Why did you decide to become a cosmologist?

Cosmologists study a truly mind-bending topic: the nature of our universe. It’s a field of study that dwarfs the scopes of many other sciences. Cosmologists, like scientists in other realms, divide themselves into different areas of study. We interviewed Neil Shah, a PhD student in cosmology at Tufts University who studies vacuum decay and alternative theories of gravity. He explains to us why he chose to study cosmology and shares some insights from the field.

Why did you choose to study cosmology?

While I love many areas of physics, I have always been most fascinated by those physics topics that deal with fundamental questions. What is the origin of the universe? How did the universe evolve? What is its intrinsic structure now? What are the laws that govern reality? What are the equations of spacetime? It is these questions that keep me up at night and drive me to explore the edges of our physics knowledge, and it is these questions that cosmology tries to answer.

Cosmology is also unique as a science as it explores the intersection of both the largest and smallest scales in physics. Cosmologists ask questions about the structure of the observable universe, which is the largest structure known to man. Since at the beginning of the universe all matter was contained in a sea of fundamental particles, cosmologists also grapple with particle physics at the smallest scales of reality. I aim to be a scientist who studies the fundamental nature of reality, and therefore I aim to be a cosmologist.

What aspect of cosmology do you study specifically?

I am a theoretical cosmologist currently pursuing two paths of study, one involving vacuum decay and the other involving alternative theories of gravity.

Vacuum decay is a cosmological phenomenon that is difficult to explain from scratch, but the basic idea can be relayed as follows: There are essentially two ways of looking at the matter content of our universe. The traditional view is that matter is composed of small fundamental particles, and the other, more modern view developed by theorists like Feynman is that the universe is filled with “fields” and what we think of as “particles” are in fact excitations in these fields. Both of these perspectives are valid in different contexts; matter looks more like particles when it’s traveling slowly and more like fields when it’s traveling fast, close to the speed of light.

There is a field for each fundamental particle, and just like particles, fields have energy. Scientists believe that the Higgs field, which corresponds to the Higgs Boson, may be in what’s called a “metastable vacuum,” which means that at some point through a process called quantum tunneling, the field may suddenly lose a large amount of energy. This shift will create a bubble of “true vacuum,” which will spread out at the speed of light. Whatever this bubble touches will be annihilated, and it is possible that the very laws of physics may change inside this bubble. In other words, this event would be catastrophic! Fortunately we can assuage our worries by keeping a few things in mind. One is that, by some estimates, this shift won’t happen for a period of time many times longer than the age of the universe. A second is that the speed of light is actually quite slow on cosmological scales, and it’s thus unlikely we’d be affected.

I also study alternative theories of gravity. While our current understanding of gravity implies the existence of a new form of matter called “dark matter,” some cosmologists have developed alternative theories of gravity that are compatible with observations without relying on the existence of dark matter. I published a paper recently that shows some fundamental issues with one of these theories. While I believe these theories have some shortcomings and personally believe in the existence of dark matter, I think the study of these theories is important to understanding the true nature of gravity.

What is something you’ve learned that you think should be more widely known?

As an undergraduate physics student, it can be easy to feel like a lot of the exploration of fundamental physics has already been done and that there is not much work left to be done to describe the equations that govern reality. As a graduate student, though, I have come to learn that this is completely false. I think people would be surprised to know just how much more work needs to be done to accomplish this enormous task. For example, we know that the standard model, which describes the 17 fundamental particles that we have discovered, has to be incomplete since it completely breaks down at very high energies. Furthermore, the standard model is missing quite a crucial physical phenomenon: gravity! The standard model is, importantly, a “quantum” model of the world – gravity, as it turns out, is highly incompatible with quantum mechanics.

In a way, I find this incompleteness of fundamental physics very exciting. It means my generation of cosmologists has a lot of work left to do. I can’t wait to make my contribution and help further push the boundaries of our collective knowledge of fundamental physics.