# An Italian Cosmologist Who Wanders in Dante’s Dark Wood

A cosmologist and data scientist with a literary bent, Valeria Pettorino thinks multiple angles and diverse points of view are needed to unriddle the nature of dark matter and dark energy.

In 2004, the Italian theoretical cosmologist Valeria Pettorino wrote her doctoral thesis on “dark energy in generalized theories of gravity.” As a side project, she translated the opening lines of Dante’s Divine Comedy into a geometry problem.

“I felt there was mathematics already within Dante’s writing,” Pettorino said recently.

Dante’s epic poem, in Mark Musa’s translation, begins:

Midway along the journey of our life
I woke to find myself in a dark wood,
for I had wandered off from the straight path.

Given a line segment AB of size equal to our life path, consider its midpoint M. If D is a man called Dante, D shall be coincident with M.
The segment AB shall be contained in a dark field DF.
Assuming that a circumference C exists, circumscribed to the dark field DF, verify that the straight line r is external to such circumference.

This reimagining, part of a creative writing group project, was published in a collection titled Faximile — an homage to admired authors and texts, in which the Pythagorean theorem became a story, The Iliad became a football match, and the Italian constitution was rendered in hendecasyllabic verse. “We liked the originals, and we wanted to play with them and understand them better,” Pettorino said.

She has approached cosmology in the same spirit, using storytelling from multiple angles as a guiding principle. After earning her Ph.D. in 2005, she traveled the world, hopping between institutions in Heidelberg, New York, Geneva and Valencia, as well as Naples, Turin and Trieste in her native Italy, alternating between observational, theoretical, methodological and statistical points of view in her study of the cosmos — a dark wood, rather like Dante’s. She considers all of these approaches necessary for unraveling the nature of dark matter and dark energy, little-understood substances that together comprise 95 percent of the universe.

It is perhaps not surprising that in 2016 Pettorino landed at the CosmoStat laboratory at CEA Saclay, a research institute 15 miles south of Paris. At CosmoStat, cosmologists and computer scientists collaborate to develop new statistical and signal-processing methods for interpreting the vast volumes of data acquired by modern telescopes. This summer, Pettorino helped complete the final analysis of data from the European Space Agency’s Planck space telescope, which mapped the early universe with unprecedented precision. Her main focus now is Euclid, the agency’s next major space telescope, set to launch in 2022. Euclid will gather 170 million gigabytes of data about billions of galaxies, slicing the universe at different epochs and tracking its evolution under dark influences.

Quanta Magazine spoke with Pettorino over Skype this summer as she helped organize the annual EuroPython conference for users of the Python programming language, among other extracurricular commitments. The interview has been condensed and edited for clarity.

### You have many interests. Tell me how you became a cosmologist.

I hadn’t thought about cosmology at all when I started physics, and even then I wasn’t very convinced about physics in itself. But physics offered me a good opportunity to combine several different interests. At the time, I was living in Naples, my city. I really wanted to follow a path that would allow me to get to know people, live in different places, and learn languages. I certainly liked logic and mathematics. And I heard about physics from my uncle, Roberto Pettorino, who was a string theorist; he told me about strings, multiple dimensions, time travel. And I loved science fiction. The authors I read most were Philip José Farmer and Jack Vance — the stories had adventure, and different technologies, and they were very realistic, creating new worlds in great detail with things that don’t exist but could very easily have existed. I liked challenges. At that time, I was taking acting classes and creative writing classes. And then I just said, “Let’s do physics!” I was curious about the whole picture, and physics looked to me like a good combination of logic, of communication, of imagination. My main goal was to learn, to increase my knowledge, to satisfy my curiosity.

### How did you eventually find cosmology?

I started physics as an undergraduate at the end of 1997, and then in 1998 there was the cosmic acceleration discovery, revealing that a lot of the universe was completely unknown, and this immediately attracted my curiosity. What happened was that independent observations by two different supernova research teams showed very surprising results: Cosmologists were expecting the universe to be expanding after the Big Bang, and since gravity attracts things toward each other, the expectation was that the universe’s expansion was decelerating. Evidence from supernova explosions showed that the expansion is, instead, accelerating — as if there is some extra form of energy that counteracts gravity and increases the velocity of the expansion. This is generically named “dark energy.”

Since 1998, many other experiments have confirmed the same picture: Normal atoms only account for about 5 percent of the total energy budget in the universe. There is an extra 25 percent that is in the form of “dark matter.” Dark matter still feels gravity, but we don’t observe it directly; it acts as a glue that allows structures, like galaxies and clusters of galaxies, to form. And then there is the rest — 70 percent — which is dark energy, and which should be responsible for cosmic acceleration.

### The ever-elusive dark energy — what is it?

That’s still unclear. The simplest way to describe it is as a kind of energy whose density is constant everywhere in time and space, termed the “cosmological constant.” This is one new parameter added to the theory of general relativity, and in practice it fits the data very well — including the final data from the Planck space satellite. Unfortunately, the problem is that the cosmological constant is not well-understood theoretically. First, we cannot predict its value, and we need to have very precise initial conditions to end up with the “right” observed value of this constant. This is the fine-tuning problem. Secondly, the cosmological constant marks our epoch as a very special time within the evolution of the universe. The density of dark energy was completely negligible in the past compared to the density of dark matter (which was higher in the past, when the volume of the visible universe was smaller.) In the future, however, dark energy will dominate over all species of matter, because the dark-matter density will continue to decrease as the universe expands. We happen to live in that epoch in which the cosmological constant is of roughly the same order as matter. That’s a big coincidence.

