Why I believe in the Multiverse
One or Many Universes? It seems a nonsense question, isn't it?
By definition, the universe is the set of everything that exists,
and therefore it is difficult for us to accept the idea that there could be others.
Yet, exotic concepts such as "Parallel Universe" and "Multiverse", which once
were used only in science fiction novels, now for several years are part of the scientific
phrasebook in use in some branches of cosmology. It happened more than forty years ago, when we
realized something strange and trivial at the same time: we live in a universe governed by the
laws of physics, but we do not know why we have these laws, and not others, and we do not know why
there are certain constants, such as the speed of light, which have certain values, and not others.
Who decided that light, in vacuum, must travel at 300.000 km per second? Who decided that a
proton should attract an electron with the force we measure, and not with double? Who decided that
the force of gravity is just this and not half? And why all this would have to do with
To understand this, it is necessary to take several steps back...
According to the standard Big-Bang theory, the birth and evolution of the universe are
phenomena that can be summarily described using the equations of physics, which in turn are based
on a few fundamental numerical constants. As far as we know until today, in nature
there are four possible interactions between the components of matter: the "gravitational" one,
that governs the motion of stars and planets and, even if it is the weakest of the four, it is the
only one that can be felt even at great distances; the "weak" one, responsible of radioactive decay;
the "electromagnetic" one, that binds electrons to the nucleus of the atom;
and the "strong" one, that keeps together protons and neutrons in the atomic nucleus.
The curious thing is that observed values of parameters that characterize these four forces
seem to be finely balanced between them. As if someone had studied
and programmed them at the table. For example, if the "strong" nuclear
force was just bigger, all the hydrogen in the universe would have been converted into helium
and now there would be no water, nor quiet and long-lived main sequence stars
that like our Sun are based on hydrogen fusion. As we know it, life would therefore not have
had the right way and time to form. Similarly, if the gravity was greater,
the stars would burn too quickly hydrogen, finishing it before life can develop,
and if it was just a little weaker stars would not shine at all: there would not be inside them, in
fact, the pressure sufficient to push the protons to overcome the force of electrical repulsion
that would tend to separate them, and you would not have the trigger of the nuclear reaction.
After all, if the electric force that binds electrons to the nucleus were a little smaller,
the electrons would flee from their orbits; and if it were a little larger, the electrons would
not bind with other atoms; and in either case, complex chemical compounds could never be created.
One could go on, but the conceptual scenario seems quite clear even with these few examples
In 1974 the Australian physicist Brandon Carter was the first to argue
that behind this remarkable series of absurdly favorable coincidences for life
there must be some physical mechanism, and spoke for the first time of the Anthropic Principle,
according to which the fundamental physical laws and the universe itself
would be constructed "ad hoc" to bring to life inanimate matter. In other words,
the aspect of our universe is what it is because, if it was different, we would not be here to
observe it. The term "anthropic" is particularly indigestible to many scientists, and with good
reason. It brings to mind concepts related to a religious conception of the universe, such as
Intelligent Design and Creationism. Something unacceptable for those who deal with science.
It was then tried to find a more rational answer to the question of "coincidences" that seem to be
built ad hoc to promote the emergence of life in our universe. And a possible solution seemed to
come from a "contradiction" in the model of the inflationary universe, theorized by the
American physicist Alan Guth in 1981 to answer in turn a question raised by the Big Bang theory:
why the temperature of the cosmic background radiation coming from regions of the sky
located in opposite directions is so similar since these regions have never been in contact
and therefore have never been able to balance their respective temperatures?
The observations of that time showed in fact that the radiation coming from
every part of the sky, the so-called "echo of the Big Bang", appears extremely homogeneous,
when instead one would expect to find differences, since regions of the universe currently very
distant from each other would have been unable to exchange thermal information.
According to Guth, the problem could be solved if it was assumed that the universe,
immediately after the Big Bang, had suddenly increased exponentially its size,
swelling at speeds much greater than that of light.
In this way, the physical properties possessed by the tiny universe that emerged from the initial
singularity would have been inherited by all regions of the post-inflationary universe.
The inflationary era would have lasted very little, just a trillionth of a second or less;
but during this phase, the universe would have increased its size by a million trillion times,
going from the size of a single proton to that of an orange. From that point on,
the history of the universe would continue in the way we know it: a gradual expansion,
resulting in a decrease in temperature and density until it reached its current levels.
There was one problem, however, that Guth first discussed publicly in the summer of 1982: what was
it that had stopped the inflation at some point? Our universe is still expanding, it is true,
but at a frighteningly slower rate than inflation. How come there was this abrupt slowdown?
And here the answer comes from Russian-American physicist Andrei Linde, who surprisingly and very
boldly hypothesize that inflation in the universe is still continuing at the same
speed and that the slowed-down expansion of our universe must be considered only as a localism!
In practice, with his hypothesis Linde gives us for the first time the vision
of a "Megaverse": a continuous boiling of a sea of matter made bigger and bigger by
a perennial inflation. A sea where our universe would be only a small wave.
"Hey, guys, just a moment before we continue... BE sure to join the Insane Curiosity Channel...
Click on the bell, you will help us to make products of ever-higher quality!"
