A
wild, compelling idea without a direct, practical test, the
Multiverse is highly controversial. But its supporting pillars sure
are stable. The theory of cosmic inflation predicts a multiverse: an
enormous number of Universes that experience hot Big Bangs, but each
of those regions where a Big Bang occurs are completely separated
from one another, with nothing but continuously inflating space
between them. We cannot detect these other Universes, but their
existence may not be avoidable in the context of inflation.
One of
the most successful theories of 20th century science is cosmic
inflation, which preceded and set up the hot Big Bang. We also know
how quantum fields generally work, and if inflation is a quantum
field (which we strongly suspect it is), then there will always be
more "still-inflating" space out there. Whenever and
wherever inflation ends, you get a hot Big Bang. If inflation and
quantum field theory are both correct, a Multiverse is a must.
When
we look out at the Universe today, it simultaneously tells us two
stories about itself. One of those stories is written on the face of
what the Universe looks like today, and includes the stars and
galaxies we have, how they’re clustered and how they move, and what
ingredients they’re made of. This is a relatively straightforward
story, and one that we’ve learned simply by observing the Universe
we see.
But
the other story is how the Universe came to be the way it is today,
and that’s a story that requires a little more work to uncover.
Sure, we can look at objects at great distances, and that tells us
what the Universe was like in the distant past: when the light that’s
arriving today was first emitted. But we need to combine that with
our theories of the Universe — the laws of physics within the
framework of the Big Bang — to interpret what occurred in the past.
When we do that, we see extraordinary evidence that our hot Big Bang
was preceded and set up by a prior phase: cosmic inflation. But in
order for inflation to give us a Universe consistent with what we
observe, there’s an unsettling appendage that comes along for the
ride: a multiverse. Here’s why physicists overwhelmingly claim that
a multiverse must exist.
The
‘raisin bread’ model of the expanding Universe, where relative
distances increase as the space (dough) expands. The farther away any
two raisin are from one another, the greater the observed redshift
will be by time the light is received. The redshift-distance relation
predicted by the expanding Universe is borne out in observations, and
has been consistent with what’s been known all the way back since
the 1920s.
Back
in the 1920s, the evidence became overwhelming that not only were the
copious spirals and ellipticals in the sky actually entire galaxies
unto themselves, but that the farther away such a galaxy was
determined to be, the greater the amount its light was shifted to
systematically longer wavelengths. While a variety of interpretations
were initially suggested, they all fell away with more abundant
evidence until only one remained: the Universe itself was undergoing
cosmological expansion, like a loaf of leavening raisin bread, where
bound objects like galaxies (e.g., raisins) were embedded in an
expanding Universe (e.g., the dough).
If the
Universe was expanding today, and the radiation within it was being
shifted towards longer wavelengths and lower energies, then in the
past, the Universe must have been smaller, denser, more uniform, and
hotter. As long as any amount of matter and radiation are a part of
this expanding Universe, the idea of the Big Bang yields three
explicit and generic predictions:
a
large-scale cosmic web whose galaxies grow, evolve, and cluster more
richly over time,
a
low-energy background of black body radiation, left over from when
neutral atoms first formed in the hot, early Universe,
and
a specific ratios of the lightest elements — hydrogen, helium,
lithium, and their various isotopes — that exist even in regions
that have never formed stars.
This
snippet from a structure-formation simulation, with the expansion of
the Universe scaled out, represents billions of years of
gravitational growth in a dark matter-rich Universe. Note that
filaments and rich clusters, which form at the intersection of
filaments, arise primarily due to dark matter; normal matter plays
only a minor role.
All
three of these predictions have been observationally borne out, and
that’s why the Big Bang reigns supreme as our leading theory of the
origin of our Universe, as well as why all its other competitors have
fallen away. However, the Big Bang only describes what our Universe
was like in its very early stages; it doesn’t explain why it had
those properties. In physics, if you know the initial conditions of
your system and what the rules that it obeys are, you can predict
extremely accurately — to the limits of your computational power
and the uncertainty inherent in your system — how it will evolve
arbitrarily far into the future.
But
what initial conditions did the Big Bang need to have at its
beginning to give us the Universe we have? It’s a bit of a
surprise, but what we find is that:
there
had to be a maximum temperature that’s significantly (about a
factor of ~1000, at least) lower than the Planck scale, which is
where the laws of physics break down,
the
Universe had to have been born with density fluctuations of
approximately the same magnitude of all scales,
the
expansion rate and the total matter-and-energy density must have
balanced almost perfectly: to at least ~30 significant digits,
it
must have been born with the same initial conditions — same
temperature, density, and spectrum of fluctuations — at all
locations, even causally disconnected ones,
and
its entropy must have been much, much lower than it is today, by a
factor of trillions upon trillions.
If
these three different regions of space never had time to thermalize,
share information or transmit signals to one another, then why
are they all the same temperature? This is one of the problems with
the initial conditions of the Big Bang; how could these regions all
obtain the same temperature unless they started off that way,
somehow?
