Gravity is one of the four fundamental forces of nature. But it’s the only one to affect all matter and energy—and it’s the weakest.

“This makes it both ubiquitous and hard to measure,” says Tristan Smith, assistant professor of physics. “Einstein’s theory of general relativity has been extensively tested over decades of research in our solar system, but gravitational effects on galactic and cosmological scales are still largely unknown.”

Smith will explore this terrain as a Cottrell Scholar, an opportunity he had coveted since first his first days at the College in 2014. Bestowed by the Research Corporation for Science Advancement, the scholarship empowers Smith to develop a new, robust test of gravity by comparing the deflection of light around galaxies to the motion of stars within those galaxies.

The award also has an educational component: Smith will use some of the funding to develop a science communication curriculum for research students at Swarthmore, building upon ongoing efforts to help students communicate science to general audiences. 

And it follows the collaboration Smith had with cosmology theorists at Johns Hopkins University during his sabbatical there last year. The group theorized that the cause of the so-called Hubble Tension in cosmology may be resolved by a new exotic form of matter in the early universe: early dark energy. The implications of their paper are vast, raising intriguing questions across fundamental physics.

Smith recently discussed his excitement for the Cottrell Scholarship, his recent research efforts, and the ways that his sabbatical year is already benefiting students at Swarthmore.

In what ways will your research branch off of Einstein’s theory of general relativity?

Those first measurements, made 100 years ago, were made of the bending of starlight around the sun due to gravity. I’m looking to develop the same basic idea, but instead of thinking about starlight passing near the sun, I’ll be looking at light from galaxies passing by other closer galaxies to us. So the physical setup involves us looking at a galaxy some cosmic distance away from the Earth, and behind this galaxy, another cosmic distance away, is a second galaxy: As the light from that farthest galaxy passes by the nearer one, its light gets slightly bent by the gravity of the intervening galaxy. The amount of bending depends on the mass of the intervening galaxy as well as the inner workings of gravity. We can estimate its mass by measuring the motion of the stars within the intervening galaxy. When we combine the motion of the stars with the amount of bending of light, we have a new fundamental test of the physics of gravity.

How and when was this interest sparked for you?

This is one of the first ideas I had as a senior graduate student that I thought was pretty good. I started to think about how we could test these modified gravity theories in different ways than other people had done before. At the time, there weren’t good enough measurements of the physics of galaxies to really explore this properly, and the right kinds of simulations were not available. So it really was a process of waiting. And so those two pieces matured enough that I think we’re at the point where we can put them together to create very stringent, fully consistent tests of gravity. So it’s been a long time coming, and it’s pretty amazing thinking back to that history of it for me.

How did the Hubble Tension paper come about?

A postdoctoral researcher, Vivian Poulin, and I really hit it off as scientific collaborators from the moment he arrived at Johns Hopkins. We wrote a general paper on the physics of scalar fields and trying to see how their presence affects the measurements that we’ve made in cosmology. It explored the possibility that these scalar fields may help with this Hubble Tension, but it didn’t look very promising. And so I had actually written it off. But Vivian kept looking into it and found that if you do the analysis that we did in a slightly different way, things looked more and more promising: that the scalar fields may play an important role in resolving this tension. It was such a simple extension of the work that we had done before that it didn’t take us very long to work things out more completely and find that there really was something very interesting happening.

It was so much fun, because as a theoretical cosmologist, most of the work I’ve done has just put upper limits on possible extensions of our standard theory of cosmology. And this is the first time that I’m actually talking about something like extra physics that nobody expected. We are not just refining our understanding a little bit in this area; here, we might actually be extending our knowledge and understanding of the universe. It’s been tremendously exciting to do that.

What fuels your interest in improving how science is communicated?

That’s something I’m attracted to as someone who was an understudy on a Broadway show as a 10-year-old, and was in a Cheerios commercial as a kid. Those are unusual experiences to bring to my role as a scientist and mentor. But the issue is universal: How do you take technical, maybe esoteric things you have been studying and put them in a context to communicate them effectively to any type of audience you may have? That’s always been a top priority of mine, and with the Cottrell funding I was able to develop a pilot program at Swarthmore this summer to provide resources to research students on the science of communication. They’ll meet, go over pertinent articles, and do exercises to practice communicating what they’re studying. And we’ll be applying a lot of this to the Sigma Xi poster session this fall, helping students make the most of the pitch they give to community members. 

How might your sabbatical experience impact your students at Swarthmore?

One of the top benefits is the opportunity you have to pursue new lines of inquiry and research. But even more important are the new relationships you can develop with other researchers, at other institutions, which have a really unbelievable impact on the kinds of opportunities that our students have through working in my group. Every summer, my research students and I take a trip to Hopkins because I want them to see what that environment is like, and to meet some of the people that we’re collaborating with. This summer we made a trip to MIT to be a part of a summer undergraduate science workshop, along with students from Dartmouth and Brown and Haverford. And I was able to make new connections to researchers at the University of Chicago who my students and I will be virtually communicating with. The sabbatical year gave me a broader palette of ways to bring students into really, really exciting areas of research.

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