The Big Bang theory, formally known as the Lambda-CDM model of the universe
describes how the universe expanded from an initial state of incredibly
high density and temperature. It is generally considered the best theory
of the origin of the universe. There is a formal theoretical model describing it,
which is supported by observations. And it is reasonably well understood by cosmologists.
But what is not well-understood is what led to the Big Bang. In other words,
what came before the Big Bang? Was there anything before this monumental event?
Or does the question itself defy our current understanding of physics and time?
We don’t have a formal theory on it. But that’s the question we
want to answer today. And in order to do this, we will need to take a speculative
journey beyond the beginning of time. Stay tuned because that’s coming up right now.
First, let's get our bearings. The Big Bang wasn't an explosion in space like
when a bomb detonates; it was an expansion of the space itself.
And it’s not clear that there was anything beyond this expansion.
So animations like the one you see here are a bit misleading because they are from the
perspective of someone outside the universe. Such a perspective may not have existed,
and if it did, it would be from another universe,
because the entire known universe was part of the big bang that you see in
the tiny spot here. But if you were somehow a witness to the Big Bang within the universe,
you would have just noticed a very rapid reduction of the density and temperature all around you.
This happened around 13.8 billion years ago, and since that time, the universe has expanded
and cooled to what we observe today. And this remarkable theory is supported by observations
such the expansion of the universe, first discovered by Edwin Hubble in 1929,
which suggests that the universe was much smaller in the past, and the cosmic microwave background
radiation which can be detected everywhere in space, first discovered by Penzias and
Wilson in 1964, which is the leftover of the shockwave of that primeval explosion.
Although we have quite a good understanding of the Big Bang, we really cannot say how or
why it occurred, let alone what came before it. General Relativity merges the concepts
of space and time into one fabric. So if the Big Bang was the beginning of space,
it was also the beginning of time. In a sense, asking what happened before the Big Bang is like
asking what's north of the North Pole. So this may be an absurd question to even ask.
What do we consider to be North of the North pole? Maybe that’s space? Well,
that’s not really more north. It’s just higher up in the air. The point is answering what happened
before is really a tricky thing, because it’s not even clear where you would start.
However, physicists are never content with unanswered questions, so have proposed several
theories. And let me stress, that what we’ll be talking about is not established science but is
an active field of research. So these are for now, some of the best guesses.
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One is the idea of a cyclic or 'bouncing' universe, where the Big Bang is just the latest
of many beginnings, in an eternal series of cosmic expansions and contractions. In such a scenario,
the universe begins from an initial tiny state, perhaps a singularity in which all the matter
and energy of the universe is contained in an infinitesimally small volume. The universe then
expanded, and after 13.8 billion years is at its current state that we observe. And
It will keep expanding for perhaps many more billion or even trillions of years,
and then at some point it will contract for another long period or time until it is tiny
again, back to a singularity. And then the cycle repeats itself over and over again for eternity.
In this scenario, the Big Bang for us would be just one of many cosmic beginnings and endings,
in an infinite an ongoing cycle. Following the logic of this hypothesis,
you could argue that time has an infinite past and will have an infinite future.
How likely is this hypothesis? Well, for this scenario to be possible, we would expect the
current rate of expansion of the universe to be slowing down at least to some degree, because
the expansion would need to stop and reverse at some time in the future. But observations indicate
the exact opposite. The expansion of the universe is not slowing down. In fact, it is speeding up,
that is, it’s accelerating. This is due to a mysterious phenomenon that we have labeled Dark
energy, for the lack of a better term. And there does not appear to be anything that can stop it.
But, here’s the caveat, we know the lambda cdm model, which is the standard model for the time
evolution of the universe, and which by the way, is based on a solution to Einstein’s
General Relativity, is incomplete. How do we know this? One of its most glaring problems is
that the model requires the existence of dark matter. The “CDM” in the name after all is an
acronym for “Cold Dark Matter” -- Cold means that the matter has to be composed of slow particles,
not fast like neutrinos. This means that the matter has to have a relatively high
mass. And dark because it’s invisible. It doesn’t absorb, reflect or emit light.
Observational evidence, such as the faster than expected rotation of galaxies, also seems to
confirm that an unseen halo of dark matter should exist to account for mass that can’t be seen.
We have known about the problem of dark matter for almost 100 years. And we have been searching
for it for at least 50 years. So far, we have found nothing. If we don’t find it,
or we learn that dark matter doesn’t exist, the whole CDM model is garbage. And if it is garbage,
then it’s quite possible that the universe could indeed collapse sometime in the future
based on some other model, not yet theorized, that might predict it.
Another interesting fact is that String Theory appears to hint at a cyclic model of the universe.
String theory, as you probably know aims to supersede our best model of particle physics, and
would solve the biggest problem in physics, the quantization of gravity. But there is no evidence
that either String Theory or the cyclic model is correct, so for now, both appear to be unlikely.
Another hypothesis about what came before the Big Band is the concept of a multiverse, where
our universe is just one bubble in a frothy sea of universes, each with its own laws of physics.
There are many types of multiverses, but this particular multiverse idea stems
from the theorized concept of eternal inflation. I made a detailed video
about what eternal inflation is here, in case you want to learn it in detail.
In a nutshell, the idea is that there exists an infinite spacetime that
is expanding faster than the speed of light. This is what we call inflation.
Inflation is believed to have occurred in our universe shortly after the Big Bang for
an extremely short period of time, perhaps for only 10^-36 seconds. But in this short time,
the universe expanded by a factor of 10^78 in size. The evidence
for inflation is the observation that our universe is homogeneous,
that is, it looks about the same everywhere in space, and appears to be geometrically flat.