### So if this big coincidence doesn’t seem like a plausible storyline, then maybe a big modification is needed?

This lack of understanding about the cosmological constant has motivated researchers to look for alternative explanations. Cosmic acceleration could be caused by a new fluid, or a new particle whose density changes in time instead of being constant, or more than one particle, or more than one fluid. Or, cosmic acceleration could be the hint that our laws of gravity (namely, Albert Einstein’s theory of general relativity) need to be modified, particularly at very large scales.

Astrophysicists have already tested general relativity at the scale of the solar system. Models referred to as “modified gravity” try to modify general relativity at very large scales to account for cosmic acceleration. Some of these modified-gravity models have been excluded already, for example after the recent detection of gravitational waves. But there are still many models fitting current data, and no clear solution of the theoretical problems associated with the cosmological-constant scenario.

### You’ve moved around a lot during your career. What effect has this had on your approach, being exposed to such a wide spectrum of people and ideas?

It was challenging to move continuously. I moved 10 times in 12 years, sometimes changing country, sometimes changing continent, sometimes for a month, six months, a year, two years — for the possibility of having funding. It was great because I wanted to work with different groups and also have the opportunity to understand different points of view on the same story. It was a bit like in 2004 when I was taking a creative writing class and we rewrote stories from the point of view of different characters or objects. Somehow I wanted to have the same feeling in science, in cosmology. I wanted to learn from different groups and different perspectives — the observational point of view and theoretical point of view. I started as a theorist and then got more and more interested in testing theories with data. I never thought I would become the best theorist or the best observer, but at some point I realized that I could talk to both theorists and observers and that was a valuable skill in itself, allowing me to grasp challenges and requirements on both sides. I wanted to know more about the methods used, the assumptions made in the data analysis, and the difficulties in working with different data sets. And I wanted to test the theories myself.

### What are the difficulties on both sides?

The challenge for theorists is in trying to formulate a coherent proposal that explains cosmic acceleration without requiring fine-tuning or coincidence. It means developing the formalism, checking that it is stable and consistent, and deriving its theoretical predictions. There are different approaches one can take in trying to test the predictions, but you have to first understand them; you have to describe them analytically with equations and with new numerical codes, capturing how every single species — matter, radiation, extra fields or forces — evolved from the beginning of the universe until today.

From the data point of view, it’s important to make sure you remove systematic effects related to the specific features of the detector, or other effects that may mimic the signal you actually want to measure. In addition, it is important to be aware of the assumptions you’re making within the whole analysis and when you’re validating your tools, for example by comparing independent numerical codes to check that they give the same result. Sometimes time constraints may limit the tests and validations we do to the simplest theoretical scenarios. Testing more exotic ideas is an additional challenge that we need to face to avoid confirmation bias — the tendency to interpret or cherry-pick data in a way that confirms pre-existing beliefs. Sometimes it feels a bit uncomfortable that the cosmological-constant model accepted as standard is also the one which is easier to test.

We need to maximize the information we can get from the large amount of data that’s becoming available and use it to improve the interpretation of the dark universe. That’s what I’m doing at CosmoStat — applying advanced statistics to the data, to improve the comparison between data and predictions from theoretical models. And recently, with Austin Peel, a postdoc here, we developed machine-learning algorithms to identify hallmarks of modified gravity.

### If you could choose, how would this cosmic storyline end?

In a sense, the story is really just starting. Overall, I feel that data, theory, methodology as well as data science are all key ingredients of the same quest, with different challenges but the same aim of understanding the universe — its evolution, its future. Communication among these diverse communities with different skills and expertise can make the difference between having access or not to new exciting discoveries.

### Speaking of diversity, along with your involvement with the EuroPython Society and the Science to Data Science training program, you also volunteer for the Supernova Foundation, which provides mentoring for women in physics. How does the Supernova mentoring work?

This began two years ago, as a pilot program started by Michelle Lochner from Cape Town, with just a few mentors and a group of mentees. It was originally intended for women in developing countries, but then Michelle got requests from mentees all over the world, and also from mentors who wanted to help. In practice, we connect women who are undergraduates in physics from different countries and also provide them with role models who they can talk to for support. Right now, I have two mentees. It’s really about giving them information about our experience and practical information about, say, how to prepare a presentation or write a CV. And it’s about helping them in difficult situations like gender harassment. It can seem simple, but it really has a lot of impact on their motivation, just the fact that they can talk to someone who has already gone through a similar career path. It’s something that personally I would have really liked at the start of my career — especially in situations in which I found myself as the only woman in the whole department.

Do you find it discouraging that so few women pursue careers in physics even now, and that women in the field still face such bias and sexism?

What I find most discouraging is that statistics show there are actually women starting careers in physics at the undergraduate or Ph.D. level, but they become increasingly underrepresented as their career progresses, as actually happens in many fields. When this happens because of bias or lack of equal opportunities, it is frustrating.

The point is not, of course, to convince women to do physics. The point is that they shouldn’t be discouraged. They should have the same possibilities and chances, if they wish to pursue a career in physics. Everyone benefits from that. Not just women. This comes back to the idea that there is a higher chance of scientific progress with diverse approaches and different perspectives when telling the story.