of each galaxy there is a huge black hole. Not a stellar-mass black hole, but what we call **a super-massive black hole** with a mass more than a million times the solar mass, even in some cases seven billion solar masses. For instance, in the center of our own galaxy –the Milky Way- we measured indirectly the mass of a black hole with four million solar masses. With such masses, these black holes are able to capture full stars and destroy them by what we call tidal forces, namely gravitational differences that elongate the star so strongly that the star is destroyed. Many years ago I worked on the process of tidal destruction of a star and I predicted **the process of pancake stars, namely the star is flattened crossing a region around the big black hole** -flattened into the orbital plane in the shape of a gaseous pancake-, it is strongly heated and it explodes and produces a flare. So I called that **the flambeed stellar pancake**. And it has finally been observed.

**So those super-massive black holes are indeed monsters?**

Now, with big telescopes, we can observe in other galaxies some flares of light, which are interpreted as tidal destructions of stars. So it is clear that very big black holes can destroy stars. And even bigger black holes –because in some galaxies we have recently measured black holes weighing more than 10 billion solar masses- which cannot destroy stars outside of them, but only inside. Stars can just drop into this black hole and disappear without producing astronomical effects. So we do have some sort of monsters.

**How are they formed?**

It is a very interesting problem to try to explain the formation of such big black holes. If the black hole is not too massive –for instance, a few million solar masses, like the one in the center of our galaxy-, we can explain the formation as the growth of an initial small black hole formed by a star -during a million or a billion years-, that grows and grows in mass and size, but just by attracting matter. But huge black holes -with for instance 10 billion solar masses- have no time to grow by this process (the Universe is 13.8 billion years old and this means that there has not been enough time to reach such a large size). So this is one of the unsolved questions now. A possible solution is that the Big Bang could have produced a first generation of massive black holes, which would explain why we can observe very big massive black holes very early in the history of the Universe.

“The fear of a black hole devouring the Earth is completely unfounded”

**In the future, will our planet be devoured by a black hole?**

When we speak about black holes, they are generally presented as space monsters that can devour everything. Some people are afraid that one day our planet or the Sun could be swallowed by a black hole. This fear is completely unfounded, for many reasons. One of them is that black holes are rather exceptional objects. We estimate that only one star out of ten thousand can become a black hole. In average, stellar-mass black holes are very far away, because they are rare. The closest stellar-mass black hole we have detected is 6,000 light years away, so it can have absolutely no influence on the Solar System. The big black hole in the center of the Milky Way is still farther away, at 30,000 light years. So its influence is apparent only in its neighborhood, not near our planet. And we cannot imagine either a kind of wandering black hole coming through the galaxy into the Solar System like a comet, this is astrophysically quite impossible. So we should not fear at all the idea of our planet being absorbed by a black hole. And I can even add that if we put a black hole replacing the Sun in the same position- with its same mass-, the black hole would be very small –with a three- kilometer radius- and the orbit of the Earth would not change by one millimeter. So the only difference is that there would be no light.

**You wrote on your blog that poetry can sometimes anticipate scientific discoveries regarding the nature of the Universe. Do you think there is a relationship between science and poetry?**

I was always fascinated by the relationship between science –which is one specific way to understand the World- and other approches, artistic ones specifically: theatre, poetry, music, painting, etc. So I worked a lot on these possible relationships, in particular the relationship between the history of the conception of the Universe (astronomy, astrophysics, cosmology) and poetry. And more than 20 years ago I published in France a big anthology, a fascinating collection of poetry texts. T**he greatest poets of all time have written –at some moment- about the Universe.** Some of them had a really fantastic intuition. Some were more or less aware of the scientific knowledge of their times and they translated this into artistic work, like poetry. Others were not especially aware, but they used their pure intuition to undestand –sometimes- very important things that would be discovered later -by other means, of course- by scholars. This is** an interesting interaction between science and poetry** In fact, in both cases it is a creative process, because when you are a scientist you are not just an experimentalist checking results of experiments with a big machine. You also create new models for understanding universes. So in some way, there is a relationship between this process of creation and that of artistic creation. **These two kinds of creation finally depend on imagination.** And I remind you -since that we are celebrating the centenary of Einstein’s General Theory of Relativity- that Albert Einstein said that imagination is more important than knowledge.

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