Let's think about Baltic's question some more and think how a quick course called "Expansion Cosmology in 15 Minutes" could be conducted. "Expansion Cosmology in 15 Minutes" (Course outline and syllabus
)
The basic equation in cosmology is Alex Friedmann's equation which tells how the Hubble Rate of expansion evolves over time. Or equivalently how its reciprocal, the Hubble Time, evolves.
But let's start with something more intuitive:
Here's a plot of the growth of a generic distance over time:
http://ned.ipac.caltech.edu/level5/March03/Lineweaver/Figures/figure14.jpg
You can see the acceleration effect begin to kick in around 7 billion years into the expansion. The best-fit curve is the heavy dark one. The other curves are what you get by varying the parameters from their best-fit values.
We don't have any evidence of a boundary, or any "space outside of space", or any large-scale non-uniformity. So to keep it simple we model
the universe as approximately uniform and boundaryless. A good 2D analogy for the 3D universe is the balloon model. All space and all existence concentrated on the 2D surface. Watch a few minutes.
http://www.astro.ucla.edu/~wright/Balloon2.html
See where the galaxies stay in same place (long. and lat.), getting farther apart all the time, while the photon wigglers travel at constant speed across the surface between them trying to cover distances which keep increasing.
The universe could also be infinite and approximately uniform---infinite space with infinite amount of matter distributed more or less evenly throughout. But the finite ("balloon analogy") model is often easier for people to grasp.
The most important step in understanding is
to get QUANTITATIVE experience with the model. A merely verbal concept of the universe doesn't make it. Cosmology means finding the math model (derived from the Einstein law of geometry/gravity) that gives the simplest best fit to all the observational data. So to get a first real feel for the subject go to the online table-making calculator:
http://www.einsteins-theory-of-relativity-4engineers.com/TabCosmo6.html
This makes a table of the history of the universe. You decide on the dimensions of the table---e.g. the number of steps---e.g. the range of expansion or time you want it to cover. And Click on the blue dots that get pop-up explanations.
Learn what the numbers in the different columns mean. The second column "a" you can think of as the size of a generic distance---normalized so that it equals 1 at the present day.
The third column "T" is the year count. You can see that the present day is about year 13.7 billion. These two numbers are what you saw plotted in figure14 at the start of this post. The growth of a generic distance---or "scale factor".
In the first column you see the scale factor's reciprocal 1/a, labeled S for "stretch". It is the ratio that distances and wavelengths have expanded since that time in the past. This stretch factor is actually a convenient way to keep track of time because we can actually TELL how much incoming light (e.g. from some galaxy) has been stretched. So we know the S-era it came from. The light is branded with the time it was emitted, and the S number is the brand. Incidentally, until around S=1090 the universe was filled with glowing hot gas, in which light could not travel very far without being scattered. S=1090 is the moment this dazzling "fog" cooled enough to clear and light could run free. So that was the origin of the CMB radiation we now see. The wavelengths have been stretched by a factor of 1090. You may have noticed this number at the top of the S column in the table.
So now a beginner has been introduced to the first three columns of the table:
S, a, and T.
For a first introduction I think there is just one more column---the Hubble Time T_Hub---that one should get acquainted with.
If you take the Hubble Time of any given era (say the present day S=1, 14 billion years), and change years to lightyears, you get that era's Hubble Distance (which you can see at the present day is 14 billion ly).
That is, at that point in time, the
distance which is growing at the speed of light. Other distances grow proportionally. A distance twice the Hubble length grows at twice c.
You can work out a bit of arithmetic. When the Hubble time is 14.0 billion years (as it is today) that simply means that distances are growing at percentage rate of 1/140 of a percent per million years. On the other hand at some point in the past when the Hubble time was 7.0 billion years that meant distances were growing 1/70 of a percent per million years.
[To get somewhat better resolution you could change the number of steps from 10 to 26 and check the "exact S=1" box. Then you will see that around year 5 billion the Hubble time was 7 billion years, as in the previous example.]
The main thing is that as you look down the fourth column of the table you can simply read off the listed numbers as
percentage growth rates of distance.
You can see, for example, that in the longterm future, many billions of years from now, the distance growth rate will be 1/165 of one percent per million years. And you can see what the growth rate has been in the past. Like that in year 5 billion the corresponding figure was 1/70 of a percent.
So it's time to check out the table-calculator and try some things with it. That could be a 15 minute introduction to cosmology.
