It’s Professor Dave, let’s talk about cosmology.
While astrophysics is the field of science that studies the universe and everything in
it, the subfield of astrophysics that specifically studies the origin and development of the
universe is called cosmology.
That means that if we are going to start our story from the beginning, we have to start
with cosmology.
How did the universe begin, and when?
Early civilizations had all kinds of mythology surrounding this question.
But ever since the 20th century, we have begun to answer it not with stories, but with science.
Actual empirical evidence that provides clues as to how it all began.
And what does this evidence tell us?
Overwhelmingly, it tells us that the universe was born around 13.8 billion years ago, from
a single point.
Scientists and laypeople alike refer to this event, the beginning of our universe, as The
Big Bang, a name coined by astrophysicist Fred Hoyle in an attempt to criticize the
model, but ironically it stuck.
Many people are incredulous of such a big bang, considering the whole idea to be completely absurd.
But the main reason for this is nothing more than a cosmic gap in understanding of principles
in physics and astronomy.
When most people imagine the Big Bang, they think of a loud kaboom, a cartoon explosion
graphic, and the present-day, fully-formed universe spilling out of it, with a comet,
and the planet Saturn, and other complex objects.
This would indeed be impossible to believe, and in fact, it could not be further from
the truth.
In this tutorial we will describe, in a basic way, our current model for the birth and early
development of the universe, as understood by modern cosmologists.
Unfortunately, we will not yet be ready to discuss the evidence that supports this model,
because understanding that evidence will require a discussion of galaxies and many other objects
that we will get to later in the series.
So for now, try to simply take this model at face value, and I promise that later on,
we will examine several independent threads of evidence that all corroborate this model
of origin and specific age for the universe to great precision.
So now, let’s start at the beginning.
To be fair, at present, we don’t know what happened at the beginning.
By the beginning, we mean t equals zero.
The first instant.
Our understanding begins to take shape at around 10 to the negative 36 seconds after
the big bang.
That’s a trillionth of a trillionth of a trillionth of a second.
From that moment forward, we start to have an increasingly solid grasp of how things
must have gone, which is pretty darn impressive when you think about it.
As for the imperceivably small increment of time before that, we must admit that our current
models just don’t work.
We will still talk about this earliest period here, but to be clear, it will be largely
speculative, including some of my own personal conjecture.
But don’t worry, we will quickly get back to the real science.
I’m already exhausted.
Let’s just start at the first moment, and picture blackness, since that’s kind of
the best we can do.
Now, the first thing, time zero.
Freeze the clock.
A single point.
The uncaused cause.
How could this have happened?
Let’s first recall some things about the Heisenberg Uncertainty Principle.
This tells us that when looking at complementary variables, like energy and time, the more
certainty associated with one, the less certainty that can be associated with the other.
This is what allows for quantum fluctuation, the very real and very measurable phenomenon
by which particles pop into and out of existence, all over the universe and at all times.
This means that the idea that something could simply appear is actually not without precedent.
Of course, to go from virtual particles to the entire universe is quite a leap, for a
number of reasons, but if you really think about it, with nothing else in existence yet
to be compared to, would this fluctuation have been large or small?
Is it a lot of energy or a little?
In comparison to what, precisely?
What if, taking into account both positive and negative energy, the net energy is very
close to zero?
Then the entire universe could be regarded as a quantum fluctuation, borrowing energy
it will pay back later.
If we take for granted that without any other frame of reference, a quantitative amount
of energy appearing from nothing can be of any arbitrary amount, then we are acknowledging
that the appearance of some energy from no energy is the only thing we must explain.
It is the emergence of an initial duality.
Plus and minus.
Yes and no.
Whatever you want to call it.
It is not planets and stars tumbling out of a kablooey graphic.
It is the emergence of the simplest possible thing that is a thing, rather than no-thing.
Everything else follows from there.
Now we get to 10 to the negative 43 seconds after the big bang.
What do we know about this time?
Still not much.
But if we recall some things about the standard model of particle physics, experiments in
particle accelerators have allowed us to begin the incredible task of unifying the four fundamental forces.
These are the electromagnetic force, the weak nuclear force, the strong nuclear force, and gravity.
In that earliest period that we know almost nothing about, also called the Planck epoch,
given the universe’s minuscule size, the temperature of the universe must have been
over 10 to the 32 Kelvin, which is nearly a billion trillion trillion degrees, far too
hot for familiar particles to exist, and all four forces must have been unified into one
single force.
The search for quantum gravity is the search for the quantum particle that would mediate
this singular force, thus it is sometimes called the search for the theory of everything.
Particle accelerators can’t yet achieve this incredible energy, so we must hope for
bigger and better technology.
But in the next epoch, from 10 to the negative 43 seconds until 10 to the negative 36 seconds,
also called the grand unification epoch, temperatures cooled down to 10 to the 29 Kelvin.
This allowed for gravity to decouple from the other three forces, which can be collectively
referred to as the electrostrong force, and which we believe could potentially be described
by a grand unified theory.
This act of forces breaking off from other forces is the result of symmetry breaking,
a phenomenon that can occur when extreme temperatures cool below certain transition temperatures.
