The standard cosmological model describes the Big Bang as an extremely dense and hot period that the Universe would have gone through about 13.8 billion years ago, to then lead to the process of inflation and expansion that the Universe we observe today. If for a long time the mathematics of general relativity ruled out any possibility of a pre-Big Bang, for several years cosmologists have known that Einstein's theory is incomplete and have been developing theoretical models proposing hypotheses concerning pre-Big Bang physics.
The first thing to understand is what the Big Bang really is. “The Big Bang is a moment in time, not a point in space says Sean Carroll, a theoretical physicist at the California Institute of Technology. So it's possible the Big Bang universe was tiny or infinitely large, Carroll said, because there's no way to go back in time to what we can't even see today. All we really know is that it was very, very dense, and very quickly it became less dense.
“No matter where you are in the universe, if you go back 14 billion years, you get to this point where it was extremely hot, dense and followed by rapid expansion adds Carroll. No one knew exactly what was happening in the Universe until 1 second after the Big Bang, when the universe cooled enough for protons and neutrons to collide and bond.
Many cosmologists believe that the Universe went through a process of exponential expansion, called inflation, during that first second. This would have smoothed the fabric of spacetime and could explain why matter is so evenly distributed in the Universe today.
It's possible that before the Big Bang, the Universe was an endless expanse of ultra-hot, dense material, persisting in a stable state until, for some reason, the Big Bang happened. This extradense universe may have been governed by quantum mechanics, according to Carroll. The Big Bang would therefore have represented the moment when classical physics took over as the main driver of the evolution of the Universe.
For Stephen Hawking, this moment is unimportant:before the Big Bang, the events are immeasurable, and therefore indefinite. Hawking called this the "limitless proposition":time and space are finite, but they have no boundaries, starting or ending points, in the same way that planet Earth is finite, but n has no edge. “Events before the Big Bang having no observational consequences, we might as well discard them from the theory and say that time began at the Big Bang said Hawking in 2018.
Or maybe there was something else before the Big Bang worthy of physicists' attention. One hypothesis is that the Big Bang is not the beginning of time, but rather a moment of symmetry. In this idea, before the Big Bang, there was another universe, identical to this one, but with increasing entropy towards the past rather than the future.
Increasing entropy, or increasing "disorder" in a system, is essentially the arrow of time, so in this mirror universe time would be the opposite of time in the modern universe and our universe would be in the past. Proponents of this theory also suggest that other properties of the Universe would be reversed in this mirror universe. For example, physicist David Sloan, a physicist at the University of Oxford, suggests that asymmetries in molecules and ions (called chirality) would be in opposite orientations to what they are in our universe.
A related theory holds that the Big Bang wasn't the start of everything, but rather a moment in time when the Universe went from a period of contraction to a period of expansion. This notion of the "Big Bounce" suggests that there could be infinite Big Bangs as the Universe expands, contracts and expands again. The problem with these ideas, according to Carroll, is that there is no explanation for why or how an expanding universe would contract and revert to a low-entropy state.
For brane cosmological theories, generally derived from string theory, the Big Bang, then our universe, would have appeared following a brane collision in the bulk of a universe with more than four dimensions. This model of the universe, called the ekpyrotic universe, implies as a natural hypothesis the existence of a multiverse. Other theories like eternal inflation suggest that universe bubbles would be created in a bigger universe in infinite inflation.
Carroll and his colleague Jennifer Chen have their own pre-Big Bang vision. In 2004, physicists suggested that perhaps the universe as we know it is the offspring of a parent universe from which a bit of spacetime broke off. “It’s like a decaying radioactive nucleus “, explains Carroll:when a nucleus disintegrates, it emits an alpha or beta particle.
The parent universe could do the same thing, except instead of particles, it emits "child universes", perhaps ad infinitum. This generation of baby universes would occur through quantum fluctuation in the parent universe. These newborn universes are "literally parallel universes", and do not interact or influence each other.
