It’s a week before classes start and I’m hip deep in preparations for teaching graduate Cosmology.

As a Professor of Astrophysics, I get to help students work through a lot of topics from stellar evolution to the formation of galaxies. But each time I’m tasked with teaching Cosmology, I feel like I’m about to get the wind knocked out of me.

It’s not the technical details that get me, though they can involve some pretty heavy lifting. The real problem for me when it comes to Cosmology is feeling that I can’t do the subject justice. That’s because, unlike most other branches of astrophysics, with cosmology it’s hard to ignore the philosophical ghosts hovering just outside classroom. There are some serious boundaries in human conceptions of reality that cosmology bumps up against and, try as I might, I can’t pretend they don’t exist.

But before we go any further, it’s important to know that the most remarkable thing about the science of cosmology is that it exists at all. Human beings have been asking how everything got here since there were human beings. But even formulating those questions about “everything” has been making people’s head spin for just as long.

When, for example, we talk about “everything,” do we only mean things we can see? Or is there stuff so far away that we can’t see it? And, of course, this leads to those persistent questions about infinity and cosmic boundaries. How can the universe have an edge? What does that even mean? And how can the universe have a beginning? What could come before the beginning of everything? And is “nothing” a thing or not?

Before the scientific version of cosmology emerged in the last century, addressing these questions was solely the purview of religion and/or philosophy. The former mainly provided justifications for how a super-being might have arranged things. The latter, however, did manage to get some good work done articulating the meaning of the kinds of cosmological questions posed in the last paragraph.

The scientific version of cosmology leapt past the older religious/philosophical inquiries by focusing on a set of specific questions for which data gathering might be possible. Thus, for example, Edwin Hubble, in his groundbreaking study 100 years ago, measured the speed of galaxies relative to us, as well as their distance.

Hubble’s data told him two things. First, all the galaxies were all receding from us. And second, the farther away the galaxies were, they faster the appeared to be receding. Thus was born the recognition that the universe was expanding.

Eventually that expansion was framed within a cosmological model in which the universe as a whole was evolving. “Creation” had started out immeasurably dense, hot and smooth. Then with time and cosmic expansion (which might be seen as the same thing), the cosmos thinned, cooled, and became lumpy with structures like galaxies, stars, and people.

The most important point of this “Big Bang” model was its focus on evolution. The universe was different in the past than it appears today. This gave astrophysicists something to focus on. The main job of cosmology was to tell us what happened.

By looking out deep into space, astronomers were also looking deep into the past. That meant their observations were like a time machine, letting us glimpse the universe in different phases of its evolution. Thus the work, both theoretical and observational, was to measure and interpret data relevant to cosmic evolution from today back to . . . well, as far back as we can push things.

There can be no doubt that this scientific cosmology has been spectacularly successful. That’s one of the first points I feel a responsibility to in my teaching. After thousands of years of just pure speculation, we now have a detailed, data-driven model that can map out 13.8 billion years of cosmic history with remarkable precision. I could spend the whole class just working to let that astonishing accomplishment sink in.

But that is not the full story. Many of the ancient cosmological questions I listed earlier are not answered by the current version of cosmology. In particular, the Big Bang was never a theory of the origin of the universe. Instead it’s a theory of what happens after the origin.

Unsolved mysteries

The true origin of everything remains unresolved. To deal with it, we can’t avoid basic philosophical questions about the nature of causation. Similar kinds of philosophical questions must be addressed in dealing with questions about the origins of time or space if these are taken to be “emergent” from other kind of basal structure.

Much of the recent controversy in cosmology and foundational physics comes exactly as researches have attempted to crash through these questions by building next generation models postulating emergent space-times, string theory vacua landscapes, and, of course, the multiverse. One criticism of these theories is that being data free (almost no specific observations can constrain them), they rely heavily on unconscious philosophical assumptions.

This is a point Sabine Hossenfelder makes well in her book Lost in Math. The irony about this situation is some leaders in the field remain adamant that philosophy is useless to cosmology. Luckily others—like Lee Smolin, Carlo Rovelli, and Sean Carroll—recognize that cosmology’s implicit philosophical ghosts need to have their say, even if they have very different ways to address those ghosts.

So as I step before my class next week, I’m going to feel the weight of two opposing urges. On the one hand, it’s worth a lot of time to unpack just how successful this endeavor has been in explaining the universe. And yet, at the same time, I feel like I could spend a whole semester exploring how cosmology’s basic questions still push at our conceptual boundaries.

I’m not sure how I’ll balance those two responsibilities. But I am sure I’m going to try.

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Frank is a professor of astrophysics at the University of Rochester.