Pedagoguery

The concept of the multiverse is a useful concept to explain many features of our universe. It can be used to explain how our universe seems so fine tuned to support life, especially since many of our fundamental scientific theories do not constrain the physical constants. Given that, however, how realistic is the notion? It depends on the type of multiverse you are considering.

The observable universe currently has a radius of 42 billion light years. That is the extent to which we can get any information because of the speed of light. While the exact distance changes as the universe ages (and may end up shrinking if the expansion of the universe accelerates) anything beyond the light travel distance since the beginning of the universe is forever beyond our knowledge. As such, it is essentially another universe. This is what is referred to as the level 1 multiverse: an assumption that the observable universe is simply a volume in a larger structure. The physical laws are probably the same throughout multiverse, but speculation on that score becomes less reliable as the distance between universes increases.

The level 2 multiverse is a much different, and much more speculative notion. The idea first arose from the idea of chaotic inflation. In such a scenario, the vast bulk of the universe is constantly expanding exponentially, and smaller universes are constantly seeding out of it. Each one would have its own unique physical laws. A level 1 multiverse can easily be embedded in a level 2 multiverse.

There are several arguments given in favor of a level 2 multiverse. The first is that space has no end. While few would dispute this in concept, it does potentially lead to speculation that cannot be supported scientifically. After all, one of the key hallmarks of a scientific theory is that it must be falsifiable – in other words it must make specific predictions that may run contrary to observational or experimental evidence. When speculating about multiverses, very little can be falsifiable.

Another claim made is that known physics predicts other domains. This is the case with chaotic inflation. Inflation is based on a scalar field called the inflaton field. (Scalar fields have only a value at all points in space, unlike most fields of which we are aware, which have both a value and a direction.) While the existence of such a field is on firm theoretical ground, the exact nature of the field is not, so it is quite speculative to say what it predicts.

Another claim is that the theory that predicts the multiverse has passed a key observational test. This specifically refers to the microwave background radiation. The detailed patterns in it suggest that the universe did indeed go through a period of inflation very early on. However, it is quite a leap to go from an inflationary period in our universe to the chaotic inflation model. The one does not necessarily imply the other.

A further claim is that the fundamental constants are finely tuned for life. While this fact is manifestly true, many times explanations for this fact implicitly assume an answer. Take, for example, dark energy. A universe with too much of it will expand so fast that complex structures cannot survive. However, statistical analyses of universe formation with varying amounts of dark energy density seem to imply that higher levels are more likely. Therefore, there should be a peak where dark energy density is high enough to be plausible, but low enough to be able to produce life. According to those analyses, our universe has precisely that density of dark energy. The problem is that this argument assumes the desired outcome because you can only do a statistical analysis if more than one of a thing exists. If ours is the only universe, that type of statistics has no meaning.

String theory is another argument used in favor of the multiverse idea. The problem with string theory is that, in its current form, it does not constrain many of the fundamental physical properties of a universe. Since it predicts that every value of these properties can exist, it loses any real predictive value at all. Until it can be better constrained, its predictive value is small.

The final argument used in favor of a multiverse is that all that can happen, does happen. In an infinite universe, this is undoubtedly true. But, once again, this strays into the realm of speculation. It implies a greater framework or principal that decides what is allowable and what is not, and we have no direct evidence of such a thing. It also fails the falsifiability criterion.

In the end the idea of a multiverse, while interesting and perhaps explanatory, falls short of scientific theory. A level 1 multiverse is plausible, but cannot be confirmed, and a level 2 multiverse is pure speculation.

Next time, new types of supernovae.