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.
