What’s Up, Doc? About the evolution of globular clusters and the behavior binary black holes

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The category  What’s Up, Doc? introduces Leibniz University’s former PhD students. Today, meet Cristián Maureira-Fredes, who wrote his thesis about computer simulations of stellar dynamics of stars and black holes.

At the Leibniz University, every doctoral thesis’ final destination is the university library. We librarians marvel at the creative, tricky, striking, and even emotional research topics that PhD fellows intimately study for years. Through ups and downs they struggled with their research and their personal growth. Symbolically we get to hold all this in our hands eventually. Too often they end up in the cellar unheeded. We wanted to make a difference by inviting graduated PhDs for an interview asking for an accessible explanation of their topic and about their experiences during their studies.

Our first article was Eine monoton steigende Funktion aus dem Tal der Tränen about the motivation development of first year maths students. In this article Cristián Maureira-Fredes takes us along his personal journey as a PhD student. He studied numerical simulations of globular clusters, for example the behaviour of gas clouds in the proximity of black holes. He considered the special scenario of two black holes closely revolving around each other, so called binary black holes. As a result he developed the computer programme  GraviDy.

Please introduce yourself.

My name is Cristián, and I come from a small city in Chile called San Antonio. I came to Germany in 2012 for six months to finish my master in Computer Science, from which I was applying my knowledge to solve astrophysical problems. After finishing that period, my adviser asked me if I would like to do continue with my doctoral studies in the Max-Planck-Institute for Gravitational Physics (Potsdam-Golm), to which I said yes. I arrived for the second time to Germany at the end of 2013, and I have been living in Berlin since then.

What are you doing now?

I spend my whole doctoral studies working with numerical integrators written in C++, from which I even wrote one from scratch, GraviDy. Complementary to the numerical integration, I was processing large amounts of data using Python. I’m currently working as a Software Engineer at The Qt Company GmbH, mainly on a project that generates Python bindings of a widely known C++ framework. This work combines two of the programming languages that I widely used in my doctorate, so it was really helpful to use them while studying.

Would you give us a summary of your doctoral thesis, please?

My thesis was about studying the dynamical evolution of stars and black holes under different circumstances. Since the topic was theoretical and not experimental, I worked under many assumptions and not laws. First, I started to study the evolution of globular clusters, which are very-large set of stars  that evolve dynamically just due to the the gravitational interaction. These systems are interesting because they can collapse, form binaries, and even host black holes.

Secondly, I studied how binary black holes would evolve by changing the environment, and affecting the properties of the system, in simple words, adding stars around them in the shape of a sphere, a disc, allowing black holes to swallow stars, and more entertainingly, throwing clouds of gas to them to see how they would react! Here you can find videos and more details of the clouds simulations. 

What has been your motivation to choose the topic?

Since I started studying Computer Science I had in mind that I would like to solve a real problem on a different field. When I was a little kid, my father was always pointing at the sky teaching me which one were planets, names of constellations, and so on. I spent my childhood watching documentaries from Carl Sagan and Stephen Hawkings, reading encyclopedias that explained properties of our solar system and universe, and of course watching many Sci-Fi movies and TV shows. My undergraduate specialization was High Performance Computing, and I was really happy when I saw a couple of papers explaining how we could achieve integration of large numbers of particles with HPC techniques, that was the moment when I decided I wanted to work with Astrophysical simulations.

Where are the boundaries of what is visually imaginable and what seems counterintuitive with regard to massive black holes?

I think black holes appeared as a „fixed theory“ for many phenomena we couldn’t explain, like observations of stars moving around „something like a star“ that was invisible to our eyes. That’s a really abstract idea, and even the formal definition of a black hole of „a place in space in which the gravitational force is so strong that not even light can get away from them“ is even more abstract. That’s why I think the concept is really attractive for scientists and science fiction in general.
I am really happy to be alive in the Gravitation Wave era, and still remember how thrilled I was when I heard the news about LIGO „observing“ the first gravitational wave that came from a merger of two stellar black holes. I truly believe it’s a matter of time for humanity to find more answers regarding all the theoretical types of black holes we currently have in literature, and each new answer will move this „separation margin“ between what is imaginable and counterintuitive regarding them.

Could you explain binaries of massive black holes, please?

We are almost certain that every galaxy will host a black hole in the middle, and since galaxies are whirling around in our universe they will meet at some point, and then these galaxies will start to interact for a long period of time. The galaxies will start to interact with each other, and at some point the two central black holes will be dynamically bound, from where they will start to evolve together, and at some point they „hopefully“ will merge.
This merge is really important, because a really large amount of energy will be given off in the form of gravitational waves, a topic that I’m sure I don’t need to explain further, due to all the detections we have been listening to, thanks to LIGO.

A daring question: What does the math concerning this model “feel” like?

To be honest, it was not really hard. Stellar dynamics does not have crazy integrals or formulas that can make you want to run away. Most of the numerical methods were clear, and easy to implement on a piece of code. I think the real challenge, due to how easy it was to make mistakes, was the implementation of a couple of Post-Newtonian terms: some of them were not two or four lines of a normal A4 paper, but the whole page. Having around 60 lines of code for only one of the terms I worked with, was something really new and frightening. On the other hand I found many formulas in which some „random“ values and constant were used historically, „because [they] made the result better“, or at least that was the answer I got from many people, maybe I would have needed better math skills to perform mathematical analysis to the stellar dynamics formulas I used [laughs].

What has been the most memorable event during your thesis?

I don’t want to lie, it was the most stressful period of my life (so far!), so it could be that I was in a position where bad things seemed really bad and good things were amazing. Every small victory: getting nice results, my first scientific talks, my publications accepted, and even some emails from people saying they liked my work made me feel that all my effort was not in vain.

I feel like applying my computational skills on a topic like astrophysics made me special among my peers, and I will always remember many researchers thanking me, because science needed people with my background to improve the computational aspect of science.

But the most memorable moment of my thesis was at the end, when after finishing my presentation I was called inside the office of one of my committee members and the whole examination board was there, and they proceeded to congratulate me for passing my examination, and every one started to give me a handshake referring to me as „Herr Doktor“. I will never forget that moment.

What would you do differently in hindsight?

I started my studies knowing only a couple of „documentary definitions“ of different astrophysical phenomena, so I spent more than one year studying concepts, theories, and models that most of my peers were used to and discussed without much trouble. I regret not having a better base because I could have used my time to finish more projects and perform more simulations. Even though it was challenging, I don’t regret my decision of doing a PhD in a whole new topic, a topic that I still find fascinating.

What piece of advice would you like to share with PhD students?

Patience, perseverance, and independence. I think my PhD marked a process of self-growth not only in an academical point of view, I needed a lot of independence to keep going with all my projects, and to handle a life in a new country. Compared to my undergraduate studies, not all my collaborators were available all the time, and I met many people that were completely lost during these periods. 

I faced „real failure“ during my studies, spending many months of experiments that were completely wrong, that’s why perseverance is something that you must have, and take each defeat as a motivational boost to keep going.

Cristián Maureira-Fredes’ doctoral thesis is freely accessible in the institutional Leibniz University’s repository

Querido Cristián, ha sido un placer!  Muchas Gracias!

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