We know we live in an expanding universe. This means that the entire universe is getting bigger with each passing day. It also means that in the past our universe was smaller than it is today. Rewind that tape enough, and physics suggests that our universe was once an infinitely small, infinitely dense point — a singularity.
Most physicists believe that this point developed in the big Bangbut because all known physics collapses under the extreme conditions that prevailed in the infancy of our universe, it is difficult to say for sure what happened in those first moments of the universe.
To go back in time
For most of the history of the universe, it was dotted with celestial objects similar to those present now – 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 its average temperature was about 10,000 kelvins. It was so hot and so dense that it was a plasma, a state of matter where atoms are torn 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 you 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.
If we look back even earlier than that, however, things get really sketchy.
When we try to make sense of the universe when it was less than a second old, we have no physical theory that can cope with the incredibly high temperatures and pressures the universe has been through. All of our theories of physics are crumbling and we don’t understand how particles, forces and fields work under these conditions.
Physicists can plot the growth of the cosmos using of Einstein general theory of relativitywhich links the contents of the cosmos to its history of expansion.
But Einstein’s theory contains a fatal flaw. If we follow general relativity to its ultimate conclusion, then at some finite time in the past our entire universe was crammed into one infinitely dense point. This is called 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 Big Bang singularity does not tell us that the universe began there. Instead, he tells us that general relativity itself has collapsed and lost its predictive and explanatory power.
Physicists have long known that general relativity is incomplete. It cannot explain high-strength 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 the ages
Unfortunately, we currently lack such physics. We have several candidates for quantum gravity, like string theory and looping quantum gravity, but these theories have not been fully developed, let alone tested.
But if either of these theories is correct, they can tell us some interesting things about the early universe.
In the case of loop quantum gravity, the singularity is replaced by a piece of spacetime of finite size. In string theory, on the other hand, our universe comes from a “landscape” of possible universes. It’s also possible that our Big Bang only exists as one of an infinite series of universes, endlessly multiplying in a multiverse. Only new advances in theoretical physics will help unravel the darkness of these possible ideas.
But there is another problem: we can never find out what caused the Big Bang. In its first moments, even our very conceptions of time and space crumble. On such extreme scales, normal, everyday concepts like “beginning” and “before” may not even make sense.