Whenever
we come up against a question of initial conditions — basically,
why did our system start off this way? — we only have two options.
We can appeal to the unknowable, saying that it is this way because
it’s the only way it could’ve been and we can’t know anything
further, or we can try to find a mechanism for setting up and
creating the conditions that we know we needed to have. That second
pathway is what physicists call “appealing to dynamics,” where we
attempt to devise a mechanism that does three important things.
It
has to reproduce every success that the model it’s trying to
supersede, the hot Big Bang in this instance, produces. Those
earlier cornerstones must all come out of any mechanism we propose.
It
has to explain what the Big Bang cannot: the initial conditions the
Universe started off with. These problems that remain unexplained
within the Big Bang alone must be explained by whatever novel idea
comes along.
And
it has to make new predictions that differ from the original
theory’s predictions, and those predictions must lead to a
consequence that is in some way observable, testable, and/or
measurable.
The
only idea we’ve had that met these three criteria was the theory of
cosmic inflation, which has achieved unprecedented successes on all
three fronts.
Exponential
expansion, which takes place during inflation, is so powerful because
it is relentless. With every ~10^-35 seconds (or so) that passes, the
volume of any particular region of space doubles in each direction,
causing any particles or radiation to dilute and causing any
curvature to quickly become indistinguishable from flat.
What
inflation basically says is that the Universe, before it was hot,
dense, and filled with matter-and-radiation everywhere, was in a
state where it was dominated by a very large amount of energy that
was inherent to space itself: some sort of field or vacuum energy.
Only, unlike today’s dark energy, which has a very small energy
density (the equivalent of about one proton per cubic meter of
space), the energy density during inflation was tremendous: some
1025 times greater than dark energy is today!
The
way the Universe expands during inflation is different from what
we’re familiar with. In an expanding Universe with matter and
radiation, the volume increases while the number of particles stays
the same, and hence the density drops. Since the energy density is
related to the expansion rate, the expansion slows over time. But if
the energy is intrinsic to space itself, then the energy density
remains constant, and so does the expansion rate. The result is what
we know as exponential expansion, where after a very small period of
time, the Universe doubles in size, and after that time passes again,
it doubles again, and so on. In very short order — a tiny fraction
of a second — a region that was initially smaller than the smallest
subatomic particle can get stretched to be larger than the entire
visible Universe today.
In the
top panel, our modern Universe has the same properties (including
temperature) everywhere because they originated from a region
possessing the same properties. In the middle panel, the space that
could have had any arbitrary curvature is inflated to the point where
we cannot observe any curvature today, solving the flatness problem.
And in the bottom panel, preexisting high-energy relics are inflated
away, providing a solution to the high-energy relic problem. This is
how inflation solves the three great puzzles that the Big Bang cannot
account for on its own.
During
inflation, the Universe gets stretched to enormous sizes. This
accomplishes a tremendous number of things in the process, among
them:
stretching
the observable Universe, irrespective of what its initial curvature
was, to be indistinguishable from flat,
taking
whatever initial conditions existed in the region that began
inflating, and stretching them across the entire visible Universe,
creating
minuscule quantum fluctuations and stretching them across the
Universe, so that they’re almost the same on all distance scales,
but slightly smaller-magnitude on smaller scales (when inflation is
about to end),
converting
all that “inflationary” field energy into matter-and-radiation,
but only up to a maximum temperature that’s well below the Planck
scale (but comparable to the inflationary energy scale),
creating
a spectrum of density and temperature fluctuations that exist on
scales larger than the cosmic horizon, and that are adiabatic (of
constant entropy) and not isothermal (of constant temperature)
everywhere.
This
reproduces the successes of the non-inflationary hot Big Bang,
provides a mechanism for explaining the Big Bang’s initial
conditions, and makes a slew of novel predictions that differ from a
non-inflationary beginning. Beginning in the 1990s and through the
present day, the inflationary scenario’s predictions agree with
observations, distinct from the non-inflationary hot Big Bang.
The
quantum fluctuations that occur during inflation get stretched across
the Universe, and when inflation ends, they become density
fluctuations. This leads, over time, to the large-scale structure in
the Universe today, as well as the fluctuations in temperature
observed in the CMB. Its a spectacular example of how the quantum
nature of reality affects the entire large-scale universe.
The
thing is, there’s a minimum amount of inflation that must occur in
order to reproduce the Universe we see, and that means there are
certain conditions that inflation has to satisfy in order to be
successful. We can model inflation as a hill, where as long as you
stay on top of the hill, you inflate, but as soon as you roll down
into the valley below, inflation comes to an end and transfers its
energy into matter and radiation.
