Hello. 
We now continue our quick overview of the 
history of cosmology. 
In the last module, we learned how 
expansion of the universe, observational 
discovery, and general activity which 
provided a theoretical framework for 
cosmology, really established it as a 
science. 
Something else however, happened in 1930 
that's of considerable importance. 
And that is the discovery of dark matter 
by Caltech physicist and astronomer Fritz 
Zwicky, who applied simple Newtonian 
mechanics to motions of galaxies and coma 
cluster. 
Their kinetic energy has to be balanced 
by the binding energy. 
And so he could deduce how much mass was 
needed in order to keep clusters 
together. 
So he found out that he needs 400 times 
as much invisible matter that does exert 
some gravitational influence that was 
actually seen in stars. 
Similar thing was found by another 
astronomer, Sinclair Smith, for Virgo 
cluster a few years later. 
But at that time, nobody took this 
seriously. 
Because this was just too outrageous 
statement and there was no other evidence 
for it. 
Until 1970's when sufficient evidence has 
accumulated that dark matter could no 
longer be ignored. 
And now, essentially, everybody believes 
in its existence for good reasons which 
we'll cover later. 
Dark matter is, today a key ingredient in 
the models of structure formation. 
But its exactly physical nature is not 
yet understood, and this is one of the 
outstanding problems of physics today. 
Meanwhile, observational astronomers 
following first, Hubble, but then his 
followers including Allan Sandage, tried 
to establish what kind of relativistic 
cosmological model we live in. 
Hubble designed a set of cosmological 
tests, how observations of distant 
galaxies can be used to determine local 
geometry. 
The foremost of those were so-called 
Hubble Diagram. 
Remember, that's the plot of relative 
distance versus velocity for galaxies far 
away from us. And near us, this is very 
close to a straight line. 
Now, as you go further deeper in space, 
relativistic effects start playing role 
and the line deviates from straight in 
way that depends on values of 
cosmological parameters. 
So, Sandage and his collaborators tried 
very hard to do this. 
Using brightest cluster galaxies is what 
they call standard candles, assuming that 
they're in general, luminosities 
constant. And that basically failed, 
because galaxies are not constant. They 
evolve, they're made of stars, stars of 
all the galaxies merge. 
And so until mid-90s, Hubble diagram was 
not considered as a serious cosmological 
test. But then, things change 
considerably. 
Galaxy evolution really became forefront 
of cosmology in 1970, or late 1970s and 
1980s. 
And this was in part with the realization 
that we must understand how constituents 
of galaxies of all, and therefore 
galaxies themselves before we can use 
them for cosmology. 
But its also containing number of 
interesting question so on and so on. 
And, studies of this were enabled by 
development of modern instrumentation, 
ending up in charge capital devices. 
And both of that was done, Palomar 
Directory. 
Here are some of the famous astronomers 
who developed early instruments to study 
distant universe. 
Meanwhile, on the theoretical front, Fred 
Hoyle, Hermann Bondi and Thomas Gold came 
up with a new idea called the Steady 
State Cosmology. 
Remember, the cosmological principle says 
that universe is same at all places and 
all directions. 
Well, they said also at all times. 
But because it expands, new matter has to 
be created in order to fill up the gaps. 
And mechanism for this was not specific 
and that was clearly seen as a weakness. 
But in that sense, universe will always 
look the same, keep expanding but always 
look the same. 
And there are cosmological test that can 
distinguish between those two. 
In part, this was trying to respond to 
the extrapolation of the galactic cosmic 
expansion to what now called Big B,ang 
which at least Hoyle found distasteful 
and give it the name. 
But, Big Bang Theory actually made some 
important predictions. 
George Gamow and his colleagues, Alpher 
and Herman, 
actually considered what one might 
thought was primitive atom and ask what 
would be the physics if universe is so 
hot and dense. 
And so, what universe will do is what 
stars do in their course, which is 
convert hydrogen into helium, heavier 
elements. 
And did the develop that along with Hans 
Bethe and called Alpher, Bethe, and Gamow 
Theory. But, it turns out that they can 
predict formation of elements all the way 
up to helium, in other words, just 
hydrogen and helium. 
However, there's really been an afterglow 
of this cosmic thermonuclear explosion 
which takes to the red shift stretching 
the photons now will not be gamma rays, 
but will be really microwaves. 
So that black body temperature of five 
degrees Kelvin or so. 
Well, this was actually measured in 1965 
by Penzias and Wilson, who deservedly got 
the Nobel Prize for this discovery. 
