Gravity vs. Pressure: The battle that formed the universe - Fabio Pacucci

Welcome one and all! It’s time to grab your seat for the biggest battle in the soon-to-be-formed universe. That’s right—the Big Bang is about to go down! In one corner is the force that brings all matter together. It acts on any particle with mass, and its range is infinite—give it up for gravity!

In the other corner, our contender can push matter away with spectacular strength. When the going gets tough, this fighter just gets tougher. That’s right, it’s pressure! Over the next several hundred thousand years, these two contenders will be wrestling for the fate of the universe. That’s right folks, the ripple effects of this historic match will shape the structure of the universe as we know it today.

But what are these powers fighting over? We’ll find out when the Big Bang hits right... now! Let’s zoom in for the play-by-play. This epic event has brought three components into our infant universe: dark matter, which only interacts with gravity; baryonic matter, which makes up all matter you’ve ever seen, is affected by both gravity and pressure; and radiation composed of innumerable particles of light, also known as photons.

In the moments just after the Big Bang, all three components are in equilibrium, meaning no one location is denser than another. But as the universe starts expanding, differences in density start to emerge. Gravity immediately gets to work pulling matter together. Dark matter begins to collect at the center of these increasingly dense regions, forming the foundations of future galaxies.

Meanwhile, pressure begins gathering its strength. In this hot, high-energy environment, protons and electrons can’t come together to form atoms, so these loose particles zip around, freely interacting with ambient photons. The result is almost a fluid of baryonic matter and radiation. But the closer these baryonic particles get, the hotter the fluid becomes, pushing photons to ping around with incredible force.

This is the power of pressure, specifically radiation pressure, battling to push things apart. With each of gravity’s vicious tugs squeezing photons and matter together, pressure exerts a forceful shove back. And as the two giants struggle, they heave this fluid back and forth—creating massive waves called baryonic acoustic oscillations.

Moving at almost two thirds the speed of light, these BAOs ripple across space, impacting the universe on the biggest scale imaginable. These rolling waves determine the distribution of matter throughout space, meaning that today—almost 14 billion years after this fight began—we're more likely to find galaxies at their peaks and empty space in their troughs.

And that’s not all. We can still see these ripples in the background radiation of the universe, a permanent reminder of this epic brawl. But after being locked in a stalemate for roughly 370,000 years, the tide of our battle finally begins to turn. After all this time, the heat from the Big Bang has dissipated significantly, cooling the universe down to a temperature at which loose electrons start to pair up with protons.

Known as the “era of recombination,” this stops electrons from recklessly pinging around. This allows light to stream freely for the first time, illuminating the universe. These photons now only exert a tiny force on the neutral atoms they interact with, gradually reducing the power of pressure.

And with that, it’s time to crown our champion! The undefeated force, the most pervasive power in the universe: it’s gravity! And yet, this rivalry isn’t over. A similar battle continues between these two sworn enemies today, within every single star. As gravity pulls a star’s gas inward, pressure increases and pushes the matter back outward.

This push and pull keeps the Sun, and all other stars, stable for billions of years. In fact, this clash of the titans is the same reason Earth’s atmosphere doesn’t collapse to the ground. So while their greatest fight might have ended, these two warriors are still to be locked in combat—even as a new challenger approaches.