What does the universe as a whole look like? Then we will take a look at the history of cosmological

research and then we start with building world models. So we need to think about

then what is the mathematical description of the universe as a whole.

So we should start with four facts which we should take into account. So first of

all what is the question that we want to answer. If we talk about cosmology not

about individual galaxies, the question that we want to talk about is how did

the universe evolve from a certain beginning when it was born up to what it

is today. So in other words we are looking for a model that in the end will

describe why galaxies look like, look the way they look like, why the stellar

populations look like as they are, how galaxies are distributed in space and

so on. This is meant by this how did the universe evolve into what it is

now. And for this we really need to take a look at four basic facts that are

important and I describe them as facts in the sense because three out

of the four have been observationally proven and the fourth one is

philosophically interesting if it was wrong. So the facts that we have is that

we live in a universe that is expanding and that means that its volume is

continually increasing in a very very specific manner. The second fact is that

the universe and well the second and the third facts are that the universe is

isotropic and homogeneous. What this means is that the universe looks the

same irrespective of what direction we are looking at. If I take a chunk of

universe in this direction the statistical properties along that cone

down into the universe, how things behave, how the universe behaves, the content of

the universe is the same as if I take a look over there or over there. Homogeneous

means the universe looks the same from all points. So if we do cosmology and do

these observations and somebody else make a parsecs away does these

observations they will be measuring the same physics. So these are three

really fundamental things. Essentially this is the Copernican principle if you

want right? Humans are not the center of the universe but we are living not at a

special position in the universe but the universe looks the same everywhere and

it looks the same from from all places. This is called the cosmological

principle which means there is no special point in the universe. We'll take

a look at that. Now we have to prove all of these and we'll be taking a look at

some of these observations today and over also over the course of the next

weeks. We'll find these points all over again and you you find essentially it's

the same. I mean for example one one more complicated example is there are

measurements where people try to measure the value of the fine structure constant

at high redshift. You can do that for example by measuring the ratio of

wavelengths of emission lines because that depends on value of Planck's

constant and we find that these ratios don't change which tells you that

physical well natural constants are constants right which is not I

priori clear. If you measure these constants today we don't live long

enough to see slow low-term changes but everything that we see indicates that

constants like the gravitational constant and Planck and so on don't

change. That's a more complicated one than just looking so but what do I

really mean by looking is also physical properties right. If we talk with

somebody in a galaxy somewhere else they will be measuring the same speed of

light. They might be using different units. I mean right tentacle monster

probably doesn't use an average human step size or 40 thousands of the

circumference of some boring planet as the length unit right but we will be

able to communicate with them and translate the units and we will be