Planck Density: Why We Can’t Know Everything about the Universe

From the Lecture Series: The Big Bang and Beyond: Exploring the Early Universe

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.

The Problem with Creating Planck Density

When we try to apply 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. Well, that’s the sort of problem science is good at solving; we just need to manufacture some Planck density matter and see how it behaves, right?

Well, here’s the problem: Planck density is about 1093 grams per cubic centimeter, or roughly what you would get if you crammed a billion galaxies into a space the size of an atomic nucleus. There is no chance of us producing this density in a lab in the foreseeable future. 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.

That 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. You 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. It’s, therefore, 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.

This article comes directly from content in the video series The Big Bang and Beyond: Exploring the Early UniverseWatch it now, on Wondrium.

Big Bang: What the Phrase Actually Means

So we need to keep in mind three things about what the phrase big bang does and doesn’t mean. 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.

The big bang did not happen at some special point in space. Rather, the big bang happened everywhere in the known universe at once. 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.

What Happened 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 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 is the history of everything that has happened since that moment.

Q: Why can’t we describe how matter behaves at or above Planck density?

As we understand them now, the laws of physics don’t work at or above Planck density. When we use equations to describe matter at such levels of density, the results are contradictory or even meaningless. Our only option would be to observe matter with such density to see how it behaves.

Q: Why don’t we know how matter behaves when it is at or above Planck density?

This is because our understanding of physics and our equations don’t work when applied to matter at Planck density. We should first understand the laws of physics when matter has such density levels. So we need to observe matter at such densities, which is currently impossible in the foreseeable future since matter with such density is equivalent to cramming a billion galaxies into the size of an atomic nucleus.

Q: What do scientists think happened before the big bang?

We cannot know what happened before the big bang, but some scientists have theories about what was before. Some believe that the entire universe came to existence with Planck density, while others believe that the universe existed at Planck density for a long time before the big bang. Also, some scientists believe that the universe was at a density lower than Planck density but contracted and expanded its density until it finally “bounced” and began to expand.