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.