And now, what we're going to see is, is that those perturbations, the
tiny ripples in the gas, left, you know, that, were there from, recombination.
Or, sorry.
Were there from, actually, the era of inflation.
And have, you know, been trying to grow with time.
Now the, fact that you have more matter in one region than in
its surrounding, allows gravity to begin to pull that matter in on itself.
The grav-, its own gravity.
Its self gravity actually can take those tiny ripples and begin to amplify them.
So we now see something really remarkable happening, which
is the beginning of what we call cosmic structure.
So [COUGH] ] what we get is, those perturbations
growing in time, becoming over-dense filaments compared to their surroundings.
And then within those filaments we actually
get, you know, there's ripples on ripples.
We get individual places that are really beginning to
accumulate matter, and they will become the first stars.
So somewhere around a z of 20 or so we
begin to see, and z, remember, is the red shift.
The cosmic microwave background the recombination occurs at around
z of 1000 or so or z of 2000.
We are now at z of zero, so looking back in time z increases.
So at around z of 20, we see the first generation of stars begin to form.
And these are very different than anything
that's around today because there were no metals.
There's no, what we call metals, there's no heavy elements.
And heavy elements really control a lot, even though there's not many of them
in a star today, they actually control a lot of the behavior of the star.
But these first generations of stars are really just hydrogen and helium.
And they do what stars do.
They start fusing.
But because there's no heavy element, they lead very different lives.
And in particular, many of them are huge, right?
It's very rare to find the star more than 100 solar masses
today, or the star that has more mass than 100 times the sun.
Whereas, probably they were quite common, these
behemoth were quire common during the early universe.
So we, simulations show us that we have lots of stars
between 100 and even 300 times the mass of the sun.
And these stars are very big, they're very bright.
And they produce ionizing radiation, ultraviolet photons.
And those ultraviolet photons go streaming out into the environment.
And they ionize the gas, the gas of neutral hydrogen.
Remember, that we started to get neutral hydrogen
at recombination about 300,000 years after the Big Bang.
And here we are, you know, a few tens of
millions of years after the Big Bang, and what we
see is, the newly born stars beginning to reionize, or
beginning to turn that hydrogen gas back into electrons and protons.
And that's going to be very important because right now, the universe is
pretty much the space empty space, well, it's not quite empty, of course.
But, you know, interstellar, intergalactic space is full of ionized hydrogen.
So there's only this brief window that we call
the dark ages, between recombination and these first stars forming.
That the universe was actually mostly neutral hydrogen.
Okay, so these stars form and then they go supernova, so they're starting
to blow some of the processed elements that were created inside those stars.
The heavier elements that are forming inside of
stars, that we talked about in week two.
They're starting to now seed the universe with heavier elements.
So that's important.
So it's from this point onward that maybe you
get the possibility of forming things like planets and such.
