Hello, again. 
Let us now review briefly the history of 
modern cosmology. 
Before we could study the universe as a 
whole in scientific sense, we actually 
discovered and understood galaxies which 
are its major constituents. 
At first, people didn't know what they 
were. 
They were just as much in sky, they were 
called nebulae, and famous catalog by 
Chales Messier. 
And then, William Herschel also cataloged 
many thousands of them, and followed by 
others. 
Now 
even before that was really properly 
understood, philosophers tried to address 
the question. And they talked about 
island universes, or what today we would 
call a galaxy. 
They had no scientific reason to believe 
one way or the other but it was an 
interesting speculation. 
Now, in 19th and 20th century, more 
catalogs or galaxies were produced but 
there was still no understanding. 
For example, the very basic question was, 
are these nebulae, 
Galaxies like the Milky Way or are there 
just some smudges within our own galaxy, 
the Milky Way, which is the universe as a 
whole? 
So, up until 1920s, the early 1920s, this 
was still an unsolved question. 
And there was a famous great debate 
between two astronomers, Harlow Shapley 
from Harvard, and Heber Curtis from Lick 
Observatory, 
whether or not galaxies or a nebula, I 
should say, are island universes just 
like the Milky Way, or are there just 
some smudges of gas inside the Milky Way? 
The debate was inconclusive. 
Shapley had wrong answer, 
that Milky Way is all of it. 
Curtis was advocating that galaxies are 
many like the Milky Way. 
And arguments were not really based on 
any solid experimental data. 
This all changed with Edwin Humble in 
1923 on using Mt. 
Wilson Observatory. 
He obtained photographs of Andromeda 
galaxy and found a variable star, 
so-called surface in Andromeda. 
Comparing its brightness to brightness of 
surface in our galaxy, immediately told 
us that Andromeda is much, much further 
away than anything in our galaxy. 
Within our galaxy, people knew about 
stars that are clusters that are 
kiloparsecs away. 
Andromeda turned out to be several 
hundred kiloparsecs away. 
Therefore, it was galaxy just like the 
Milky Way, and then all the others which 
are much fainter, presumably even further 
out. 
So, all of a sudden, the picture changed 
from Milky Way being the entire universe 
to a much, much bigger universe, 
populated with galaxies like Milky Way 
and others. 
This is how Hubble really became famous. 
He did something else later on, 
discovering expansion of the universe. 
But, before we get into this, theory 
actually made some important advances. 
If you want to understand universe at 
large from physical terms, the only 
interaction that actually matters is 
gravity. 
Because all other forces are short range 
except for electromagnetic force, 
but charges are mixed so well, positive 
and negative ones, so that net 
electromagnetic field is pretty much a 
zero in any average sense. 
Not so with gravity. 
Gravity cannot be cancelled or 
compensated. 
and so, if you want to understand how 
things happen on large scales, you need 
the theory of gravity. 
This came in the form of theory of 
relativity, and in particular general 
theory of relativity. 
So, Einstein came up with the way that 
describes that, 
and we'll talk a little more about it a 
couple lectures from now. 
But he immediately understood that this 
can be actually applied to study of the 
universe as a whole and even taught a 
class about that from 1919, this was 
before it was actually proven that the 
theory was correct. 
He assumed that it was just normal matter 
like stars and so on. But it's finite in 
spatial size, and we'll talk about how 
that's possible. 
In 1917, Einstein made first cosmological 
models. 
He believed in his theory, of course, 
even though it was experimentally proven 
only in 1919. And, he had something 
interesting. 
He, he found out that universe has to 
either collapse on itself under its own 
gravity, or maybe expand forever. 
Collapse under its own gravity seemed 
like a natural thing, and in order to 
balance that, he introduced a force 
called cosmological constant. 
This turned out to be a very unstable 
cosmological model, and it's wrong 
anyway. 
at the same time, Dutch physicist Willem 
De Sitter developed similar model and ob, 
obtained equations for expanding 
universe. 
Now, later on, two of them came up with 
the different model in the beers names 
but that is a different story. 
Back to observations, discovery of the 
expanding universe was probably one of 
the greatest scientific discoveries of 
all times. 
Two people need special credit here 
especially Vesto Melvin Slipher who was 
an astronomer at Lowell Observatory in 
Arizona who measured velocities, radial 
velocities, 
of galaxies. 
And obtained the data that were much 
later that were later used by Hubble. 
That's roughly same time Knut Lundmark 
and Carl Wirtz in Sweden and Germany 
obtained similar results. But, their plot 
of velocity versus estimated distance to 
the galaxies didn't show anything. 
A little bit later, 
Edwin Hubble plotted distances to 
galaxies which are measured little better 
using relative brightness as a measure of 
distance, against velocities most of 
which are obtained by Slipher, 
and found this remarkable trend that the 
further away galaxy was the faster it was 
going away from us. 
This is now known as the Hubble diagram 
and was immediate evidence for an 
expanding universe. 
Here is a brief explanation of the 
expansion of the universe. 
Imagine just a piece of the volume of the 
universe populated with galaxies, more or 
less uniformally distributed. 
If the space expands in itself, it 
carries the galaxies apart. 
And, the further apart from, from each 
other they are to begin with, the larger 
the distances there will be next time. 
So, since the, the growth in the distance 
between any two galaxies is proportional 
to the distance itself, and its time 
derivative is the velocity. 
So therefore, you get the velocities of 
recession from galaxies from each other 
is proportional to their distance which 
is Hubble's Law. 
Now, Einstein go to see this for himself. 
He visited Pasadena numerous of times in 
1930s. 
Here, he is on Mt. 
Wilson with Edwin Hubble and Walter 
Adams, another famous astronomer, looking 
though an eyepiece of the 100 inch 
telescope. 
This was completely fake and posed 
picture because that's not what 
astronomers do. 
Allegedly, Einstein declared failure to 
expan, the, the to predict expanding 
universe as the greatest mistake of his 
career. 
And remember, he was, invented 
cosmological constant as a means of 
preventing the expan, collapse of the 
universe for that wasn't really needed. 
He failed to predict expansion of the 
universe, 
wven though that was implicitly contained 
in his equations. 
And that was probably the greatest 
scientific prediction anyone could make. 
So, this is why he wasn't so happy. 
In the meantime, two theorists really 
developed modern relativistic 
cosmological models. 
Alexander Friedmann in Soviet Union 
developed relativity-based expanding 
universe models. 
Then, he came up with the equation that 
bears his name that we will use very 
heavily. 
Then, roughly at the same time or few 
years later, George Lemaitre in Belgium 
developed similar cosmological models. 
And also, he said, okay, if the universe 
is expanding now what if we just look 
back. 
It all must have started with very dense 
state which is probably very hot and he 
called us the Cosmic Egg. 
And so, 
that was essentially a first notion of 
the Big Bang, but it was not taken very 
seriously because he was little too much 
of a extrapolation and he was a Jesuit 
priest and you know that sounds a little 
suspiciously like biblical creation. 
So, that was forgotten for a little 
while. 
Then in 1930's, relativistic cosmology 
really started to flourish. 
And here are some of the more important 
contributors to it. 
Milne developed a model that is based on 
special relativity. 
It includes no matter. 
It's just the dynamics of space and time 
itself. 
And Eddington who was the person who'd 
actually proven that general relativity 
is right promoted their own models and 
tried to think about the interfaces, 
interfaces between quantum theory and 
relativistic cosmology. 
Robertson and Walker are two 
mathematicians who developed a 
mathematical description that will 
actual, that's actually still being used 
in explaining or describing expanding 
universe.