Multiverso es un término usado para definir los múltiples universos posibles, incluido nuestro propio universo. Comprende todo lo que existe físicamente: la totalidad del espacio y del tiempo, todas las formas de materia, energía y cantidad de movimiento, y las leyes físicas y constantes que las gobiernan.
La idea de que el universo que se puede observar es sólo una parte de
la realidad física dio lugar al nacimiento del concepto de multiverso.
Los diferentes universos dentro del multiverso son a veces llamados universos paralelos.
La estructura del multiverso, la naturaleza de cada universo dentro de
él, así como la relación entre los diversos universos constituyentes,
dependen de la hipótesis de multiverso considerada.
El concepto de multiverso ha sido supuesto en cosmología, física, astronomía, filosofía, psicología transpersonal y ficción, en particular dentro de la ciencia ficción y de la fantasía. El término fue acuñado en 1895 por el psicólogo William James. En estos contextos, los universos paralelos también son llamados
«universos alternativos», «universos cuánticos», «dimensiones
interpenetrantes», «mundos paralelos», «realidades alternativas» o
«líneas de tiempo alternativas».
In the 1980s, physicist Alan Guth offered an enhanced version of the
big-bang theory, called inflationary cosmology, which promised to fill
this critical gap. The centerpiece of the proposal is a hypothetical
cosmic fuel that, if concentrated in a tiny region, would drive a brief
but stupendous outward rush of space—a bang, and a big one at that. In
fact, mathematical calculations showed that the burst would have been
so intense that tiny jitters from the quantum realm would have been
stretched enormously and smeared clear across space. Like overextended
spandex showing the pattern of its weave, this would yield a precise
pattern of miniscule temperature variations, slightly hotter spots and
slightly colder spots dotting the night sky. In the early 1990s, NASA’s
Cosmic Microwave Background Explorer satellite first detected these
temperature variations, garnering Nobel Prizes for team leaders John
Mather and George Smoot.
Remarkably, mathematical analysis also
revealed—and here’s where the multiverse enters—that as space expands
the cosmic fuel replenishes itself, and so efficiently that it is
virtually impossible to use it all up. Which means that the big bang
would likely not be a unique event. Instead, the fuel would not only
power the bang giving rise to our expanding realm, but it would power
countless other bangs, too, each yielding its own separate, expanding
universe. Our universe would then be a single expanding bubble
inhabiting a grand cosmic bubble bath of universes—a multiverse.
It’s a striking prospect. If correct, it would
provide the capstone on a long series of cosmic reappraisals. We once
thought our planet was the center of it all, only to realize that we’re
one of many planets orbiting the sun, only then to learn that the sun,
parked in a suburb of the Milky Way, is one of hundreds of billions of
stars in our galaxy, only then to find that the Milky Way is one of
hundreds of billions of galaxies inhabiting the universe. Now,
inflationary cosmology was suggesting that our universe, filled with those billions of galaxies, stars, and planets, might merely be one of many occupying a vast multiverse.
Yet, when the multiverse was proposed back in the
1980s by pioneers Andrei Linde and Alexander Vilenkin, the community
of physicists shrugged. The other universes, even if they existed,
would stand outside what we can observe—we only have access to this
universe. Apparently, then, they wouldn’t affect us and we wouldn’t
affect them. So what role could other universes possibly play in
science, a discipline devoted to explaining what we do see?
And that’s where things stood for about a decade, until an astounding astronomical observation suggested an answer.
Although the discovery that space is expanding was revolutionary, there
was one aspect of the expansion that most everyone took for granted.
Just as the pull of earth’s gravity slows the ascent of a ball tossed
upward, the gravitational pull of each galaxy on every other must be
slowing the expansion of space.
In the 1990s, two teams of astronomers set out to
measure the rate of this cosmic slowdown. Through years of painstaking
observations of distant galaxies, the teams collected data on how the
expansion rate of space has changed over time. And when they completed
the analysis, they all nearly fell out of their chairs. Both teams
found that, far from slowing down, the expansion of space went into
overdrive about 7 billion years ago and has been speeding up ever
since. That’s like gently tossing a ball upward, having it slow down
initially, but then rocket upward ever more quickly.
