4.2 Testing the Multiverse Review

summary

The uniformity of our universe is explained by inflation.

Exponential stretching makes our part of the universe almost exactly uniform… locally.
On a much greater scale, the same theory predicts that the universe is 100% non-uniform (the multiverse).
This actually gives credence to the anthropic principle. The universe produces itself in all possible versions, and we live in those parts of the multiverse where we can live.

How many different universes can be created by inflation?

The universe is “immortal” as it keeps reproducing itself.
Once, the universe was thought to be a sphere. The inflationary multiverse can be thought of as a balloon fractal, constantly branching out and creating new universes.
Each time the universe grows $e$ times, quantum fluctuations divide it into $e^3$ different parts with slightly different temperature.
$N$ such steps create $e^N$ independent universes, because each domain $\frac{1}{H}$ is independent, and the “box” has length $\frac{1}{H} \times e^N$.
The total number of combinations is on the order of $$e^{e^{3 \times 60}} \gg 10^{500}$$much greater than the total number of vacua in the string theory landscape.

How many universes can we observe?

The Bekenstein bound is the limit of entropy—or, in a sense, “information”—that can be contained in a given system.
For any observer of size $R$ and mass $M$, the total entropy is bounded by $2\pi M R$. Therefore, the number of records any observer can keep is no more than $$e^{2\pi MR} \sim 10^{10^{45}}$$
In fact, it appears that a normal brain can absorb no more than 10¹⁶ bits of information in a lifetime, so the number of “universes” one can experience is even more limited to $$10^{10^{16}}$$ which is still much greater than the total number of vacua in the landscape 10⁵⁰⁰.
All vacuum states in string theory are metastable. After a very long time, they will decay. At that time, our part of the universe will become ten-dimensional, or it will collapse and disappear. But, because of eternal inflation, the universe as a whole is immortal.
All of this depends on our understanding of not only classical physics but also of quantum cosmology.

Can we test the multiverse theory?

Is this physics or metaphysics? Can it be experimentally tested?
This theory provides the only known explanation of numerous anthropic coincidences: extremely small vacuum energy, strange masses of elementary particles, etc. In this sense, it was already “tested.”
There are some who say that in order for a scientific theory to be sound, it must be falsifiable. This philosophy is critical of the multiverse theory in that it becomes difficult to falsify that which can produce an infinite number of different results.
Another, perhaps more appropriate, way of looking at scientific theories such as the multiverse theory is that the theory can be sound if we eliminate all other possible explanations. As Sir Arthur Conan Doyle wrote, “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.”

Can we still return to the old concept of a single universe?

In order to propose a true alternative to the theory of the inflationary multiverse, one would have to achieve several incredibly difficult goals:
- An alternative to inflation, and also an alternative to the most developed versions of string theory
- An explanation as to why only one vacuum of string theory can actually exist and why all other 10⁵⁰⁰ vacua are forbidden
- An alternative solution of the cosmological constant (vacuum energy) problem and many other coincidence problems, which have so far only been addressed in the context of the multiverse theory
Scientists have often remarked on the unreasonable effectiveness of mathematics in our universe:
- “The unreasonable efficiency of mathematics in science is a gift we neither understand nor deserve.” —Eugene Wigner
- “The most incomprehensible thing about the universe is that it is comprehensible.” —Albert Einstein
If the universe is the same everywhere, then the effectiveness of the laws of physics applying universally is this “undeserved gift.”
The inflationary multiverse does away with this line of reasoning. The laws of mathematics and physics are efficient only if they allow us to make reliable predictions.
There are many places in the multiverse where our laws of physics are useless and mathematics is inefficient, and we cannot exist in these places.