Why inflation ended within the short time that it did, is not known. It’s possible
that on scales much larger than our observable universe, the universe is not so homogenous.
Since quantum mechanics ensures that there will always be some randomness, it’s possible
that inflation could last a bit longer or shorter than expected in different parts of the universe.
In the 1980s, Paul Steinhardt, Andrei Linde and Alexander Vilenkin realized that the exponential
expansion of cosmic inflation, although it stopped in our part of the universe,
could continue without end in other unobservable parts of the universe. And if that’s the case,
then the universe we are familiar with, may be a vanishingly small fraction of all that exists.
So here is the astonishing part. Let’s say, space expanded exponentially,
by at least 10^78x or more, and stopped in one part of an eternal universe. This could
be our universe, or any arbitrary universe. But it would be like a bubble in larger reality.
But space could continue to keep expanding exponentially
in another part of the universe, and after another 10^-32 seconds,
it stops in another half of the just expanded universe, forming another bubble universe.
So now we have two universes, but the space between these two universes keeps
expanding exponentially. And as parts of this newly created space stop inflating,
more bubbles form. But again the space between these bubbles keeps expanding exponentially.
In eternal inflation, since new space is being created all the time,
quantum fluctuations would ensure that new universes keep spawning in an ever-inflating
universe. This is why it’s called ETERNAL inflation. For all practical purposes each
of these parts where inflation stopped looks like a separate mini-universe,
or bubble, and is independent of what happens in other parts of the universe.
Our universe then would be a bubble of spacetime, in a preexisting primordial
spacetime of some kind that is forever inflating. Our universe would be nothing but a very tiny part
of an unimaginably larger whole. So the beginning of our universe would be of no
particular significance, like the formation of a molecule of water in an eternal ocean.
Theoretically, this is possible.
So far, the two theories we’ve talked about require a preexisting universe of some kind,
either an eternal cyclic universe, or a bubble universe in a larger reality. But
a third intriguing hypothesis outlines how our universe, starting with the Big Bang,
could come from nothing, if we define “nothing” the way physicists define it.
What am I talking about? One of the known characteristics of quantum
mechanics is that even in empty space containing no particles or any kind,
at the subatomic level, particles are popping in and out of existence all the
time. These are a kind of quantum fluctuation that results in virtual particles. They borrow
energy from the vacuum and give it right back so quickly that no conservation laws
are violated. Energy is conserved because the temporary imbalance is not even measurable.
Quantum mechanics, specifically the Heisenberg Uncertainty principle, allows this to happen,
as shown by this formula. Although we cannot measure these particles
individually, even if we tried, we can measure their collective pressure in something called
the Casimir effect which is evidence that this phenomenon does indeed occur.
Cosmologists have speculated that even in a universe where no matter,
space or time exists, as long as the laws of quantum mechanics exist,
spacetime itself could have emerged in a quantum fluctuation. How is this possible?
Well, in quantum mechanics, anything that is not forbidden by conservation laws necessarily
happens with some finite probability. If we live in a closed universe, like a sphere is
a closed universe, then all the positive energy of matter is perfectly balanced by gravity, which
has negative energy. This is similar to the way we think all the positive charge in the universe
is perfectly balanced by all the negative charge, making the overall charge of the universe neutral.
Just as virtual particles come in and out of existence without breaking any conservation laws,
a small empty space could come into existence probabilistically due to quantum fluctuations.
And since time is connected to space, time would follow in this nucleation.
When this happens, there are two possibilities. If this bubble of space does not expand rapidly,
it disappears again almost instantly, just like virtual particles. But if the bubble can
expand to a large enough size very quickly, then a universe is created in a way that is irreversible.
A scientific paper authored by 3 Chinese physicists, titled “spontaneous creation
of the universe from nothing,” was published in 2014, which showed a mathematical proof
of how this could happen. The idea is that while quantum fluctuations cannot usually be measured,
they can under certain conditions with inflation form fluctuations so strong that they materialize.
Essentially if you roll the dice enough times for near eternity,
you will eventually hit the jackpot and a universe will form. In this case what came before the
big bang would not really be nothing because quantum mechanics would be that “something.”
In other words, the laws of quantum mechanics have to exist in order for this to happen.
If this theory is correct, then there is no reason to think that this would have happened only once.
There is no reason that it couldn’t have happened multiple time, and the intriguing possibility,
unlike the first two hypotheses that I talked about,
is that we might not be completely isolated from other universes that formed the same way.
You’ll notice that none of these ideas makes our universe particularly unique or special
in some way. We are either one bubble in vast frothing sea of bubbles, or merely the most
current bubble in an infinity of bubbles that have come before us and will come after us,
You might think, wait a minute, we have all these models that describe what happened
fractions of a second
after the big bang so well. Why can’t we turn the clock back just a little bit further and
figure out what happened just a tiny fraction of a second before that? What is the problem?
The problem is that our current understanding of physics breaks down at the singularity,
the moment that predates the Big Bang. Beyond that singularity all our theories tell us nothing.
The laws of physics as we know them cease to apply. It's an informational boundary. This is
where new theories like quantum gravity may shed some light, and why its so important to pursue.
So we don't know for sure what happened before the Big Bang. But this uncertainty isn't a defeat;
it's a siren call to scientists, philosophers, and dreamers. Each unanswered question is a doorway
to deeper understanding, and a vault waiting to be unlocked. If you enjoyed this video,
then be sure to check out my short video on how a universe can come from nothing. The
link is coming up in 5 seconds. I’ll see you in the next video my friend.