Edwin Hubble building upon
generations of work in astronomy
and lots of new evidence,
came up with a very simple idea
about the Universe,
and the idea was the Universe
is expanding.
But when you start thinking
about that idea,
it's really mind-boggling.
For example, what it means
is that everything
in the Universe-- every galaxy,
every star, every planet,
every atom in your body--
was squashed into a tiny space
probably smaller than an atom,
certainly smaller
than the smallest dot
you could make
on a piece of paper.
Now that was an idea so strange
that even many scientists
in Hubble's time
struggled with it.
But some scientists thought
the evidence was so compelling
that they started looking
at this very carefully,
and slowly, using logic
and the evidence available,
and sometimes new evidence,
they began to figure out
what might have happened
in such a Universe.
Hubble had already figured out
that if you could calculate
the speed at which the Universe
was expanding,
you could calculate
when it was formed.
Now think about it;
that's actually quite amazing.
It means he was saying
you could calculate
the Universe's birthday.
That's fairly amazing.
Some scientists then began
to think, "Can we figure out
what things might have been
like at the Big Bang?"
And they figured out
pretty soon that if you have
all the energy
and all the mass of the Universe
in one tiny space,
it had to be incredibly hot,
billions of degrees hot.
It also had to be very dense,
and it had to be expanding
so fast that it would have been
a bit like an explosion.
Now it was this image
that encouraged
an English astronomer,
Fred Hoyle,
who was always a skeptic
about this theory,
to describe this, jokingly,
as the "Big Bang" theory.
Well, he was being satirical,
but the name has actually stuck.
Then some scientists
began to try to figure out
what matter and energy
would be doing
under these extreme conditions.
They got a lot of help
because during World War II
a lot of people worked on
atomic weapons,
and atomic weapons are all about
extreme conditions.
Einstein had already shown
that under extreme heat
and temperature, matter and
energy are interchangeable.
They change into each other.
So this was the first thing
they found out.
At the very beginning,
the Universe must have been
a sort of blur
of energy and matter.
They also realized
that as the Universe expanded,
it would have cooled.
And they knew that
matter and energy
behave in different ways
at different temperatures
and pressures.
Slowly they began to figure out
the precise temperatures
and pressures
in the first few moments
of the Big Bang.
And in this way
they managed to construct
a good, logical,
evidence-based story
of what happened
during the Big Bang.
We can't explain the exact
moment of the Big Bang,
what happened before,
or why the Big Bang happened.
Cosmologists have lots of ideas
about this
but, frankly, no real evidence.
So Big Bang cosmology
can't do any better
than any traditional
origin story
in explaining why
the Big Bang happened
or what happened at the instant
of the Universe's creation.
But from a split second
after that
they can tell a very good,
evidence-based, logical story.
We believe that everything
appeared in the Big Bang,
including even time and space.
And at first things are
happening incredibly quickly.
We begin our story, in fact,
a billionth of a billionth
of a billionth of a billionth
of a second
after the Universe
first appears.
Everything's off the charts.
The Universe is gazillions
of degrees hot,
it's incredibly dense,
and it's expanding
as fast as you can imagine.
But as it expands it cools,
and as it cools
distinct forms of energy
begin to appear.
Four main forms of energy--
we call these the four
fundamental forces.
The first is gravity;
that's the force, remember,
that Newton identified.
It appears a billionth
of a billionth of a billionth
of a second after
the Universe is created.
Then electromagnetism appears;
that comes with positive
and negative charges.
And of course it's the force
we're all familiar with:
it's basically electricity.
Then we get the third
and fourth forces,
the strong and weak
nuclear forces;
these operate
over tiny distances,
but they bind the center
of nuclei together in atoms.
Now some of this energy
congealed to form
the first matter.
Remember, energy is what makes
things happen.
Matter is the "stuff"
of the Universe,
its basic
constructional material.
The first forms of matter
were probably quarks.
But quarks instantly
combined in triplets
to form protons--
which have positive charges,
electrical charges--
and neutrons,
which have no charges at all.
Protons and neutrons
will make up the nuclei
of all atoms.
Very quickly electrons
also appeared;
these are much lighter
than protons and neutrons,
and they have a negative charge.
But, despite the fact
that protons and electrons
have opposite charges,
they can't yet combine
because there's just too much
going on,
there's too much energy.
So we enter what scientists call
a plasma Universe.
All of this happened
in just a second or two.
The Universe is now
a mere ten billion degrees hot.
It's still very dense.
It's probably about
a hundred thousand times
as dense as a piece of rock.
So if I were to grab a piece
of the Universe
the size of this rock,
it would probably weigh
as much as 25 elephants.
The Universe we've seen
is also a plasma.
All the matter is in the form
of a plasma.
That's to say, it's dominated
by charged particles--
protons and electrons.
And because they're charged,
it's as if the Universe
was full of velcro,
and they sort of cling
onto photons of light,
photons of electromagnetic
energy,
as they try to pass through.
