If you thought everything was limited to what we found beyond the cosmic horizon, get ready to change your mind.
“It's hard to build inflation models that don't lead to a multiverse. This is not impossible, so I am sure more research is needed. But most inflation models do lead to a multiverse, and the evidence for inflation will push us towards serious acceptance [of multiple universes],”said Alan Guth, the American physicist and cosmologist who first proposed the idea of inflation, or cosmic expansion, once.
Imagine that the universe that we observe - from end to end - is just a drop in the cosmic ocean. That beyond what we see, there is more space, more galaxies, most of all, countless billions of light-years farther than we can ever reach. And as vast as the universe can be, just as countless can be the number of similar universes - some larger and older, some smaller and younger - scattered throughout space-time. And just as quickly as these universes expand, the spacetime containing them expands even faster, pulling them further apart and ensuring that no two universes will ever meet. Sounds like science fiction? This is the scientific idea of multiverse, or multiple universes. But if the scientific view that we take today is correct,this idea will not only be adequate, but also an inevitable consequence of our fundamental laws, says physicist Ethan Siegel.
The idea of multiple universes is rooted in the physics needed to describe the universe we see and inhabit today. Throughout the sky, we see stars and galaxies clustered in a large cosmic web. But the further into space we look, the further back in time we get. The farther away the galaxies are, the younger they are and, therefore, less developed. Their stars have fewer heavy elements, they appear smaller because there are fewer mergers, more spirals, and fewer ellipses (because the latter take time), and so on. If we move to the limits of the visible, we will find the very first stars in the universe, and beyond them - a region of darkness in which there is only one light: the afterglow of the Big Bang.
The Big Bang itself - which happened 13.8 billion years ago - was not the beginning of space and time, but rather the beginning of our observable universe. Before that, there was an era known as cosmic inflation, when space itself was expanding exponentially, filled with the energy inherent in the fabric of spacetime. Cosmic inflation is itself an example of a theory that came and replaced the one that came before it:
“It was consistent with all the successes of the Big Bang theory and covered the entire modern cosmology.
“She explained a number of problems that the Big Bang could not explain, including why the universe was everywhere the same temperature, why it is spatially flat, and why there were no high-energy relics like magnetic monopoles left.
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“And she made many new predictions that could be observationally tested, most of which were confirmed.
There was, however, one consequence, which the inflation theory predicted. We don't know if we can confirm it or not: multiple universes.
Inflation leads to exponential expansion of space, which can very quickly result in the fact that any pre-existing curved space will appear flat.
Inflation causes space to expand exponentially. That is, you take anything that existed before the Big Bang and becomes much, much, much more than it was. So far, this suits us: it explains how we got a homogeneous and huge universe. When inflation ends, the universe is filled with matter and radiation, the appearance of which we see as a red-hot Big Bang. And this is where the oddities begin. For inflation to end, no matter what quantum field is responsible for it, it needs to go from a high-energy unstable state to a low-energy and stable state. This transition and "slide" down into the valley is what brings inflation to an end and causes the Big Bang.
But no matter which field is responsible for inflation, as in all other areas obeying the laws of physics, it must be inherently a quantum field. Like all quantum fields, it is described by a wave function, with the probability of wave propagation over time. If the magnitude of the field slowly rolls into the valley, the quantum spread of the wave function will be faster than the roll-off, meaning that it is possible - even likely - inflation will gradually lead to the Big Bang.
If inflation were a quantum field, the field's magnitude would diverge over time, with different regions of space taking on different realizations of the field's value. In many regions, the value of the field will fall to the bottom of the valley, ending inflation, but in many others, inflation will continue as long as you like in the future.
Since space expands at an exponential rate during inflation, this means that exponentially more regions of space are created over time. In some areas, inflation will end: where the field slides into a valley. But in others, inflation will continue, creating more and more space around each area where inflation ends. The inflation rate is much faster than even the maximum expansion rate of the Universe filled with matter and energy, therefore, in the shortest possible time, the inflation areas cover everything. According to the mechanisms that provide us with enough inflation to create the universe that we see, our region of space where inflation has ended is surrounded by much more other regions - where inflation continues or did not end immediately.
Inflation continues indefinitely, despite the areas where it ended
This is where the phenomenon known as eternal inflation occurs. Where inflation ends, the Big Bang is born and the universe - like the one we see - is similar to our own. But where inflation does not end, more inflationary space is born, which gives rise to other regions in which there will be big bangs, separate from ours, and other regions in which inflation starts.
How big our universe is, this is just a small fraction of everything that really is
This picture of vast universes, much larger than the meager part that we are able to observe, constantly created by the swelling space, is the multiverse. It is important to understand that the multiverse is not a scientific theory in and of itself. It does not make predictions and observable phenomena that we can reach. No, the multiverse itself is a theoretical prediction that follows from the laws of physics that we have deduced to date. Perhaps this is even an inevitable consequence of these laws: if you take an inflationary universe governed by quantum physics, you get this.
Perhaps our understanding of this state before the Big Bang is wrong, and that our understanding of inflation is completely wrong. In such a case, the existence of multiple universes will not be the ultimate consequence. But the prediction of eternal inflation, containing countless pocket universes, is a direct consequence of our best theories, if correct.
What is the multiverse, then? It can go beyond physics and become the first physically motivated "metaphysics" we have ever encountered. For the first time, we understand the limits of what our universe can teach us. Until then, we can predict, but we cannot confirm or deny the fact that our universe is only one small part of a larger realm: the multiverse.
Ilya Khel