Today, let’s take a walk on the wild side and suppose, for the sake of argument, that our universe isn’t the only one out there. Consider that there are many other universes, perhaps infinitely many. The entirety of these universes, including our own, is what cosmologists call the multiverse. More like a fable than a scientific hypothesis, this conceptual naughty inspires some while irritating others.
How far can we push the theories of physics?
The controversy began in the 1980s. Two physicists, Andre Lind of Stanford University and Alex Vilenkin at Tufts University, have independently suggested that if the universe underwent very rapid expansion early in its existence – we call this an inflationary expansion – it wouldn’t be our only universe.
It is assumed that this inflationary phase of growth occurred at a trillionth of a trillionth of a trillionth of a second after the beginning of time. That’s about 10-36 Seconds after the “explosion” when the clock describing the expansion of our universe began to tick. You might ask, “How do these scientists feel comfortable talking about ridiculously small times? Wasn’t the universe ridiculously dense in those times?”
Well, the truth is that we don’t yet have a theory that describes the physics under these conditions. What we have is an extrapolation based on what we know today. This isn’t ideal, but due to our lack of empirical data, it’s the only place we can start. Without data, we need to push our theories as far as we consider reasonable. Of course, what is reasonable for some theorists will not be for others. And this is where things get interesting.
The assumption here is that we can apply essentially the same physics to energies about a thousand trillion times greater than those we can examine at the Large Hadron Collider, the giant accelerator at the European Organization for Nuclear Research in Switzerland. And even if we can’t apply exactly the same physics, we can at least apply physics with similar actors.
Intermittent water, quantum fields
In high energy physics, all characters are fields. Fields here mean disturbances that fill the void and may or may not change over time. The initial image of the field is the image of the water filling the pond. Water is ubiquitous in a pond, with certain properties taking values at each point: temperature, pressure, and salinity, for example. Fields have an excitation that we call particles. The electron field contains the electron as an excitation. The Higgs field contains the Higgs boson. In this simple image, we can visualize the particles as ripples of water spreading along the surface of the pond. This is not a perfect picture, but it helps the imagination.
The most famous protagonist driving inflationary expansion is the scalar field – an entity with properties inspired by the Higgs boson, discovered at the Large Hadron Collider in July 2012.
Subscribe to get unexpected, surprising and touching stories delivered to your inbox every Thursday
We don’t know if there are scalar fields in cosmic infancy, but it is reasonable to assume that they exist. Without them, we’d be terribly stuck trying to visualize what happened. As mentioned above, when we don’t have data, the best we can do is build reasonable hypotheses that we hope future experiments will test.
To see how we use the scalar field to model inflation, imagine a ball rolling downhill. As long as the ball is at a height above the bottom of the hill, it will roll down. It has stored energy. At the bottom, we set its energy to zero. We do the same with the standard domain. As long as it is shifted below the bottom line, it will fill the universe with its energy. In large enough areas, this energy induces the rapid expansion of space which is a sign of inflation.
Lind and Flinken added quantum physics to this picture. In the quantum world, everything is tense. Everything vibrates endlessly. This is the origin of quantum uncertainty, an idea that defies common sense. So, as the field rolls down the slopes, it is also experiencing these quantum jumps, which can push it down or more. It is as if the waves in the pond are irregularly creating peaks and valleys. Intermittent water, these quantum fields.
Here comes the torsion: when a large enough space is filled with a given energy field, it will expand at a rate related to that energy. Think about the temperature of the water in the pool. Different areas of the field space will have different elevations, just as different areas of a pond can contain water at different temperatures. The result of cosmology is a large number of insanely inflated regions of space, each expanding at its own rate. Very quickly, the universe would consist of countless inflating regions growing, unaware of their surroundings. The universe turns into a multiverse. Even within each region, quantitative fluctuations may drive a sub-region to inflate. The image, then, is one of an eternally proliferating universe, filled with bubbles within bubbles. Our country will only be one of them – one bubble in a frothy multiverse.
Is the multiverse testable?
This is very inspiring. But is this science? For a hypothesis to be scientific, it must be testable. Can you test the multiverse? The answer, strictly speaking, is no. Each of these inflated regions – or inflated ones, where there can also be failed universes – lies outside our cosmic horizon, the region that determines the distance that light has traveled since the beginning of time. As such, we cannot see these cosmoids, nor receive any signals from them. The best we can hope for is to find a sign that a neighboring universe has hit our space in the past. If this happens, we’ll see some specific patterns in the sky — more precisely, in the radiation left after hydrogen atoms formed about 400,000 years after the Big Bang. So far, no such signal has been found. And quite frankly, the chances of finding one are remote.
Thus we are stuck with a plausible scientific idea that seems untestable. Even if we find evidence of inflation, it won’t necessarily support an inflationary multiverse. what shall we do?
Various different types in the multiverse
The multiverse suggests another component – the possibility that physics differs in different universes. Things get ambiguous here, because there are two kinds of “different” to describe them. The first is different values of the constants of nature (such as the charge of an electron or the force of gravity), while the second raises the possibility that there are completely different laws of nature.
In order to house life as we know it, our world must obey a series of very strict requirements. Small deviations in the values of the constants of nature are not tolerated. But the multiverse raises the question of nature, or how common our universe and its laws are among the many universes that belong to the multiverse. Are we the exception or follow the rule?
The problem is that we have no way of knowing. To find out if we’re in common, we need to know something about other universes and the kinds of physics in them. But we don’t. We also don’t know how many universes there are, and that makes it very difficult to estimate how widespread we are. To make matters worse, if there are an infinite number of cosmoids, we can’t say anything at all. Inductive reasoning is useless here. Infinity makes us tangled in knots. When everything is possible, nothing stands out and nothing is learned.
This is why some physicists worry about the multiverse to the point of loathing. There is nothing more important to science than its ability to prove ideas wrong. If we lose that, we undermine the very structure of the scientific method.