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Yoann recently submitted his thesis in early-universe cosmology, a field that uses observations of the present-day universe to investigate the physics of its earliest moments, including particles and gravitational phenomena that we cannot directly observe today. "My PhD was about improving this modelling in the period called inflation," he explains, "when the universe expanded really fast with unknown particles interacting, mixing collider and spacetime physics."

One of the most common misconceptions about his field, Yoann says, is that people confuse astrophysics with cosmology. "Astrophysics looks at much smaller scales where there is a lot of mass condensed, like stars, black holes or galaxies. I couldn't tell you the order of the planets in the solar system, which is a problem every time I go to a pub quiz!"

But while his research looks at some of the largest scales imaginable, Yoann is also increasingly concerned with something much closer to home: the environmental impact of academic research. For example, he tracks the emissions associated with the computer simulations he runs and deliberately limits how many he undertakes.

"Industry-wise, cosmology has no short-term applications – even when Einstein invented his theory of relativity, it took quite a few decades for it to be applied to the GPS. It's not clear that the applications of cosmology will save the earth. But typically, we are encouraged to use all our computational resources to still be provided them the following year. At every scale of the system, people push you to consume.” He smiles. “Call me a rebel, but I simply don't want to. Sometimes you can use proxies like pen and paper instead of hundreds of test simulations."

This attitude also shaped the way Yoann approached academic travel. Having always dreamed of taking an Interrail trip across Europe, he became increasingly uncomfortable with the expectation that academics should routinely fly to conferences and meetings. Rather than simply reducing his own flights, Yoann decided to make the alternative visible. He organised a 'green tour' of theoretical physics institutions and conferences, travelling around Europe by train. "I was mostly silent and doing my own thing, but then I thought, if everybody is silent, then nothing is going to change. So I organised this trip, and every time I gave a talk on my research, I would take one or two minutes to tell people about what I was doing and why we should take the train more." The response was overwhelmingly positive.

The logistical challenges were not insignificant, and Yoann ironically cites his inability to sleep on sleeper trains among the difficulties, but says the climate benefits – alongside the ability to work while travelling – outweigh the drawbacks. "I consider myself rather adventurous. I think the biggest challenge was finding the money, and I was lucky to have ����Vlog�ٷ�." ����Vlog�ٷ� supported the journey through a travel grant, while the institutions that hosted Yoann ultimately covered half of his living and travel expenses. In the end, his calculations showed that the per diem cost was similar, if not less, than usual airplane-based trips to single academic events and visits.

The success of the trip has prompted Yoann to consider boycotting large conferences in favour of settings that encourage what he terms 'academic intimacy'. Rather than large international gatherings that require researchers to fly across continents for a few days, he believes there are other ways to create meaningful connections. "There's no reason why international conferences can't be organised in continental hubs. You could waive fees if people travel by train, these things have started appearing a bit. Or we could develop more Visiting Fellowships, where people go to an institution for a few weeks or months rather than a few days." He suggests that individual action is ultimately about accepting imperfection while still trying to influence the people around you.

"You don't have to be perfect. That's not the point. The point is that everyone needs to do their bit and be aware, because this is how you make a change."

He shrugs. "My friends or colleagues think that I'm that annoying guy, and that's exactly what I want. Because then sometimes they feel bad, because they have my voice in their head. In 10, 20 years, these people will be in power and be in the position to make change."

Yoann's approach to climate action is perhaps fitting for someone whose research is concerned with the largest scales imaginable. "I like going as far as possible – I think there’s something pure about it, and that's why I do cosmology, because there's nothing bigger you can do in terms of scale." He is now preparing to begin a new chapter as a postdoctoral researcher in Poland. When asked what he is looking forward to, he smiles. "What do I look forward to? Being an adult, really. Not having to pass a degree anymore."

"When I came with my two cases to ����Vlog�ٷ� five years ago, it was an adventure. Now it feels like a new adventure, and I like that. It's probably going to be one of the best times of my career in the sense that I'll be the most free."

He considers for a moment. "This is probably why all of us academics keep going – you are free. You think. That's your job. And it’s pretty cool."

You can see the details of Yoann’s European journey on , including a map with journal entries from each stop.