Welcome to the Coursera class on Galaxies & Cosmology.
My name is George Djorgovski and I am a professor of California Institute of
Technology. These classes offered to the second year
physics and astronomy students of Caltech,
therefore, it requires certain amount of preparation and we'll come to that in a
moment. But first of all, let us define what is
cosmology. It is the science of the universe as a
whole and its major constituents, how they work, and how they evolve, how
they form? It is different from cosmetology and if
you made that mistake, this'll be a really good time to stop.
Well. It's not an easy task.
Universe is very big and we can not reach it all.
We can just sit here and watch and so we have to make some assumptions.
The basic assumption that we make is that laws of physics are same everywhere and
at all times. This is a very reasonable assumption and
if it weren't true, it would be very hard to do any kind of science.
But we can actually test some aspects of this proposition.
Moreover, sometimes, while studying the universe we discover things in physics
that we didn't know about. notable examples are nature and existence
of dark matter and dark energy and we'll talk about them later in the class.
Unlike most other sciences, cosmology has only one object to study.
There is only one universe in which we live.
And all we can do is sit and watch or in technical terms, we can look at the
surface of the past light cone. So there are parts of the universe that
are a priori, not observable to us. We pretty much sure that they do exist
and they're probably not very different from the one part that we do see, but
it's something to keep in mind. And also, things are far away and
therefore, or they're faint and small and we need most advanced technology to do
this. This is why cosmology really flourished
as a science in the latter part of the twentieth century,
because before then we simply did not have good enough tools to do it right.
But even so, we have to be always aware of possible biases and selection effects.
For example, there could be some very faint galaxies that we're missing.
Cosmology responds to the basic human need to understand the big picture.
Where is it all coming from? How does it work?
Where's it going? And it evolve through time.
In a pre-scientific days, creation myths were made to answer these questions, but
they really were just made up stories. starting from beginnings of modern
science in the Renaissance and the Enlightenment,
things started to get little more reverse.
First, the Copernican revolution which was a major shift in the way people think
about universe or the world. That positive that we're not at a special
place. The earth is not the center of the
universe, it's just a random spot. Cosmology actually uses this in so called
cosmological principle that we'll define in a bit.
But to, not only that, universe is not static and unchanging, we see the change.
And just like biology and planet earth, universe evolves on its own and following
laws of physics, major constituents of the universe, galaxies, stars and them
have also evolved and we can understand how that works.
And just like anything else in science, it became subject of study and therefore
it is always improvable. We are not declaring anything with
absolute certainty, but we're actually trying to understand what's going on.
And like any other science, other science, the more we push further, the
more we learn, the more new questions we open up.
And that's the nature of science and this is very good.
So, where, really cosmology started as a
branch of astronomy, astronomy as a whole and certainly cosmology became a branch
of physics. Because we use, laws of physics and
principles of physics to obtain our measurements to understand them to
interpret them make predictions. Alright, now let us find out how well
prepared you are for this class. First we'll do few quizzes.
Let us now resume our journey to the universe and dust off some of the
astronomical units of measurement. The most basic unit in astronomy and
cosmology is distance to the, from the sun to the earth.
It's one astronomical unit and it's about 150 million kilometers.
There is of course the light year which is the distance that light travels over
one year and one light year is approximately ten to eighteen cm.
Now astronomers almost never use light-years.
We use parsecs, which is the distance from which one astronomical unit is seen
as an angle of one arcsecond. And that turns out to be roughly 200,000
astronomical units or 3 * 10^18 centimeters.
So when we talk parsecs or kilo parsecs and mega parsecs and giga parsecs, you
can multiply that by 3.26 to get number, corresponding number in light years.
Two basic properties of objects that we need to understand are masses and
luminosities. And for convenience and historical
reasons, we use solar mass as a unit. And that is approximately 2 * 10^33 grams
and solar luminosity which is close to 4 * 10^33 ergs/s.
So you can convert them to other units as you need to.
You should really remember these numbers, because we'll constantly refer to
distances and masses and luminosities using these units.
Now what we observe are fluxes. We mostly observe electromagnetic
radiation, although we now have forms of astronomy that are not dependent on
electromagnetic radiation like cosmic rays, neutrinos and [INAUDIBLE] or
gravitational waves. But flux is what's measured and we have
detectors that can operate in full range of wavelengths from radio to gamma rays
and they are usually measured over some finite bandpass.
So, spectral energy distribution which is the shape describes the spectrum of an
object is defined to be energy per unit time per unit second per unit for
frequency or wavelength and we never observe that, that's a differential unit.
Instead of that, it's always observed or some finite bandpass like optical filter
or bandwidth in [INAUDIBLE] astronomy. One unit that is often used is jansky
which was introduced by radio astronomers and it's 10^-23 ergs per second per, per
square per hertz. Optical and [INAUDIBLE] astronomers are
beginning to use janskies as well although, we are typically talking about
microjanskies and nanojanskies. So this is often called flux density and
get really the power of object. One has to integrate it over all
bandwidth and then multiply it by the area that's correct.
