If you thought that everything is limited to what we found behind the cosmic horizon, prepare to change your mind.
“It is difficult to build inflation models that do not lead to a multiverse. This is not impossible, so I am confident of the need for additional research. But most inflation models do lead to a multiverse, and evidence of inflation will push us toward serious adoption of [multiple universes], “said Alan Guth, the American physicist and cosmologist, who first proposed the idea of inflation, or cosmic expansion.
Imagine that the Universe that we observe, from end to end, is just a drop in the cosmic ocean. What is beyond what we see is more space, more galaxies, most of all, countless billions of light years further than we can ever reach. And how vast the Universe can be, the number of universes similar to it can be as innumerable – some more and older, some smaller and younger – scattered throughout space-time. And just as quickly as these universes expand, space-time containing them expands even faster, takes them further away from each other, and ensures that no two universes will ever meet. It looks like science fiction? This is the scientific idea of the multiverse, or multiple universes. But if the scientific view that we are taking today is true, this idea will not only be an adequate, but inevitable consequence of our fundamental laws, believes physicist Ethan Siegel.
The idea of multiple universes is rooted in the physics needed to describe the universe, which we see and in which we live today. Throughout the sky, we observe stars and galaxies grouped in a large cosmic web. But the further we look into space, the further back in time we get. The farther away galaxies, the younger they are and, consequently, less developed. There are fewer heavy elements in their stars, they seem less, because there are fewer mergers, more spirals and fewer ellipses (because the latter takes time), and so on. If we move to the limits of the visible, we will find the very first stars in the universe, and behind them – the 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 expanded exponentially, filled with energy inherent in the fabric of space-time. Cosmic inflation is in itself an example of a theory that came and replaced the one that was before it:
It was consistent with all the successes of the Big Bang theory and covered all modern cosmology.
She explained a number of problems that the Big Bang could not explain, including why the universe was everywhere of the same temperature, why it was spatially flat and why there were no high-energy relics like magnetic monopoles.
And she made many new forecasts, which could be tested observationally, most of which were confirmed.
There was, however, one consequence that the theory of inflation predicted. We do not know whether we can confirm it or not: multiple universes.
Inflation leads to the fact that space expands exponentially. That is, anything that existed before the Big Bang is taken, and it becomes much, much, much more than it was. So far, this is okay with 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 observe as a red-hot Big Bang. And here, strange things begin. To stop inflation, regardless of what quantum field it is responsible for, it needs to move from a high-energy unstable state to a low-energy and stable one. This transition and “rolling” down into the valley – that’s what brings inflation to an end and causes the Big Bang.
But regardless of which field is responsible for inflation, as in all other areas that obey the laws of physics, it must be a quantum field by its very nature. Like all quantum fields, it is described by a wave function, with the probability of wave scattering with time. If the magnitude of the field slowly slides into the valley, the quantum spreading of the wave function will be faster than rolling, meaning that, perhaps – even likely – inflation will gradually lead to the Big Bang.
If inflation was a quantum field, the magnitude of the field will scatter with time, and different areas of space will assume different realizations of the field 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 much as necessary in the future
As space expands at an exponential rate during inflation, this means that an exponentially larger number of areas of space are created over time. In some areas, inflation will end: where the field slides into the valley. But in others, inflation will continue, creating more and more space around each area where inflation ends. The rate of inflation is much faster than even the maximum rate of expansion filled with matter and the energy of the universe, therefore in the shortest possible time, the sectors of inflation capture everything. According to the mechanisms that provide us with sufficient inflation to create the universe we see, our area of space where inflation has ended is surrounded by much more other areas – where inflation continues or did not end immediately.
It is here that a phenomenon known as eternal inflation occurs. Where inflation ends, the Big Bang and the Universe, like the one we observe, are similar to our own. But where inflation does not end, more inflationary space is born, which gives growth to other regions in which there will be large explosions separated from ours and other regions in which inflation begins.
This picture of the 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 itself. It does not make predictions and observable phenomena, to which we can reach. No, the multiverse itself is a theoretical forecast, which follows from the laws of physics, which we derived to date. Perhaps, this is even an inevitable consequence of these laws: if we take an inflationary universe governed by quantum physics, then this is obtained.
Perhaps our understanding of this state, which was before the Big Bang, is wrong, and that our ideas about inflation are completely wrong. In this case, the existence of multiple universes will not be the final consequence. But the prediction of eternal inflation, containing an infinite number of pocket universes, is a direct consequence of our best theories, if they are true.
What is a multiverse, in that case? It can go beyond physics and become the first physical motivated “metaphysics” that we have ever encountered. For the first time, we understand the boundaries of what our Universe can teach us. Until then, we can predict, but we will not be able to confirm or deny the fact that our universe is only one small part of a larger kingdom: the multiverse.