If you
do this, you’ll find that there are certain “hill-shapes,” or
what physicists call “potentials,” that work, and others that
don’t. The key to making it work is that the top of the hill need
to be flat enough in shape. In simple terms, if you think of the
inflationary field as a ball atop that hill, it needs to roll slowly
for the majority of inflation’s duration, only picking up speed and
rolling quickly when it enters the valley, bringing inflation to an
end. We’ve quantified how slowly inflation needs to roll, which
tells us something about the shape of this potential. As long as the
top is sufficiently flat, inflation can work as a viable solution to
the beginning of our Universe.
The
simplest model of inflation is that we started off at the top of a
proverbial hill, where inflation persisted, and rolled into a valley,
where inflation came to an end and resulted in the hot Big Bang. If
that valley isn’t at a value of zero, but instead at some positive,
non-zero value, it may be possible to quantum-tunnel into a
lower-energy state, which would have severe consequences for the
Universe we know today.
Here’s
where things get interesting. Inflation, like all the fields we know
of, has to be a quantum field by its very nature. That means that
many of its properties aren’t exactly determined, but rather have a
probability distribution to them. The more time you allow to pass,
the greater the amount that distribution spreads out. Instead of
rolling a point-like ball down a hill, we’re actually rolling a
quantum probability wave function down a hill.
Simultaneously,
the Universe is inflating, which means it’s expanding exponentially
in all three dimensions. If we were to take a 1-by-1-by-1 cube and
call that “our Universe,” then we could watch that cube expand
during inflation. If it takes some tiny amount of time for the size
of that cube to double, then it becomes a 2-by-2-by-2 cube, which
requires 8 of the original cubes to fill. Allow that same amount of
time to elapse, and it becomes a 4-by-4-by-4 cube, needing 64
original cubes to fill. Let that time elapse again, and it’s an
8-by-8-by-8 cube, with a volume of 512. After only about ~100
“doubling times,” we’ll have a Universe with approximately
1090 original cubes in it.
If
inflation is a quantum field, then the field value spreads out over
time, with different regions of space taking different realizations
of the field value. In many regions, the field value will wind up in
the bottom of the valley, ending inflation, but in many more,
inflation will continue, arbitrarily far into the future.
So
far, so good. Now, let’s say we have a region where that
inflationary, quantum ball rolls down into the valley. Inflation ends
there, that field energy gets converted to matter-and-radiation, and
something that we know as a hot Big Bang occurs. This region might be
irregularly shaped, but it’s required that enough inflation
occurred to reproduce the observational successes we see in our
Universe.
The
question becomes, then, what happens outside of that region?
Wherever
inflation occurs (blue cubes), it gives rise to exponentially more
regions of space with each step forward in time. Even if there are
many cubes where inflation ends (red Xs), there are far more regions
where inflation will continue on into the future. The fact that this
never comes to an end is what makes inflation ‘eternal’ once it
begins, and where our modern notion of a multiverse comes from.
Here’s
the problem: if you mandate that you get enough inflation that our
Universe can exist with the properties we see, then outside of the
region where inflation ends, inflation will continue. If you ask,
“what is the relative size of those regions,” you find that if
you want the regions where inflation ends to be big enough to be
consistent with observations, then the regions where it doesn’t end
are exponentially larger, and the disparity gets worse as time goes
on. Even if there are an infinite number of regions where inflation
ends, there will be a larger infinity of regions where it persists.
Moreover, the various regions where it ends — where hot Big Bangs
occur — will all be causally disconnected, separated by more
regions of inflating space.
Put
simply, if each hot Big Bang occurs in a “bubble” Universe, then
the bubbles simply don’t collide. What we wind up with is a larger
and larger number of disconnected bubbles as time goes on, all
separated by an eternally inflating space.
An
illustration of multiple, independent Universes, causally
disconnected from one another in an ever-expanding cosmic ocean, is
one depiction of the Multiverse idea. The different Universes that
arise may have different properties from one another or they may not,
but we do not know how to test the multiverse hypothesis in any way.
That’s
what the multiverse is, and why scientists accept its existence as
the default position. We have overwhelming evidence for the hot Big
Bang, and also that the Big Bang began with a set of conditions that
don’t come with a de facto explanation. If we add in an explanation
for it — cosmic inflation — then that inflating spacetime that
set up and gave rise to the Big Bang makes its own set of novel
predictions. Many of those predictions are borne out by observation,
but other predictions also arise as consequences of inflation.
One of
them is the existence of a myriad of Universes, of disconnected
regions each with their own hot Big Bang, that comprise what we know
as a multiverse when you take them all together. This doesn’t mean
that different Universes have different rules or laws or fundamental
constants, or that all the possible quantum outcomes you can imagine
occur in some other pocket of the multiverse. It doesn’t even mean
that the multiverse is real, as this is a prediction we cannot
verify, validate, or falsify. But if the theory of inflation is a
good one, and the data says it is, a multiverse is all but
inevitable.
You
may not like it, and you really may not like how some physicists
abuse the idea, but until a better, viable alternative to inflation
comes around, the multiverse is very much here to stay. Now, at
least, you understand why.
by
Ethan Siegel at bigthink.com on December 30, 2021
