1. Cosmology What is Cosmology? Study of the universe as a whole
Its past, present, and future
Is the Universe finite or infinite?
Did it have a beginning?
Will it ever end?
Where did everything come from?
Stellar systems
Stellar Clusters
Galaxies
Galaxy Groups and Clusters
Galaxy Superclusters
The Universe

2. The Shape of the Universe
In Einstein’s General Theory of Relativity, space and time can be curved
The amount of matter and its motion affects the curvature of space-time
If there’s a lot, then the universe is like a sphere and finite: “Closed”
If there’s less than that, it’s infinite: “Open”
If there’s just the right amount, it’s infinite: “Flat”

3. The Ultimate Number:  Q. 104: The Flat Universe
The density, denoted , is the amount of stuff in the Universe
The critical density c, is the amount of stuff you need to make the universe flat
The ratio is called : (aka 0)
If  > 1, universe is closed and finite
If  < 1, universe is open and infinite
If  = 1, universe is flat and infinite
Q. 104: The Flat Universe

4. What is ? First Attempt
Add up contribution from everything we know of:
Stuff Cont. to 
Stars
Gas, Dust 0.036
Dark Matter 0.259
Neutrinos 0.001
Total
Total  < 1*
Universe is Open*
Universe is Infinite*
*It’s lies, all lies

5. Hubble Expansion Revisited
Universe is expanding
In the past, everything was together
When was everything together?
Will it expand forever?

6. Age of the Universe: Round 1
To find age of universe, find how long ago any galaxy left us
Let its distance be d
Hubble’s Law gives us the velocity: v = H0d
If you move at constant speed, then: d = vt0
Solve for t0:
Compare: oldest known stars
About 13 Gyr
Q. 105: Age of Universe Flawed Argument

7. Age of the Universe: Round 2
We assumed the Universe always expanded at a constant rate
But there is gravity
Gravity slows things down
So the universe was expanding faster in the past
Which means it should be younger than we previously calculated
The formula gets a little more complicated:
If  = 0.30, t0 = 11.7 Gyr
If  = 1, t0 = 9.6 Gyr
Younger than the oldest stars: The age problem

8. Age of the Universe: Round 2
Now
If you ignore gravity, you get one age
When you include it, universe is younger
No gravity
distance
Matter,  = 1
t = 9.6 Gyr
t = 14.4 Gyr

9. Measuring Deceleration
White dwarf supernovae are so bright you can see them more than half way across the universe
You are seeing the universe at an earlier era
Universe may have been expanding at a faster rate
Should be able to see this
How you do this:
Measure distance to very distant white dwarf supernovae
Compare to Hubble’s Law
Fit to curve for various 
Drum roll please . . .

10. And the Results Are In No model (with only ordinary matter) worked
It looked like universe has recently been accelerating
Best fit: Universe decelerated early on, but now accelerating
m = 0.0,  = 0.0
m = 0.3,  = 0.7
m = 0.3,  = 0.0
m = 1.0,  = 0.0

11. Dark Energy Universe is currently accelerating
This is impossible with ordinary matter or dark matter
There must be another unknown contribution to the universe’s gravity
This mysterious stuff is called “Dark Energy”
We know almost nothing about Dark Energy
But dominant view is that it is
Cosmological Constant
Also known as Vacuum Energy Density

12. Vacuum Energy Density
Best guess for nature of dark energy is vacuum energy density
Particle physics: A combination of relativity and quantum mechanics
Predicts you can create particles and anti-particles out of nothingness
For a brief time
For this reason, “empty” space (the vacuum) has energy
We don’t know how to calculate how much
Vacuum energy density predicted to have repulsive gravity
Empty space repels itself – causes expansion

13. What is ? Second Attempt
Add up contribution from everything we know of:
Stuff Cont. to 
Stars
Gas, Dust 0.036
Dark Matter 0.259
Neutrinos 0.001
Dark Energy 0.693
Total
Total   1*
Universe is Flat, or close to Flat*
Universe is Infinite or very large*
*No longer lies
Q. 106: The Universe Is Made of

14. Age of the Universe: Round 3
At present, universal expansion is accelerating
Vacuum energy density is the most important contribution
Therefore, in the past, the expansion was slower
Early on, universal expansion was decelerating
Matter had a high concentration and was most important
Therefore, early on, the expansion was faster
By a numerical coincidence, the average speed is almost exactly the current speed
Our first estimate of age was pretty close: t0 = 13.8 Gyr
Oldest stars: 13 Gyr
No age problem

15. Age of the Universe: Round 3
Now
Matter & Energy m = 0.3 e = 0.7
No gravity
distance
Matter,  = 1
t = 13.8 Gyr

16. The Standard Cosmological Model
The standard cosmological model: CDM
Big bang started it all
The universe is formed from:
Dark energy ()
Cold (slowly moving) dark matter (CDM)
Ordinary matter
The total density parameter is  = 1 (or close to 1)
All structure grew from tiny density perturbations
With a simply described distribution

17. Composition of the Universe
H0 = 21 km/s/Mly
t0 = 13.8 Gyr
 = 1