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.