Katie Mack, astrophysicist: When we talk about plasmas and things evolving later in the universe, there's a process called recombination where something becomes neutral again after having been reionized.

John Green, very curious: Mm. Mhm.

Katie: But anyway it- it's- it's called recombination but it's this is this first moment when when neutral atoms are are able to form and so that recombination era that moment when the first neutral atoms form that begins what we call the dark ages of the cosmos

mhm okay and the reason is called the
Dark Ages is because now the primordial
plasma has cooled down but there's no
stars yet the universe is just hydrogen
gas mostly with a little bit of helium
in it and it's just cooling it's just
gas cooling down and the universe is
expanding and at the same time you know
gravity is still pulling together clouds
right so you have these clouds of cold
gas and those clouds are are starting to
condense so the Dark Ages goes on for
for a while
wait like like two seconds or like 10
years no like like like millions and
millions of years oh great okay yeah hey
we cannot have 10 to theg 35 seconds and
then like 10 to the5 seconds being a
while and then have like several million
years be a while this is the thing right
like the the time scales get weird right
because you count it based on how much
is happening and and you can have
millions of years years of cold hydrogen
gas and not a lot is happening in that
cold hydrogen gas this is great for us
Katie because okay suddenly it's looking
like we can get through this season I
I'm starting to believe in us we just
skipped way ahead yes exactly yes all
right so we've got these millions of
years where there are some neutral atoms
but it's just clouds of gas slowly
clumping together slowly getting colder
yeah exactly so yes so this is the
Cosmic Dark Ages and the physics of it
is very simple it's there you can write
down equations for gas Cooling and it
just it's just doing that but over time
because we had these little variations
in density some of the clouds of gas
start to get a little bit more dense
than other clouds of gas and so whoa
whoa whoa whoa whoa if we'd never had
these little variations in density if
our observable universe had been
entirely uniform there never would have
been stars in
galaxies oh um I don't know that they're
never well you would still have
fluctuations just based on the random
movement of particles So eventually
something would happen but it would it
would happen differently it would take a
lot longer okay that's helpful and a
little mind-blowing yeah okay all right
so we're we're only here in this current
state because of those early Quantum
fluctuations that came along with
inflation yes yeah otherwise I wouldn't
be be here yeah yeah this is this is why
there is structure in the universe in
the way that we great yeah okay I'm
going to not panic I'm just going to I'm
just going to listen
John: Okay that was a clip from The Universe, a new limited series podcast from Crash Course where Dr Katie Mack, a theoretical astrophysicist, walks me through the entire history of the universe including the parts that haven't been written yet. It's available now both on the Crash Course YouTube channel and wherever you get your podcasts.