By Gary Felder, Smith College
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. In order to understand why, we need to delve into how we know what was happening billions of years ago.
The Laws of Physics
One can describe the universe 13.8 billion years ago by observing what it’s doing today and by using the laws of physics to figure out what it must have been doing before that.
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.
The ‘Expanding’ Universe
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’.
Here the word expanding does not mean that our Sun, our solar system, our entire galaxy, or other galaxies are getting bigger. It means that the different galaxies in the universe are moving away from each other. This fact is expressed by saying that the universe is becoming ‘less dense’, meaning the average distance between galaxies is increasing.
Thus, the universe 5 billion years ago was more dense than the universe today as the galaxies were closer to each other then; the universe 10 billion years ago was even denser than that.
Planck Density
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. Projecting backwards, we conclude that 13.8 billion years ago, the universe was at what’s called Planck density, a higher density than we have ever observed.
And before that? We don’t know. The basic laws of physics we know are quantum mechanics and general relativity, and they can describe the behavior of matter in every condition we have ever observed. But when we try to apply those physical laws to describe how matter behaves at or above Planck density, we get contradictory or meaningless results.
The laws of physics we know don’t work in those conditions, and we don’t know the right set of laws to describe matter at those densities. Thus, understandably, until we know how matter behaves at or above Planck density, we can’t say what happened before that critical moment 13.8 billion years ago.
The ignorance about physics above Planck density is the key to what’s wrong with describing the big bang as an explosion from a state of infinite density.
One can sometimes find timelines of the universe that show the big bang as a moment when the universe was infinitely small and show the moment of Planck density occurring a fraction of a second later. But those timelines—in fact, any timeline that shows events before Planck density—are based on equations that we know are incorrect during that period.
What Big Bang Means
It’s more accurate to say that the moment of Planck density is the earliest moment we can talk about with our current theories and call that moment the big bang. So we need to keep in mind three things about what the phrase big bang does and doesn’t mean.
Firstly, the big bang is not a moment when the universe had zero size or infinite density. Rather, the big bang is a moment of an incredibly high but finite density that we call Planck density. Secondly, the big bang did not happen at some special point in space. Rather, the big bang happened everywhere in the known universe at once. Lastly, and perhaps most importantly, the big bang is not necessarily the beginning of the universe. The big bang is, rather, the earliest moment we can currently talk about.
Contrary to popular belief, the big bang model is not a theory of how the universe began. Science doesn’t currently tell us how or why the universe came to exist, or what caused it to start expanding.
The big bang model is a theory of how the universe has evolved over the last 13.8 billion years. That history is well understood and has been verified by many lines of evidence.
This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early Universe. Watch it now, on Wondrium.
Before the Big Bang
What might have happened before the big bang? Some physicists who’ve studied this issue believe that literally nothing happened before that moment; the entire universe popped into existence at Planck density. If that’s true, then the big bang actually was the beginning of the universe.
But other physicists believe that the universe existed for a long time at Planck density, not expanding, and then started to expand at the moment we call the big bang.
And still other physicists believe that the universe before the big bang was at a lower density, but it was contracting and increasing its density. Then, when it reached Planck density, it ‘bounced’ and began to expand. In that view, the big bang was the moment when contraction changed to expansion.
Whatever did or didn’t happen before then, the big bang was the moment when the universe was at Planck density, and that’s the earliest moment we can talk about using our currently known physics. The big bang model, thus, is the history of everything that has happened since that moment.
Common Questions about Big Bang: The Very Beginning
The word expanding, in the term ‘expanding universe‘, refers to the fact that the different galaxies in the universe are moving away from each other. The universe in this context is becoming ‘less dense’, meaning the average distance between galaxies is increasing.
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. Projecting backwards, we conclude that 13.8 billion years ago, the universe was at what’s called Planck density, a higher density than we have ever observed.
Contrary to popular belief, the big bang model is not a theory of how the universe began. The big bang model is a theory of how the universe has evolved over the last 13.8 billion years.
