Hi,welcome back to the Cosmic Classroom.
Today I want to talk to you about the
Big Bang
and the Cosmic Microwave Background
Radiation.
But before I do so, I'll revise
something that we talked about already
the expansion of the universe.
And, if you feel like this is not enough
then you can go back and check that that
video before you come back to this
one.
So if you look at this first slide
that I have here,
it's a beautiful picture taken by
the Hubble Space
Telescope,
that shows many galaxies almost
everything you see in this picture is a
galaxy with exception
of a couple stars here, everything
else is a galaxy
and here
I chose three galaxies that look very
similar they look like face on spirals
and if you notice some of them, the
blue one appears bigger
the red one appears
smaller so it's pretty clear
that the red one it's probably further
away from us
than the blue one assuming they all
have the same
size
what Hubble noticed is that
galaxies that are further away from us
like the red one
are moving away from us at
fast speeds
while galaxies that are closer to us
are also movong away from
us at slower speeds
and that
is independent of the direction in the
sky did you look at
so Hubble measured distances to
galxxies and velocities
uh...
and uh... the galaxies velocity with
respect to us
and plotted
his very famous
diagram which is
called a Hubble Diagram or Hubble Law
showing that
for on average the recessional
velocities of galaxies increase as
they move
as they are further and 
further away from us
this is evidence for the
expansion of the universe
if you think about it the
universe is expanding
it was smaller in the past
being smaller in the past
is is evidence that there was a
time when the universe start
a time
of the beginning of the universe
a big bang time
so the expansion of the universe
is direct evidence for the big bang
and using this evidence
and and many others that I
won't to go in detail here,
we can
see determine the history of the
universe if we look at the other
slide that we have here
uh...
now the universe is full of galaxies up
here the universe is full of galaxies
and in the past
well the galaxies weren't very 
formed yet
and as the universe was younger and
younger and younger it was hotter and
hotter and denser
so go very briefly to this
phases
and then I'll talk to you about the
cosmic microwave background radiation
so the very first era
is what we call the
Planck Era
before the Planck time of of ten to the
minus forty three seconds we don't know
anything so we can say anything about
the universe before that time
but this is a pretty small time and
we can see a lot since then
so after the
Planck Era
there are there are a couple
other eras that I will
skip, but I'll just mention
GUT Era
and the Electroweak Era we can go
into more detail some other time
and then
Particle Era that's when
that's when protons start to
form
all the matter and anti-matter
annihilate
and we have
then we start to have particles form.
after that as the universe cools
down more
the atom the the nuclei excuse me
start to combine into atoms so this
is what they call the Era of Nuclear
Synthesis the very small era very
important era
because that's when protons
and the neutrons
combine together to start
the form
atoms
and after that
we continue to form
continue to form
elements
but it's still very hot and very
dense in this environment
the universe continues to
cool down
and until the age of three hundred
and eighty thousand years
the universe is so hot and dense
that no atom can stay stable the
atoms break apart
and and become fundamental
particles again
at this time
light that is emitted
because of particles are moving around
is trapped it can't escape
we say that at this time the universe
was opaque
there were particles they were forming
atoms and then they will breaking
apart again because it was
so hot and so dense
but the photon couldn't travel much
because the density was so high
that they were trapped
to interact with something else it's only
after this time at about three hundred
and eighty thousand years
that atoms
that the universe is cool enough that
the atoms form and remained as atoms
that allows the photons then to escape to
they are not trapped anymore and those
photons we can observe because they
escape that era
after that
there is the era of galaxies
that we live in
that live in today from an age of about
one billion years op
to today
so let's keep this in mind
and go back to that image
the three galaxies
one further away one further away one
at an average distance one closer to
us I can represent them as
three object that I believe to be
very similar
three candles but I believe to be the
same
well the first one appears smaller
because it's further away from us
the closest one appears to be a bigger
but there is more to that here
if we think about it the light
that left this red galaxy here
must have left the galaxy of very long
time ago
it...it.. because the galaxy so far
away from us the light
has been traveling for a very long time
until we receive it
the galaxy close to us well
the light
maybe left just
a billion years ago or so
and it's already reaching us
so what really happens is that we see
those galaxies at different ages at
different stages of evolution
the galaxy that's the furtherist way
we have seen it as it's was when it was
a baby, when the universe was first form
the galaxy that's close to us the
blue galaxy
has evolved a lot the
candle has burned
it's an older a
galaxy
and we see it now has more evolved
if we extrapolate this idea
and we keep looking at things that are
further and further away
we'll eventually get back
to that special time three hundred
and
eighty thousand years
in the life of the universe
where
the
photons started to be free and move
uh...
freely in space until they
reach us today
so if we move back even more
we see a picture like this
this is the cosmic microwave background
it's a baby picture of the universe
it's the most distance
the most distant
picture that we can take
because
light from before this time
wasn't able to reach us
it just scattered around and wasn't able
to reach us
so if we observe the sky
not in the optical wavelength like we
did for the pic..., the beautiful picture
of
of galaxy that I showed you
but if instead we observe it at
the different wavelength at the radio
wavelength
then what we see this is this baby
picture of the universe
where galaxies weren't formed yet
but do you see it that there's little
bit of structure
starting to form
this little bit of structure at least
that we see
is what will become
galaxies and clusters and so on
So, I hope that helps you understand what
cosmic microwave background is
I suggest you also check
my YouTube video on the expansion of the
universe which is very much relevant
to this subject
and there's uh... a free online textbook
about astronomy
that I
highly recommend
that you should also check.
see you next time.
