We know that we live in an expanding universe. That means the whole universe is getting bigger every day. It also means that our universe was smaller in the past than it is now. Rewind that tape far enough and physics suggests our universe was once an infinitely small, infinitely dense point — a singularity.
Most physicists believe that this point has expanded into the big bangbut because all known physics collapses in the extreme conditions that prevailed in the infancy of our universe, it is difficult to say for sure what happened in those earliest moments of the universe.
Go back in time
For most of the universe’s history, it was dotted with similar celestial bodies to those present today – they were just closer together.
For example, when our universe was less than 380,000 years old, the volume of the universe was about a million times smaller than it is today, and it had an average temperature of about 10,000 Kelvin. It was so hot and dense it was a plasma, a situation where atoms are torn apart into protons, neutrons and electrons. However, we encounter plasmas in many other situations in space and on Earth, so we have a pretty good understanding of how they work.
But the further back we go, the more complex the physics becomes. When the universe was only a dozen minutes old, it was an intense soup of protons, neutrons, and electrons, still governed by the same physics we use to understand nuclear bombs and nuclear reactors.
However, if we look back even earlier, things get really sketchy.
If we try to understand the universe when it was less than a second old, we don’t have a physical theory that can handle the insanely high temperatures and pressures the universe experienced. All our physical theories are broken and we don’t understand how particles, forces and fields work under those conditions.
The genesis of the singularity
But Einstein’s theory contains a fatal flaw. If we follow general relativity to its final conclusion, our entire universe was crammed into a single, infinitely dense point at some finite time in the past. This is known as the Big Bang singularity.
The singularity is often presented as the “beginning” of the universe: but it is not a beginning at all.
Mathematically, the singularity at the Big Bang doesn’t tell us that the universe began there. Instead, it tells us that general relativity itself has broken down and lost its predictive and explanatory power.
Physicists have long known that general relativity is incomplete. It cannot explain the large-scale or small-scale gravity, known as quantum gravity. In other words, to fully understand the earliest moments of the universe, we need new physics.
A question for all ages
Unfortunately, we currently lack such physics. We have several candidates for quantum gravity, such as string theory and loop quantum gravity, but these theories have not yet been fully developed, much less tested.
But if any of these theories are correct, they could tell us interesting things about the early universe.
In the case of loop quantum gravity, the singularity is replaced by a finite chunk of space-time. In string theory, on the other hand, our universe emerges from a “landscape” of possible universes. It is also possible that our Big Bang exists as just one of an infinite series of universes, multiplying endlessly in a multiverse. Only further advances in theoretical physics will help clear up the murkiness of these possible ideas.
But there’s another problem: maybe it is never know what caused the Big Bang. In the earliest moments, even our conceptions of time and space disintegrate. At such extreme scales, normal, everyday concepts like “beginning” and “before” may not even make sense.