The result sent scientists across the globe
scurrying to explain the cosmic speedup. What force could be driving
every galaxy to rush away from every other faster and faster? The most
promising answer comes to us from an old idea of Einstein’s. We’re all
used to gravity being a force that does only one thing: pull objects
toward each other. But in Einstein’s general theory of relativity,
gravity can also do something else: it can push things apart. How?
Well, the gravity exerted by familiar objects like the moon, the earth,
and the sun is surely attractive. But Einstein’s equations show that if
space contains something else—not clumps of matter but an invisible
energy, sort of like an invisible mist that’s uniformly spread through
space—then the gravity exerted by the energy mist would be repulsive.
Which is just what we need to explain the
observations. The repulsive gravity of an invisible energy mist filling
space—we now call it dark energy—would push every galaxy away from
every other, driving the expansion to speed up, not slow down.
But there’s a hitch. When the astronomers deduced
how much dark energy would have to permeate every nook and cranny of
space to account for the observed cosmic speedup, they found a number
that no one has been able to explain. Not even close. Expressed in the
relevant units, the dark-energy density is extraordinarily small:
.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
000000000000000000000000000138.
At the same time, attempts by researchers to
calculate the amount of dark energy from the laws of physics have
yielded results that are typically a hundred orders of magnitude larger, perhaps the greatest mismatch between observation and theory in the history of science.
And that has led to some soul searching.
Physicists have long believed that with
sufficient hard work, experimentation, and industrious calculation, no
detail about the fundamental makeup of reality would lie beyond
scientific explanation. Certainly, many details still lack an
explanation, such as the masses of particles like electrons and quarks.
Yet the expectation has been that in due course physicists will find
explanations.
The spectacular failure of attempts to explain
the amount of dark energy has raised questions about this confidence,
driving some physicists to pursue a radically different explanatory
approach, one that suggests (once again) the possible existence of a
multiverse.
The new approach has scientific roots that stretch back to the early
1600s, when the great astronomer Johannes Kepler was obsessed with
understanding a different number: the 93 million miles between the sun
and the earth. Kepler struggled for years to explain this distance but
never succeeded, and from our modern perch the reason is clear. We now
know that there are a great many planets, orbiting their host stars at a
great many different distances, demonstrating the fallacy in Kepler’s
quest—the laws of physics do not single out any particular distances as
special. Instead, what distinguishes the earth-sun distance is simply
that it yields conditions hospitable to life: were we much closer or
farther from the sun, the extreme temperatures would prevent our form
of life from taking hold. So, although Kepler was on a wild goose chase
in seeking a fundamental explanation for the earth-sun distance, there
is an explanation for why we humans find ourselves at such a distance.
In seeking an explanation for the value of dark
energy, maybe we’ve been making a mistake analogous to Kepler’s. Our
best cosmological theory—the inflationary theory—naturally gives rise
to other universes. Perhaps, then, just as there are many planets
orbiting stars at many different distances, maybe there are many
universes containing many different amounts of dark energy. If so,
asking the laws of physics to explain one particular value of dark
energy would be just as misguided as trying to explain one particular
planetary distance. Instead, the right question to ask would be: why do
we humans find ourselves in a universe with the particular amount of
dark energy we’ve measured, instead of any of the other possibilities?
This is a question we can address. In universes
with larger amounts of dark energy, whenever matter tries to clump into
galaxies, the repulsive push of the dark energy is so strong that the
clump gets blown apart, thwarting galactic formation. In universes
whose dark-energy value is much smaller, the repulsive push changes to
an attractive pull, causing those universes to collapse back on
themselves so quickly that again galaxies wouldn’t form. And without
galaxies, there are no stars, no planets, and so in those universes
there’s no chance for our form of life to exist.
And so we find ourselves in this universe and not
another for much the same reason we find ourselves on Earth and not on
Neptune—we find ourselves where conditions are ripe for our form of
life. Even without being able to observe the other universes, their
existence would thus play a scientific role: the multiverse offers a
solution to the mystery of dark energy, rendering the quantity we
observe understandable.
Or so that’s what multiverse proponents contend.