According to the so-called theory of Eternal Inflation, there would exist,
in practice, many universes that appear (and disappear?) continuously,
as bubbles in a primordial cosmic substrate in expansion and subject to sporadic changes.
Each of these bubbles, after being formed, would expand in turn in ways dictated by
the initial conditions, triggering the evolution of a physical world in itself.
We would live in one of these worlds in which, among the infinite possible conditions,
the right ones have been established to make us be as we are. In this vision of many universes,
ours would not be the result of a single event, but only one of many possible universes,
each with its own physical laws, its own values of constants, and its own space-time dimensions.
The majority of universes would be in conditions of great disorder and not suitable for life.
In some worlds, however, would be created by chance those conditions essential to the emergence
of life. And obviously only these last universes, which are a very small part, would be perceived by
living and conscious beings. Our universe. In short, according to Linde could
simply be a region located within a super-region in eternal expansion.
And, although it is not currently verifiable, this is the concept of
"multiverse" that I feel to support and in which I believe without any hesitation.
There are in fact a number of good reasons to believe in multiple universes as a
physical reality, mainly because we have absolutely no reason to assume that our
observable universe is all that exists Be careful though... Physicists casually talk
about parallel universes, but it's not always clear what they mean. I firmly believe in the
multiverse suggested by Linde, for the reasons that I will explain later, but I reject for
example the interpretation "Many worlds" based on certain diabolical properties of quantum physics.
according to which there is an infinite number of alternative realities,
in which every possible quantum choice is realized infinite times As well as I reject the idea that
Linde's multiverse can lead to believe in the existence of infinite universes, where at some
point it would be possible to find an exact copy of ourselves. All these are only mathematical
forcing that physical reality would know well how to defuse... And after all, in expressing this
opinion I feel in good company. Even according to the latest work of Stephen Hawking, in fact, the
alternative universes that make up the multiverse may not be too different from each other, let
alone be numerically infinite: the set of possible universes could, in other words, be circumscribed.
Linde's multiverse, although there is no observational evidence capable of
motivating my support, fully respects that philosophy that has been guiding the search
for scientific truth for centuries. Namely, the Principle of Mediocrity, or Copernican.
In cosmology, the Copernican principle, which takes its name from Nicolaus Copernicus,
the astronomer who first dared to say that the Earth is a planet like any other - implies the
concept of "non-particularity" of any event: any observed phenomenon can occur
or have occurred many other times in the universe. A principle that is opposed to the hypothesis of
the rarity of the Earth, according to which the birth and evolution of life on Earth required an
extremely unlikely combination of astrophysical and geological events and circumstances.
In this sense, it is equivalent to the principle of mediocrity, with relevant
implications in the philosophy of science. Our history is made up of episodes in which
we became aware that the world we inhabited was far larger than we had believed until then.
Sometimes this happened with a slow, almost continuous process, sometimes with considerable
jumps. We began by discovering that the world was not limited to the Pillars of Hercules but
extended beyond. Then, that there were other lands beyond the oceans. We broke free from the idea
that everything revolved around the Earth. Once we realized how large the planet we inhabit is,
we measured with increasing precision the space surrounding it and the distance
separating it from the other planets, the Sun, and the stars. From the study of the distance and
distribution of stars, we derived the size of our Galaxy, which until the 1920s we believed to be
the entire universe. Then Hubble showed us that outside the Milky Way there was a space populated
by billions of galaxies like ours. In not even two hundred years, between
the late 1700s and the late 1900s, we went from a static universe consisting of just our Galaxy
to a 14 billion-year-old universe, spanning 46 billion light years and continuously expanding.
And now,? Now, again, we are struggling with the possibility of a further decentralization: maybe
we do not live in the only existing universe, but only in one of the many suitable for life.
And I don't find anything strange in that. I see it as perfectly normal, and in line with
the continuous downgrading of role suffered by the human species during the last centuries.
However, the multiverse theory is still in its infancy, and
several conceptual problems await resolution. Or maybe we will never solve them, who knows?
It is important to clarify that all the theories that we have exposed are pure speculations and
should be preceded by calls for caution. However, to ask if there are unobservable universes is
not absurd, nor meaningless. The question, it is obvious, can not be solved by direct observation,
because nothing can come out or enter in our Universe, not even an electromagnetic signal,
but it is certainly possible to look for relevant empirical data that could lead us to an answer.
On the other hand no one has ever seen black holes or nuclear synthesis reactions that
occur within the Sun and other stars; however, we are convinced of their existence because
the theories on which these phenomena are based are well understood by the scientific community.
But let's not expect too much. This time the step to understanding "what
is out there" is really very long and challenging. Our condition, in the end,
is that of the woodworm that lives in the burrows of a precious Louis XV style piece of furniture
or of a moth that lives in the texture of a canvas painted by Leonardo.
What are the chances that the woodworm will be able to understand the true origin of the artifact
it is in, and above all the meaning of the piece of furniture and the history of that King?
And what are the chances that the moth will be able to understand the artistic
and historical meaning of those strokes of color on which it walks, defecates, and reproduces?
Maybe the same ones that we have to arrive tomorrow to give a sense to what we observe.