Previously, we talked about the theory of inflation and how that leads to Max
Tegmark’s Level 1 Multiverse. Today, we will be looking at the next level of the multiverse,
or the Level 2 Multiverse, as proposed by Tegmark and some of its defining characteristics.
Now, we saw in the previous video, that inflation generally refuses to stop and keeps creating
more space. This idea is taken a step further with the theory of chaotic eternal inflation.
The phrase “chaotic eternal” refers to what happens on the very largest of scales.
According to this model, space as a whole is stretching and will continue doing so forever, but
in some regions, it may stop stretching and form distinct bubbles, like gas pockets in a loaf of
rising bread. Infinitely many such bubbles emerge. Each of these is an embryonic Level I multiverse
which is infinite in size and filled with matter deposited by the energy field that drove
inflation. So, an infinite set of these distinct Level I multiverses, some perhaps with different
spacetime dimensionality and different physical constants constitute a Level 2 Multiverse.
But these bubbles are more than infinitely far away from Earth, in the sense that you would
never be able to get there even if you traveled at the speed of light forever. The reason for
this is that the space between our bubble and its neighbors is expanding faster than you
could travel through it. So, your descendants will never get to see their identical copies
elsewhere in the Level 2 Multiverse. For the same reason, if cosmic expansion is accelerating, as
observations now suggest, then they might not even see their alter egos in the Level 1 Multiverse.
As mentioned before, some of these bubbles, or Level 1 Multiverses may have different laws of
physics. Now, that may seem impossible at first, but not according to Max Tegmark’s
definition of these laws. He separates the laws of physics into two distinct categories:
the fundamental laws of physics, which by definition hold anywhere and anytime
giving rise to a complicated physical state of affairs, and the effective laws of physics
inferred by self-aware observers that may vary from place to place.
For example, a fish that has lived its entire life in the ocean might make the mistake of thinking of
water not as a substance, but as empty space. What a human would think of as a property of water,
say, the friction when swimming through it, the fish might misinterpret it as a fundamental law
of physics. It may even come up with statements such as “a fish in uniform motion ends up at rest
unless flapping its fins.” So, this fish would probably have no idea that water can exist in
three different phases; solid, liquid, and gas. And that what it believes to be “empty space”
is simply the liquid phase which is a particular solution to the equations describing water.
Similarly, there is a ton of evidence that this is exactly how things are for
us. Not only does our “empty space” seem to be a sort of medium,
but it appears to have way more than three phases, perhaps about 10^500, or even
infinitely many, which opens up the possibility that, in addition to curving, stretching,
and vibrating, our space may even be able to do something analogous to freezing and evaporating!
So, the Level 2 multiverse is far more diverse than the Level 1 multiverse. The bubbles vary
not only in their initial conditions but also in seemingly unchanging aspects of nature.
The prevailing view in physics today is that the dimensionality of spacetime,
the qualities of elementary particles, and many of the so-called physical constants are not
built into physical laws but are the outcome of the processes known as symmetry breaking.
For instance, theorists think that the space in our universe once had eleven dimensions,
all on an equal footing. Early in cosmic history, three of them partook in the cosmic expansion and
became the three dimensions we now observe. The others are now unobservable, either because they
have stayed microscopic with a doughnutlike topology or because all matter is confined
to a three-dimensional surface or a membrane, or simply “brane”, in the eleven-dimensional space.
Thus, the original symmetry among the dimensions broke. The quantum fluctuations that drive
chaotic inflation could cause different symmetry breaking in different bubbles. Some might become
four-dimensional, while others could contain only two rather than three generations of quarks.
In addition to such discrete properties as dimensionality and fundamental particles,
our universe is characterized by a set of dimensionless numbers
known as physical constants. Examples include the electron/proton mass ratio,
and the cosmological constant, which appears to be about 10^−123 measured in Planck units. There are
models where also such continuous parameters can vary from one post-inflationary bubble to another.
But the question still remains. How can we be sure that the Level 2 Multiverse really exists?
Well, for Tegmark, although we cannot directly interact with other Level II parallel universes,
cosmologists can infer their presence indirectly,
because their existence can account for unexplained coincidences in our universe.
Suppose you check into a hotel and are assigned a room at random and note that this is the year
you were born. At first, this may seem like a coincidence but after a moment of reflection,
you would conclude that this is not so surprising after all. The hotel has hundreds of rooms, and
you would not have been having these thoughts in the first place if you had been assigned one with
a number that meant nothing to you. The lesson is that even if you knew nothing about hotels,
you could infer the existence of other hotel rooms to explain the coincidence.
Another example Tegmark provides is relating to the mass of our sun. The mass of a star
determines its luminosity, and using basic physics, we can compute that life as we know
it on Earth is possible only if the sun’s mass falls into the narrow range between 1.6 x 10^30
and 2.4 x 10^30 kilograms. Otherwise, Earth’s climate would be colder than that of present-day
Mars or hotter than that of present-day Venus. The measured solar mass is 2.0 x 10^30 kilograms.
At first glance, this apparent coincidence of the habitable and observed mass values
appears to be a wild stroke of luck. Stellar masses run from 10^29 to 10^32 kilograms,
so if the sun acquired its mass at random, it had only a small chance of falling into
the habitable range. But just as in the hotel example, one can explain this apparent
coincidence by postulating an ensemble, in this case, several planetary systems,
and a selection effect, the fact that we must find ourselves living on a habitable planet.
Similarly, what applies to hotel rooms and planetary systems applies to parallel
universes as well. Most, if not all, of the attributes set by symmetry breaking, appear to be
fine-tuned. Changing their values by the tiniest amounts would have resulted in a qualitatively
different universe. So, although the degree of fine-tuning is still debated, these examples
suggest the existence of parallel universes with other values of the physical constants.
So, if the Level 2 Multiverse covers all possible initial conditions and symmetry breakings,
then surely this is the point beyond which there should be nothing else right?
Well not according to Tegmark! He goes even beyond a Level 2 Multiverse,
which he refers to as a Level 3 Multiverse. But that is a discussion best left for another time!
If you enjoyed this video, do leave a like, share, and subscribe to our channel for more
content ahead. Is our universe truly infinite as predicted by inflation, or just extremely
large and finite? Let us know your thoughts down below! Until then, we’ll see you in the next one!