In this way, we can understand that all the disparate fundamental particles in the universe
were once part of the same thing, that only manifested as different objects as a result
of a series of successive symmetry breakings while the universe cooled.
Next, we enter the earliest era for which theoretical physics has some reasonable basis,
taking place from 10 to the negative 36 seconds until 10 to the negative 32 seconds after
the Big Bang.
This is called the electroweak epoch.
It is marked by the decoupling of the strong nuclear force from the electrostrong force,
leaving only the electromagnetic and weak nuclear forces together, which we call the
electroweak force.
This is possible now with the universe having cooled to a frosty 10 to the 28 Kelvin.
Roughly concurrent with the electroweak epoch is what we call the inflationary epoch.
This was a brief period in which the universe expanded by an incredible factor, around 26
orders of magnitude, triggered by the separation of the elecrostrong force into the strong
nuclear force and electroweak force.
We don’t know the precise size of the universe before and after this phase, but the magnitude
of the expansion would be like inflating something the size of a small molecule up to something
ten light years across, or about 60 trillion miles.
This nearly instantaneous expansion dispersed the earliest fundamental particles around
this much larger volume quite evenly, after which the immense potential energy from the
inflation was released, producing a hot plasma of quarks, anti-quarks, and gluons.
Plodding along to around 10 to the negative 12 seconds, or one trillionth of a second
after the Big Bang, we enter a period called the quark epoch.
Here, things have cooled enough for the third and final symmetry to break, decoupling the
electromagnetic and weak nuclear forces, thus resulting in the four distinct forces we know today.
As a result, the Higgs field bestows existing particles with mass for the first time, but
things are still too hot for protons and neutrons to form.
This is also the highest-energy epoch that we can currently probe with particle accelerators,
which means we are now transitioning from theoretical cosmology to experimental.
At around 10 to the negative 6 seconds after the Big Bang, things finally cool down enough
for the quark-gluon plasma that permeates the universe to congeal into hadrons, which
are particles made of quarks.
This includes baryons like protons and neutrons, which will eventually make up all the atoms
in the universe.
This is called the hadron epoch, lasting until one full second after the Big Bang.
The action will slow down a bit from here, but before we move on, take a moment to imagine
how much has happened in just one second, the void yielding merely some energy and a
singular force, which sequentially breaks into four forces, in turn yielding a sea of
massive particles.
From one second to ten seconds after the big bang lasted the lepton epoch.
Hadrons and antihadrons largely annihilate leaving leptons and anti-leptons to dominate,
which in turn also largely annihilate, and thanks to a slight asymmetry in favor of matter
over antimatter, this leaves just a fraction of the original matter behind.
From here, the start of the photon epoch, things start to look a little more familiar.
For about seventeen minutes, the universe is cool enough for baryons to be stable, but
also hot enough for them to fuse, so protons and neutrons fuse together to make lots of
hydrogen and helium, and trace amounts of other light nuclei, an era called big bang
nucleosynthesis.
After that, it gets too cold and sparse, so fusion halts, locking the universe into a
three-to-one ratio of hydrogen to helium by mass.
At this point, the universe is about 600 light years across, so we are no longer dealing
with a tiny universe.
Over the next several hundred thousand years, as the universe continues to expand, all of
these hydrogen and helium nuclei begin to collect in little patches, due to the effects
of gravity.
This force, now understood through Einstein’s general relativity to be the warping of spacetime
by massive objects, attracts all objects with mass towards one another.
So by virtue of the mass of these nuclei, and more so, all the dark matter around the
universe, which we will discuss later, the universe takes on a sinewy, filament-like structure.
Denser regions become more dense, and empty regions become more empty, as the photon epoch
draws to a close.
During the next era, recombination and photon decoupling, about 377 thousand years after
the big bang, things are finally cool enough for electrons to combine with nuclei to form
neutral atoms for the first time.
Electrons are captured, relax down to the ground state, and emit photons in doing so.
This marks the first time that the universe is actually visible, in the sense that we
consider something to be visible to our eyes.
It is no longer opaque, but transparent, with electromagnetic radiation now moving freely
over large distances, and with a diameter for the universe of nearly 100 million light
years, the distances are large indeed.
Then for around 150 million years, not much happened.
We call this era the dark ages.
There was plenty of hydrogen and helium around, and photons were traveling everywhere, but
there were no stars to produce all the light we see in the night sky today.
Things continued to cool, from around 4,000 Kelvin, down to 300 Kelvin, which incredibly
would allow for liquid water, if any were to exist, all the way to around 60 Kelvin,
a temperature that is finally cold enough by human standards to somewhat resemble the
cold outer space we normally conceive of.
But slowly, ever so slowly, all the clouds of hydrogen and helium gas continue to collect.
Over millions of years the minute gravity exerted by these particles, combined with
the more significant gravity exerted by surrounding dark matter, pulled matter together into clumps,
little dense pockets of matter.
Closer and closer, until all the atoms are pushing right up against one another.
What happens when you get enough atoms all together in the same place?
Ignition.
So let’s move forward and see what happened next.