Many others find this explanation unsatisfying,
silly, even offensive, asserting that science is meant to give
definitive, precise, and quantitative explanations, not “just so”
stories.
But the essential counterpoint is that if the
feature you’re trying to explain can and does take on a wide variety of
different mathematical values across the landscape of reality, then
seeking a definitive explanation for one value is wrongheaded. Just as
it makes no sense to ask for a definitive prediction of the
distance at which planets orbit their host stars, since there are many
possible distances, if we’re part of a multiverse it would make no
sense to ask for a definitive prediction of the value of dark energy, since there would be many possible values.
The multiverse doesn’t change the scientific
method or lower explanatory standards. But it does ask us to reevaluate
whether we’ve mistakenly posed the wrong questions.
Of course,
for this approach to succeed, we must be sure that among the
multiverse’s many different dark-energy values is the very one we’ve
measured. And that’s where a third line of investigation, string
theory, comes to the fore.
String theory is an attempt to realize Einstein’s
dream of a “unified theory” capable of stitching all matter and forces
into a single mathematical tapestry. Initially formulated in the late
1960s, the theory envisions that deep inside every fundamental particle
is a tiny, vibrating, stringlike filament of energy. And much as the
different vibrational patterns of a violin string yield different
musical notes, so the different vibrational patterns of these tiny
strings would yield different kinds of particles.
Pioneers of the subject anticipated that string
theory’s rigid mathematical architecture would soon yield a single set
of definitive, testable predictions. But as the years passed, detailed
analysis of the theory’s equations revealed numerous solutions, each
representing a different possible universe. And numerous means numerous. Today, the tally of possible universes stands at the almost incomprehensible 10500, a number so large it defies analogy.
For some string-theory advocates, this stupendous
failure to yield a unique universe—ours—was a devastating blow. But
to those advancing the multiverse, string theory’s enormous diversity
of possible universes has proven vital.
Just as it takes a well-stocked shoe store to
guarantee you’ll find your size, only a well-stocked multiverse can
guarantee that our universe, with its peculiar amount of dark energy,
will be represented. On its own, inflationary cosmology falls short of
the mark. While its never-ending series of big bangs would yield an
immense collection of universes, many would have similar features, like
a shoe store with stacks and stacks of sizes 5 and 13, but nothing in
the size you seek.
By combining inflationary cosmology and string
theory, however, the stock room of universes overflows: in the hands of
inflation, string theory’s enormously diverse collection of possible universes become actual
universes, brought to life by one big bang after another. Our universe
is then virtually guaranteed to be among them. And because of the
special features necessary for our form of life, that’s the universe we
inhabit.
Pero la pregunta de Einstein no era teológica. En realidad, el padre
de la relatividad quería saber si las leyes de la física producen
necesariamente un universo único —el nuestro— lleno de galaxias,
estrellas y planetas. O si, como ocurre cada año con la gama de nuevos
autos que se ofrecen en las concesionarias, las leyes podían permitir
que existan universos con una amplia variedad de características
diferentes. Y si es así, ¿acaso la imponente realidad que hemos llegado a
conocer —a través de poderosos telescopios y enormes colisionadores de
partículas— es el producto de algún proceso aleatorio, un juego de dados
cósmico que seleccionó nuestras características de entre un amplio menú
de posibilidades? ¿O existe una explicación más profunda de por qué las
cosas son como son?
En la época de Einstein, la posibilidad de que nuestro universo
podría haber sido distinto era una idea perturbadora que los físicos
"serios" sólo tomarían en cuenta una vez resueltos asuntos más
importantes. Pero, recientemente, la pregunta pasó de los márgenes de la
física a la corriente principal. Y en lugar de sólo imaginar que
nuestro universo pudo haber tenido propiedades diferentes, tres avances
científicos autónomos sugieren que existen otros universos,
independientes del nuestro, la mayoría de los cuales están conformados
por distintos tipos de partículas y gobernados por diferentes fuerzas.
El cosmos, según esta novedosa interpretación, sería sorprendentemente
extenso. Mucho más de lo imaginable.
ご意見、ご質問等ございましたら、 <ernestotaju@yahoo.co.jp> へ。
