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 - gravity - with its infinite range. In the other corner, our contender can push matter away with spectacular strength - pressure. Over the next several hundred thousand years, these two contenders will be wrestling for the fate of the universe.
The Big Bang brings three components into the infant universe: dark matter, which only interacts with gravity; baryonic matter, which makes up all matter you’ve ever seen, and 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; however, as the universe starts expanding, differences in density start to emerge. Gravity immediately gets to work pulling matter together, while dark matter begins to collect at the center of increasingly dense regions, forming the foundations of future galaxies. Pressure begins gathering its strength, creating a fluid of baryonic matter and radiation.
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. This creates massive waves called baryonic acoustic oscillations, which ripple across space and determine the distribution of matter throughout space.
After being locked in a stalemate for roughly 370,000 years, the tide of our battle finally begins to turn. 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, allowing light to stream freely for the first time and 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. Though their biggest battle may have concluded, these two warriors will remain in combat - even as a new adversary draws near.