By Gary Felder, Smith College
The description of how our universe has evolved from the moment of the big bang until today is known as the big bang model. The universe right after the big bang was unimaginably hot, dense, and filled with many types of particles that don’t exist in the universe today. And it was also changing rapidly.
The Universe’s Baby Steps
In the time it is taking you to read this sentence, the temperature of the universe dropped by a factor of more than a trillion trillion, and over 99.9999999% of all the particles in the universe were destroyed. This huge change in temperature and the annihilation of almost all particles are just two of many examples of the incredibly rapid and dramatic changes that took place in the very early universe.
If you could travel back in time 13.8 billion years, you would find all of space filled with a seething fluid far hotter and denser than the core of the sun is today. There was no vacuum of space, and there were no structures of any kind. There were just particles constantly moving and colliding and emitting gamma rays, x-rays, light, and every other known kind of radiation.
Those particles that existed a moment after the big bang are the building blocks of all structures in the universe today, but at that time, the particles were all separate, like a fresh box of Legos with nothing stuck together.
A Cool Universe
Over time, the particles in that chaotic swirl were getting, on average, farther away from each other. In other words, the universe was expanding. And as the universe expanded, it cooled down. And as the universe cooled down, particles began to stick together.
The smallest particles combined into bigger particles, which then combined into atomic nuclei. Those nuclei combined with electrons to make atoms. Those atoms coalesced into stars and galaxies, eventually leading to the universe we see today.
The Bigger Picture
It turns out that if things had happened just slightly differently almost 14 billion years ago, the universe as we know it today could not have developed. For instance, if the universe had started out expanding a little more slowly than it did, everything would have collapsed again almost instantly.
Or, if the universe had started out expanding a little more quickly than it did, atoms wouldn’t have combined into stars and galaxies. The universe could even have ended up with different kinds of particles and forces that wouldn’t have allowed atoms to exist at all. And it’s theoretically possible that in distant parts of the universe, each of those things really did happen or may be happening right now.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
The Real Explanation of the Big Bang Model
Many people have in their minds an image of the big bang that goes something like this: Billions of years ago, the universe was infinitely small, and then it exploded. That description is wrong. To explain why it’s wrong, let’s talk about how we know what was happening billions of years ago.
We can describe the universe 13.8 billion years ago by observing what it’s doing today and using the laws of physics to figure out what it must have been doing before that. For example, suppose I take a video of a flowerpot falling past my office window. I may only see the flowerpot for a brief time, but I can observe how fast it moves as it goes by my window. And knowing the laws of Newtonian physics, I can then figure out how long it was falling before it reached the window.
Similarly, when we look at how all the galaxies are moving today, we can figure out where they were and how they were moving a billion years ago, 10 billion years ago, or more. And the most important thing we notice when we measure the motion of galaxies today is that they are almost all flying away from each other. We describe that motion by saying, “the universe is expanding”.
How Is the Universe Expanding?
So now, let’s clarify what we mean by the word expanding in this context. The room you’re sitting in right now is not getting any bigger. The Earth is not getting any bigger. Similarly, our Sun, our solar system, and our entire galaxy are not getting any bigger. Other galaxies aren’t getting bigger either.
What is happening is that, in general, the different galaxies in the universe are moving away from each other. We express this fact by saying that the universe is becoming “less dense”, meaning the average distance between galaxies is increasing.
The universe 5 billion years ago was denser than the universe today—meaning galaxies were closer to each other than they are now. And the universe 10 billion years ago was denser than that. About 13.5 billion years ago, there were no galaxies. All of space was filled with an extremely dense gas. At even earlier times, that gas was even denser.
Common Questions about the Big Bang Model
The big bang model describes the events after the big bang itself, telling us how the universe evolved from 13.8 billion years ago until today. According to the Big Bang model, the universe was unimaginably hot after the big bang. The particles that would later form the universe’s building blocks became all separate from each other.
We can describe the universe at that time thanks to the big bang model. To do this, we need to observe what’s going on now in the universe, and based on our observation and our knowledge of the laws of physics, we are able to predict how the universe was acting before that.
The big bang model describes the expansion of the universe as the universe becoming less dense. The expansion of the universe does not mean that galaxies or planets, or other things are themselves expanding. Rather, the average distances between galaxies are increasing.
