As scientists and researchers make sense of some of the major discoveries from the turn of the twenty-first century, “the present moment in the science of cosmology is one of consolidation,” writes the theoretical physicist Sean Carroll in the Review’s August 21, 2025, issue. “Yet crucially important questions remain unanswered,” he continues, “especially about what exactly happened at the onset of the expanding space that evolved into our contemporary universe.”
Why does the universe appear to be “flat”? Is it one, or many? What started the whole thing off? Answers to some of these questions are addressed by the model of the “inflationary universe,” which posits that the early universe was primarily composed of “a superdense kind of dark energy” that caused “rapidly accelerated expansion for a short period,” flattening out the universe’s fabric. Some cosmologists even posit that, as quantum fluctuations throughout the cosmos suggest, this high-energy “inflaton field” persists in certain places, effectively creating a multiverse by expanding parts of the universe so far away from each other that they can never interact.
Carroll, a professor at Johns Hopkins University, has made these profound mysteries and the search to resolve them accessible to the general public in a series of books on quantum worlds, the origins of life, and the nature of time, in addition to his podcast Mindscape. His current research focuses on the foundations of quantum mechanics and the evolution of entropy and complexity. I wrote to him this week to ask about dark energy, Trump’s attack on science research, and if quantum mechanics can be grasped in simple language.
Willa Glickman: What drew you to study cosmology?
Sean Carroll: I fell in love with the universe at a young age, some time in elementary school. I just read books about the Big Bang, black holes, and particle physics in my local public library. These ideas were fantastical but also real, and I couldn’t wait to learn more about them.
In your essay you mention “the inherent ambiguity of using ordinary language to describe novel scientific concepts.” How close do you think everyday language can get to explaining cosmological or quantum mechanical concepts?
Everyday language can actually convey cutting-edge physics concepts very well, if only we can let go of the everyday connotations that some terms carry with them. In reality, that turns out to be extremely hard to do! Part of the art of science communication is deploying familiar terms in very precise ways, and trying to be as clear as possible about the limitations and extensions of what they are supposed to mean.
Could you walk us through the prevailing theories about what dark matter and dark energy might be? What sort of data are physicists looking at to try to find answers?
The natures of dark matter and dark energy are some of the most pressing issues in modern physics, although in slightly different ways. We know a good amount about dark matter—how much of it there is, roughly how it is distributed through space, its velocity, and how much it interacts with ordinary matter (not at all, as far as we have seen). But that knowledge isn’t nearly enough to identify a unique candidate for what it could be. In fact, far from it: there is an embarrassment of riches when it comes to theoretical proposals for dark matter, each of which requires very different experiments to test.
For a long time the most popular model has been WIMPs, a playful acronym for Weakly Interacting Massive Particles. The word “weak” here refers specifically to the weak interactions of particle physics—one of the fundamental forces of the universe. The good news is that there is a “favorite” energy scale for the masses and interactions of weakly interacting particles, which conspire to produce an excellent dark matter candidate if there is an electrically neutral, stable particle that hasn’t yet been discovered. The bad news is that we know how to look for weakly interacting particles, and we still haven’t found one with the right properties to be dark matter.
The other popular idea for dark matter is a particle called the axion, which was proposed in the 1970s as part of a theoretical explanation for properties of the strong nuclear force—another fundamental force, in this case the one that holds a nucleus together. (It’s always considered a bonus when a new particle helps solves more than one problem at once.) Axions make a great dark matter candidate, but are harder to detect experimentally, so they haven’t received as much attention. But the continued absence of experimental signals for WIMPs has increased the interest in axions, so more experimental searches are starting up.
For dark energy, we’re lucky to have a single obvious model that has been very successful: vacuum energy, or Einstein’s cosmological constant. The idea is that space itself comes with a small, fixed energy density that pushes the universe apart. That idea seems to fit a wide variety of data, but there have been hints from some recent observations that the fit might be imperfect. The alternative is some kind of almost-but-not-quite constant energy that fills all of space. There is a simple way to get that, using what’s called a “scalar field” slowly changing over time, but specific models tend to come with flaws. Keeping an eye on this is going to be a fun aspect of cosmology in the years to come.
Are you convinced by the inflationary universe theory, or are there other cosmological theories you find more or equally plausible?
I’m not “convinced,” simply because inflation represents an enormous extrapolation away from the kinds of physics that we’ve directly probed in experiments. Plus, there are some deep conceptual questions about how inflation gets started in the first place. At the same time, inflation provides a wonderfully coherent picture of how the universe arrived at the conditions we observe on large scales. And frankly there are no competing models that do that very well at all. So inflation is still very promising, in part by default.
You describe yourself as a poetic naturalist—could you tell us a bit about the “poetic” side of that equation?
The motto of poetic naturalism is that there is only one reality, physical reality (“naturalism”), but there are many valid ways of talking about it (“poetic”). Some of those ways of talking are purely scientific, and their success is measured by how well they fit the data. There is fundamental physics, and then there are also “emergent” levels of chemistry, biology, psychology, and so on. All of these have to be compatible descriptions of the same underlying phenomena.
But aside from that, there are also ways of talking that are not fixed by the data, ways that involve human judgments: morality, values, aesthetics. These can be perfectly valid ways of describing reality, and they need to be compatible with the underlying physical reality, but because they are not determined by experiments there is room for different people to have mutually incompatible views. Sometimes that’s just the way it is.
How has physics research fared under the Trump administration?
Things are still very much in flux, but overall the Trump administration has been an unmitigated disaster for scientific research in the United States, physics included. Both NASA and the National Science Foundation are important sources of research funding for physics that have been dramatically cut, or at least had their budgets threatened. The effects of the cuts are not very well targeted, since universities are scrambling to cover as much as they can with significantly less total funding coming in. At Johns Hopkins, for example, the administration has instituted both a general salary freeze and a hiring freeze. Last year I was on a search committee for a new theoretical physicist; we chose an excellent candidate but were ultimately unable to make an offer, as the university had to retrench. This is not to mention the atmosphere of fear and uncertainty that is leading both young researchers and established ones to leave the country. The damage that has been done to our world-leading scientific research infrastructure will take decades to repair, if we are ever able to.
Have there been any recent research findings that you’re particularly excited about?
There always are, but as a theorist the kinds of progress I care about tend to be rather esoteric and incremental. On the experimental/observational side, I am cautiously intrigued by recent hints that our standard cosmological model may need some upgrades. The basic picture is completely solid—a hot Big Bang fourteen billion years ago, followed by expansion and cooling and formation of stars and galaxies, with a mixture of ordinary matter, dark matter, and dark energy. But there is room to improve our understanding of the specifics, and there is tentative evidence that we may need to make some adjustments to the model.
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