So the Universe is very
different from today's Universe.
Light cannot move freely
through it,
and you cannot form atoms,
which are the basic building
blocks of our Universe.
Then, about 380,000 years
after the Big Bang,
the plasma ends.
This is a very important
sort of mini-threshold
in the story for us
for two reasons:
first, when the plasma ended
you could form atoms;
and secondly, the ending
of the plasma
provided a powerful new piece
of evidence
for Big Bang cosmology.
Let's look at the first reason
why the ending of the plasma
is so important for our story:
the creation of atoms.
About 380,000 years
after the Big Bang,
the temperature of the Universe
has dropped
to about 3,000 degrees;
that's about the temperature
at the surface of cooler stars.
At that temperature,
the charges of protons
and electrons
are powerful enough
to bind them together.
So suddenly,
instead of a plasma,
the Universe fills up
with electrically neutral atoms
because those two charges
cancel each other out
in each atom.
Now let's pause for a moment
to think about atoms.
The first two types of atoms
we get are hydrogen
and helium atoms.
Hydrogen atoms have one
positively charged proton
at the center
and sometimes a neutron.
Helium atoms have two
positively charged protons
at the center
and usually two neutrons.
And whizzing around the centers 
in both types of atoms
we have electrons,
generally as many electrons
as there are protons,
which is why
the charges cancel out.
I'd like to read you a wonderful
description of an atom,
by Natalie Angier,
which will give you some sense
of its structure.
She writes:
ﾓIf the nucleus of an atom
were a basketball
located at the center of Earth,
the electrons
would be cherry pits
whizzing about
in the outermost layer
of Earth's atmosphere.ﾔ
So that's the sort of image
of atoms you should have
in mind
when you think about them.
Now because atoms are neutral,
suddenly photons of light
can move freely
through the Universe.
The velcro's gone;
they don't get tangled up
with charged particles.
And that leads
to the second reason
why the ending of the plasma
is so important for our story.
It provided great evidence
in support
of Big Bang cosmology.
Back in the 1940s,
some scientists had already
figured out
that as the Universe cooled
there'd be a moment
when suddenly all the matter
went electrically neutral,
and at that point
photons of light
would be able to move freely
through the Universe.
And they figured out
there'd be a sort
of flash of energy,
and some even said,
"Why not look for that flash?
It'll be powerful support
for Big Bang cosmology."
But, strangely,
no one went looking for it.
And that's probably a sign
that most scientists
still regarded the idea
kind of skeptically.
Then, in the 1960s,
two astronomers--
Arno Penzias
and Robert Wilson,
who were trying to build a
very sensitive radio receiver--
suddenly stumbled upon
this flash of energy.
They're pointing... wherever
they point their radio receiver,
suddenly they've got
this sort of hiss of energy,
it comes from everywhere
in the Universe,
and it's extremely uniform.
Now think about it for a moment;
that is very strange.
If you point to the Universe,
and you point towards a galaxy,
you expect to detect energy.
But even empty space?
That was really weird.
And at first
they couldn't understand it.
Then they talked around
to one or two astronomers,
and finally someone said,
"I think you've found
"the flash of energy
that was predicted
back in the 1940s."
It's a very exciting moment
in science.
Now this was extremely powerful
evidence in support of
Big Bang cosmology
because what it supported
was a very strange prediction
made back in the 1940s.
And no other theory
could explain why
there should be this energy
or where it could come from.
And that's the moment
at which most astronomers
finally decided, yes,
Big Bang cosmology is real,
it's telling a real story
about the real Universe.
Since then, many other forms
of evidence in support
of Big Bang cosmology
have appeared,
but still today
Hubble's evidence
and the evidence of the cosmic
background radiation
are the most powerful
single pieces of evidence
to support Big Bang cosmology.
The story we've just seen
is one we're going to see
over and over again
in the history of science.
Someone comes up
with a new claim about reality,
and it's based on logic
and it's based on evidence,
but there's not quite
enough evidence.
So people around them
treat it as interesting
but don't take it
terribly seriously.
And then, gradually,
new evidence appears,
and at a certain point
suddenly everyone thinks,
"Oh yeah, I think this is the
way things really happened,"
and then their claim
becomes a new orthodoxy.
You're going to see it
over and over again
in this course.
Now I'd like you to think
about the story
we've just been telling.
It's actually amazing.
We humans,
by sharing information
over many generations,
have slowly constructed
a good, powerful,
evidence-based story,
not about what happened
ten years ago,
or a hundred years ago,
or even 10,000 years ago,
but 13.7 billion years ago,
at the moment the Universe was
created.
Now, I don't know about you,
but I think that is quite
mind-blowing.
Okay, so let's sum up:
The Big Bang created
everything around us,
all the matter and energy.
And so it created
the foundations for building
further complexity later on.
And that's why it counts
as the first major threshold
in our course.
After all,
the move from nothing,
before the Big Bang,
to something,
after the Big Bang,
has to count as an increase
in complexity.
