Christmas Tree Meets Cosmic Shadows

I’ve always wished I had more time to dive into the intricacies of observational astronomy. I want to understand mechanisms that seem too absurd to reason about. What are the margins of error in our knowledge? What does it mean to have an acoustic wave in the early baryonic universe (Baryonic Acoustic Oscillation)? What does it mean to have an inflation of space itself happening everywhere at once (Cosmic inflation)?

A common version of the Big Bang theory goes like this: In the beginning, the universe was incredibly small and unimaginably hot—so hot that it existed in high-energy states where even light couldn’t travel freely. Then came rapid inflation, which cooled the universe. During this cooling, lower-energy particles like baryons (matter) and photons (light) began to dominate. For the first time, light could travel through the universe. Because the universe wasn’t evenly distributed, this light wasn’t evenly dispersed. We observe this uneven first light today as the Cosmic Microwave Background (CMB). Over time, the cooling baryonic universe formed galaxies, stars, planets—and eventually dinosaurs.

Over Christmas, as I sat in my living room with a toddler asleep on my lap, I found myself staring at the shadows cast by our mini Christmas tree in the corner. The warped shadows on the back wall baffled me—it was nearly impossible to deduce the structure of the object creating them. The light source was within the tree itself, small bulbs nestled among the leaves. The overlapping leaves caused the light to pass through layers of complete and partial occlusion. By the time it reached the edge of the tree, the light had lost much of its original shape.

To the human eye, the resulting patterns appeared fractal. Trying to capture the intricacies was overwhelming—an exercise in frustration that could easily induce a headache. Setting aside the view of the leaves themselves, and focusing solely on the shadows, the tree’s mystery deepened. I could discern the shapes of one or two branches, but describing the overall structure, size, or density of the tree felt impossible.

I believe astronomers face a similar challenge when studying the CMB. The patterns in the CMB are like the shadows of the early baryonic universe, just translucent enough for light to travel through. As light passed unevenly through this dense "baryonic forest," much like the branches and leaves of the Christmas tree, it left clues about the universe’s early structure.

But the universe didn’t pause. It continued inflating everywhere at once, carrying the baryons with it. A vast baryonic sphere of unknown shape and dimension emerged, giving rise to the observable universe. We’re made of those baryons and live within that sphere. Looking around, much like a squirrel inside my Christmas tree, we only see the distorted shadows on the wall—the light warped and twisted by countless occlusions.

I marvel at how astronomers make sense of the fascinating chaos of the CMB. How difficult it must be to draw conclusions from the shadowy fragments of the universe’s earliest light!