The cosmic micro background remains as 
one of the touchstones of cosmology. 
Today, we can measure it with space 
instruments, and its spectrum is as 
predicted by theory, pure black body 
radiation with which is now measured with 
exquisite precision. 
And might be actually the purest black 
body spectrum that we have measured 
anywhere. 
Another prediction of the Big Bang Theory 
is that the abundances of very liked 
elements. 
I said, you will only make helium, but 
that's not quite true. 
It will also make trace amounts of 
lithium and maybe a touch of baron and 
beryllium. 
But also, different isotopes of helium 
and hydrogen, deuterium, and helium 
three. 
So, here we have on the left plot of the 
helium mass fraction in star-forming 
galaxies, plotted against their oxygen 
abundance. 
Oxygen is only made in stars. 
So if both helium and oxygen were made in 
stars only, then this plot would be line 
going through the origin of zero point. 
But instead of that, there is an 
intercept. 
The zero point from which it starts is 
0.24 mass fraction of helium. And 
therefore, stars must begin with that 
much helium, and the only place that 
helium can come from was from primordial 
nucleosynthesis. 
Models of primardial nucleosynthesis have 
been developed with great precision and 
now they're compared with observations. 
The plot on the right shows theoretical 
predictions as functions of the density, 
Baryon density of the universe. 
And the blue band shows where they the 
measured the value is. 
So, it all is consistent with what we now 
believe are the right cosmological 
parameters. 
And this is also seen as one the great 
pieces of evidence in favor of the Big 
Bang cosmology. 
In the meantime, another important 
discovery happened, 
or set of discoveries really. 
After the World War II, thanks to the 
radars, 
radio astronomy was born. 
And radio astronomers start mapping the 
sky in radial wavelengths, seeing their 
sources with nature was at first unknown. 
And astronomers like Walter Baade and 
Rudolph Minkowsky obtained optical 
counterparts, some of these sources. 
And discovered some of them are actually 
quite far away, implying that given the 
observe flux internal power luminosity of 
these objects must be enormous. 
This was a very surprising and important 
discovery. 
Given the optical, signs of something 
interesting were already there. 
In 1940's Carl Seyfert wrote his PhD 
thesis, 
observed bright nuclei nearby spiral 
galaxies and found to have these somewhat 
unusual spectra with very broad emission 
lines. 
And their nature was not understood at 
the time. 
But it took really combination of optical 
and radio astronomy with identification 
of quasars to really drive his point 
home. 
Cyril Hazard was one of the radio 
astronomers who obtained precise 
measurements of few quasi stellar sources 
and their positions. 
Allan Sandage and others at Palomar 
obtained their optical counterparts, 
and Maarten Schmidt and his collaborators 
figured out what's going on. 
Namely, from the shift of lines in this 
spectrum, 
they figure out they must be very far 
away. 
Remember, the faster objects we see, the 
further away they are. 
Well, 
this implied enormous distances to 
quasars, which then implied that the, 
their internal luminosities are huge. 
So they have an object that may be ten 
times or hundred times or thousand times 
more luminous that entire galaxy of 
stars. 
And that luminosity comes from regions 
smaller than solar system. 
Maarten Schmidt made it to the cover of 
Time Magazine. 
I think he was the second astronomer with 
that honor, 
the first one was Harlow Shapley. 
Meanwhile, another important line of 
study was happening. 
Namely, discovery of the large scale 
structure of the universe. 
It really began in 1930's with Harlow 
Shapley, Zwicky, and their collaborators 
starting to map how galaxies clump in 
space. 
It was clear that they're not purely 
randomly distributed. 
And through the 50s,' the 70s' more 
evidence was accumulated and was obvious 
that galaxies are clumped in clusters and 
less dense but more extensive structures. 
However, the really most important new 
development was measurements of red 
shifts, 
which imply distances to vast numbers of 
galaxies. 
First thousands, but now there are really 
hundreds of thousands. 
Gerard De Vaucouleurs, who was a famous 
informational astronomer, also pointed 
out that our immediate extra-galactic 
neighborhood forms what we call the local 
super cluster of which will one part, and 
virgo cluster in the center with some 
elongated structure in the sky as seen in 
the projection here. 
And this was an indication of structures 
that are larger than anything seen 
before. 
So today, of course, we know this with a 
much greater precision, and here is the 
projected map of density of galaxies from 
the Sloan Digital Sky Survey, a modern 
perchet survey. 
And shows these filaments and voids and 
bubbles which are important features of 
the galaxy distribution that we will 
study later on.