Today, let’s walk through the wilderness and assume, to argue that our universe is not the only one that exists. Consider that there are many other universes, perhaps an infinite number. The completeness of these universes, including our own, is what cosmologists call the Multiverse. It sounds more like a myth than a scientific hypothesis, and this conceptual troublemaker inspires some while others outrage.
How far can we move physical theories?
The controversy began in the 1980s. Two physicists, Andrei Linde of Stanford University and Alex Vilenkin of Tufts University, have independently suggested that if the universe had undergone a very rapid expansion at the beginning of its existence – we call it an inflationary expansion – then our universe would not be the only one.
This inflationary phase of growth probably took place a trillionth of a trillionth of a trillionth of a second one after the start of time. That’s about 10-36 seconds after the “bang”, when the clock describing the expansion of our universe began to tick. You may ask, “How come these scientists feel comfortable talking about times so ridiculously small? Wasn’t the universe ridiculously dense at the time? ”
The truth is that we do not yet have a theory that describes physics under these conditions. What we have are extrapolations based on what we know today. This is not ideal, but due to the lack of experimental data, this is the only place we can start. Without data, we need to move our theories as far as we see fit. Of course, what is reasonable for some theorists will not be for others. And here it starts to get interesting.
Here it is assumed that we can apply essentially the same physics to energies that are about a thousand trillion times higher than those we can probe at the Great Hadron Collider, a giant accelerator located at the European Organization for Nuclear Research in Switzerland. And while we can’t apply exactly the same physics, we can at least apply physics to similar actors.
Turbulent waters, quantum fields
In high energy physics, all figures are fields. The fields here indicate faults that fill the space and may or may not change over time. The rough image of the field is an image of the water filling the pond. Water is everywhere in the pond and has certain properties that take on values at every point: for example, temperature, pressure and salinity. Arrays have excitations that we call particles. The electron field has an electron as excitation. The Higgs field has a Higgs boson. In this simple picture, we could imagine particles as ripples of water spreading along the surface of a pond. It’s not a perfect picture, but it helps the imagination.
The most popular protagonist who controls inflation expansion is the scalar field – an entity with properties inspired by the Higgs boson, which was discovered in the Great Hadron Collider in July 2012.
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We do not know whether a scalar field existed in cosmic childhood, but it is reasonable to assume that it existed. Without them, we would be stuck trying to imagine what had happened. As mentioned above, when we don’t have data, the best we can do is make reasonable hypotheses that will hopefully verify future experiments.
To see how we use a scalar field to model inflation, imagine a ball rolling downhill. As long as the ball is above the bottom of the hill, it will roll down. It has stored energy. Below we set its energy to zero. We will do the same with the scalar field. As long as it is pushed out of its minimum, it will fill the universe with its energy. In sufficiently large areas, this energy encourages the rapid expansion of space, which is a sign of inflation.
Linde and Vilenkin added quantum physics to this picture. In the quantum world, everything is nervous; everything vibrates endlessly. This is the root of quantum uncertainty, an idea that goes beyond common sense. So as the field rolls downhill, it also experiences these quantum leaps that can kick it further down or up. It is as if the waves in the pond irregularly form ridges and valleys. Turbulent waters, these quantum fields.
Here comes the turning point: When a sufficiently large area of space is filled with a field of a certain energy, it will expand at a speed corresponding to that energy. Think about the water temperature in the pond. Different areas of space will have fields at different altitudes, just as different areas of a pond may have water at different temperatures. The result for cosmology is a number of insanely inflating areas of the universe, each expanding at its own pace. The universe would very quickly consist of innumerable inflated areas that grow without being aware of their surroundings. The universe is transforming into a multiverse. Even within each region, quantum fluctuations can cause the subregion to inflate. The image is thus one of the eternally replicating cosmos, filled with bubbles in bubbles. Ours would be just one of them – the only bubble in the foamy Multiverse.
Is the multiverse testable?
That’s wildly inspiring. But is it science? For a hypothesis to be scientific, it must be testable. Can you test the Multiverse? The answer in the strict sense is no. Each of these inflating regions — or shrinking, because there could also be failed universes — is beyond our cosmic horizon, an area that defines how far light has come since the beginning of time. As such, we cannot see or receive these cosmoids. The best we can hope for is to find a sign that one of our neighboring universes has destroyed our own space in the past. If that happened, we would see some specific patterns in the sky – more precisely, in the radiation left after the formation of hydrogen atoms about 400,000 years after the Big Bang. No such signal has been found yet. The chances of finding one are, frankly, slim.
So we are stuck with a credible scientific idea that seems untestable. Even if we find evidence for inflation, it would not necessarily support the inflationary Multiverse. what should we do?
Different species different in a multiverse
The multiverse suggests another component – the possibility that physics is different in different universes. Things are pretty vague here because there are two types of “different” ones that can be described. The first is the different values of natural constants (such as electron charge or the force of gravity), while the second increases the possibility that there are completely different laws of nature.
In order to hide life as we know it, our universe must follow a number of very strict requirements. Small deviations are not tolerated in the values of natural constants. But the Multiverse raises the question of nature or how common our universe and its laws are among the innumerable universes belonging to the Multiverse. Are we an exception or do we follow the rule?
The problem is that we have no way of saying it. To know if we are common, we need to know something about other universes and the kinds of physics they have. But we don’t. We don’t even know how many universes there are, so it’s very difficult to estimate how ordinary we are. To make matters worse, if there are an infinite number of cosmoids, we can’t say anything at all. Inductive thinking is useless here. Infinity entangles us in knots. When everything is possible, nothing stands out and nothing is learned.
That’s why some physicists fear the Multiverse for disgusting them. There is nothing more important to science than its ability to prove that thoughts are wrong. If we lose it, we will undermine the very structure of the scientific method.
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