1. Galaxies What is a galaxy?
How many stars are there in an average galaxy?
About how many galaxies are there in the universe?
What is the name of our galaxy?

2. Galaxy
Gravitationally bound stars, gas, dust, neutron stars, black holes, etc.
Billions of stars in a galaxy and billions of galaxies in the universe
Milky Way: 1,000 LY thick and 100,000 LY across!

3. Active Galactic Nuclei (AGN)
Normal Galaxies – one million to one trillion solar luminosities
AGN - several hundreds of trillions of solar luminosities!
Spectrum is not black body! What does this mean?

4. AGN
What is the power source that generates such enormous luminosities in AGNs?

5. AGN Energy Source Rapidly varying intensity => small size
High luminosity and small size => supermassive black hole
Billions of solar masses!
Consumes entire stars!

6. Cosmic Distance Scale
Closest galaxy: Andromeda – 2.5 million light years away!
“Standard Candles”
Extremely bright events with well-known luminosities
Can you think of an example of such an event?

7. Cosmic Distance Scale “Standard Candles”
Extremely bright events with well-known luminosities
Ex. Type Ia (carbon- detonation) supernovae
Works out to ~ 3 billion light years!

8. Structures of Galaxies
On intermediate scales, galaxies form identifiable structures.
Groups
A few to a few dozen galaxies bound together by their combined gravity.

9. Clusters
Largest (known) gravitationally bound structures
- typically containing thousands of galaxies.
The Virgo Cluster of about
2500 galaxies (central part shown).
The center of the Hercules Cluster

10. Superclusters
Larger structures made up of clusters and groups. 10,000's of galaxies.
The Local Supercluster consists of the Virgo Cluster,
the Local Group and several other groups.

11. Cosmological Structure?
We have seen that galaxies are organized into clusters and superclusters on intermediate scales in the universe.
How is matter distributed in the universe on the very largest scales?

12. No Structure on Largest Scales (Galaxies distributed fairly uniformly)
Surprising given observations of Groups, clusters, and superclusters on smaller scales
Cosmological Principle
Universe is homogeneous and isotropic
Homogeneity => ?
Isotropy => ?

13. No Structure on Largest Scales (Galaxies distributed fairly uniformly)
Surprising given structure on smaller scales
Cosmological Principle
Universe is homogeneous and isotropic
Homogeneity => universe has no edge
Isotropy => universe has no center

14. Olbers` Paradox If the universe is homogeneous, isotropic,
infinite, and unchanging,
then the night sky should be
as bright as the sun.
Why isn't it?

15. The universe is expanding!
=> Assumption that universe is unchanging is
incorrect.
=> Doesn’t matter whether universe is finite or
infinite, we only see a finite part.
=> Light from objects greater than 14 billion light
years away has not had time to reach us!

16. Cosmological Scales
How do we know that the universe is expanding?

17. Almost all galaxies exhibit show redshifted spectral lines
=> they are receding from us!
Size of universe is not static, nor shrinking due to influence of gravity.
Universe is expanding.

18. Redshift or recessional velocity is proportional to distance:
=> the farther away a galaxy is, the faster it is receding.
V = H0 x D (Hubble's Law)
velocity (km / sec) Distance (Mpc)
Hubble's Constant (km / sec / Mpc)
1 pc ~ 3 Light Years
Or graphically. . .
Current estimate:
H0 = km/sec/Mpc

19. Hubble Expansion Analogy
1 Mpc
6 Mpc
12 Mpc
1 pc ~ 3 Light Years
2 Mpc
12 Mpc
24 Mpc
Accuracy increases with distance.
The farther out , the faster the expansion.
Works for any chosen point of reference.
As the ruler cools, all points converge.

20. The Fate of the Universe
What property determines the ultimate fate of the universe?

21. Affect of Density on the Universe

22. Density Determines the Eventual Fate of the Universe
How is the expansion rate changing with time?
Will the universe expand forever?

23. Question
What is it that is causing the expansion of the universe to accelerate?

24. We aren't sure, but we call it dark energy .
Repulsive nature suggests that universe may expand forever.
Largest contributor to cosmic density, more than normal matter or even dark matter!
Recent Progress!
Cosmological constant – One possible source of dark energy
Effect of nonzero vacuum energy (pair creation)
Knew it was repulsive, but couldn't calculate strength
Recent work yields value consistent w/ observations

25. Review Question The spectrum of an AGN is _____.
A) mainly due to starlight
B) very dim
C) non-blackbody
D) highly stable

26. Review Question Hubble's law tells us that the ______.
A) closer a galaxy is, the faster it moves away
B) farther a galaxy is, the faster it moves away
C) farther a galaxy is, the slower it moves away
D) farther a galaxy is, the larger it is

27. Review Question Isotropy implies that the universe has no _____.
A) center
B) curvature
C) horizon
D) edge