Astronomers use magnitudes which are a logarithmic measure of flux defined by
the formula written here. The magnitude is -4 decibals.
The minus sign tells you that the higher the number, means the lower the flux.
And because its a log, its a relative measure of flux relative to some unit
flux. In log that comes as a additive constant.
Typically, flux is measured over some finite bandpass like B-band filter
centered on 5500 angstroms and then log of that times -2.5 plus a constant zero
[INAUDIBLE] second gives you the actual magnitude.
If for some reason you could integrate oh, the flux over the entire spectrum,
then this would be called the bolometric magnitude.
Now, magniute zero points are another part of astronomical craziness that's
unfortunately well established and hard to change.
For historical reasons, again, Vega which is alpha Lyrae was declared, declared to
have zero magnitude and then everything is measured relative to it.
Notice that because we're talking about logarithms of fluxes,
talking about ratios of fluxes, and so it's all relative to some unit.
Well, that unit for magnitude systems is usually Vega.
And unfortunately, its spectrum is not flat,
it looks like this. There is also a more rational kind of
magnitudes that's been introduced. It's called AB new magnitudes, which are
with the fixed zero point as the formula on the bottom shows.
But typically, people use Vega based magnitudes.
And it's a handy way to remember it that zero magnitude star corresponds to almost
1000 photons per square centimeter per second per angstrom and you can scale
from there. Now, magnitudes are measured.
It's apparent magnitude. What we'd like to do is know how luminous
an object really is, and therefore, we need to know how far it is.
So an absolute magnitude was introduced, which is the apparent magnitude object
would have if you observe it from a distance of ten parsecs.
Why, why ten and not one is anybody's guess, but this is the definition.
And so if you put sun or sun-like star, ten parsecs away from us, it would have
apparent magnitude of roughly +5. Different in different bands because the
spectrum shape is different from that one of Vega.
So as a handy measure of the distance, sometimes we use the difference of
apparent and absolute magnitude which is called the distance modules and it's
equal to five times log of the distance divided by ten parsecs.
So this is how you can convert. Well, let us begin with what's probably
the first cosmological experiment, and it's Olbers paradox, which is stated in a
misleadingly simple fashion. Why is the sky dark at night?
And here is why this is a paradox. We'll assume that the universe is
infinite and it's more or less uniformly filled with stars and it never changes,
which is pretty much what people thought back in the early 19th century.
Well then, the flux from a star declines as a square
of the distance and its surface area on the sky also declines a square of the
distance. So the surface brightness, which is light
per unit area in the sky or solid angle I should say, remains constant.
Now if you have infinite amount of stars, sooner or later, line, line, your line of
sight is going to actually intersect one of those and so all of the sky should
always be bright as a surface of the sun and obviously this is not true.
So trying to understand why this is, was one of the first important paradoxes in
cosmology. Notice that exactly same reasoning can
apply to gravity because it too declines as a square of the distance.
But if everything is symmetric then there'll be infinite forces pulling in
all directions in same way so you would still feel no net force. However,
stars are units and they're randomly distributed in their fluctuation and
fluctuations of infinite gravitational forces are also infinite.
So there'll be infinite tidal shears trying to tear you apart.
Well, alright how's this resolved? One possibility is that, well, maybe
universe is not transparent. Okay, maybe that, say that it was filled
with interstellar dust. You will not see the light but the
energy's still there. So still be infinitely bright, in some
wave length. [SOUND] Another solution that was
proposed early in the twentieth century after people realize that the universe is
expanding because of the redshift, the decline of
energy of protons due to the cosmological expansion, you will actually not have
same amounts of energy from more distant stars.
Well, that helps but it's not enough by itself.
Another very clever explanation was that in, matter in the universe is not really
distributed randomly, but it's clusters and in fact this is true.
And it's clustered in such a way that its fractal dimension is less than -2,
therefore, the integral will not leverage.
This is a very clever solution but universe turns out not to be a fractal.
It is kind of close but not really. All of these things will help but the
real explanation is, the universe is finite page and finite extent.
We can only see this many light years out of the years since the Big Bang.
And therefore, there was a finite number of stars that can contribute to the
brightness of the night sky. And there simply isn't enough energy that
was ever generated through the age of the universe by stars to make the star sky
infinitely bright. But actually the sky is bright only not
very much and it's in microwaves and has black body temperature of 2.7 degree
Kelvin and that's certainly not bright enough to cause any damage.
So, it turns out that actually all star light, ever made by all stars through
history of universe so far is just a few percent of the energy density of a cosmic
microwave background which has a very subtle effect in of itself.
Alright. So, this is it for now and the next time, we'll start talking about
history of cosmology, from the earlier days to the present time.