2. A galaxy like the Milky Way contains about 10 billion stars.
A galaxy is a large, gravitationally bound collection of stars and gas.
A galaxy like the Milky Way contains about 10 billion stars.
There are hundreds of billions of galaxies in the universe.
Intro. Fig 2

3. Great Debate between Harlow Shapley and Heber Curtis.
Are galaxies just nebulae inside the Milky Way or separate “island universes?”
Inconclusive—not enough data, until Hubble.
Fig. 19.1 3

4. He used Cepheid variable stars.
Fig. 19.2
Hubble settled the debate after 1920 by finding the first accurate distance to Andromeda.
He used Cepheid variable stars. 4

5. Cosmology is the study of the universe, including:
Structure: how matter is arranged.
History: how stars, galaxies, and structure change.
Origins: conditions at early times.
Fate: the ultimate future of the universe.

6. Homogeneous: the same in all places.
Cosmological principle: The universe is assumed to be homogeneous and isotropic.
Homogeneous: the same in all places.
Isotropic: the same in all directions.
True on large scales.
Fig. 19.3a 6

7. The Universe Is Expanding
The universe (the space between the galaxies, specifically) is expanding.
It follows from two observations:
Galaxies are moving away from us.
Their speeds are proportional to their distances; galaxies farther away are moving away more quickly.
We’ll discuss each consideration at length.

8. We measure speeds with the Doppler shift.
All galaxies except the nearest have a redshift: observed wavelength > rest wavelength.
Redshifted spectral lines = movement away.
Fig. 19.4 8

9. dG is the distance to the galaxy.
Speeds and Distance
Galaxies farther away are moving more quickly (have greater redshifts).
If we measure distances and velocities, we find Hubble’s law: vr = H0  dG
H0 is a number called the Hubble constant, and measures how quickly the universe is expanding.
dG is the distance to the galaxy.

10. This is incorrect! There is no center.
It might appear that we are in the center of the universe, with all galaxies moving away.
This is incorrect! There is no center.
Simple model: paper clips on a rubber band.
Fig. 19.5 10

11. All observers see the same view.
All see other galaxies moving away, with the ones farther away moving more quickly.
The universe is expanding uniformly.
Fig. 19.5 11

12. We need to have distances to galaxies.
The Hubble constant H0 is a fundamental number: It also tells the age of the universe.
We need to have distances to galaxies.
We need things inside galaxies with known luminosity to continue the distance ladder.
Fig. 19.6 12

13. Type I supernovae are especially good distance indicators.
Very luminous standard candles.
H0 currently: 70 km/s per Mpc.
Fig. 19.7b 13

14. It is space that is doing the expanding.
Distant galaxies have a large look-back time.
Light has a large, but finite speed; it takes 27,000 years for the light from an object 27,000 light-years away to reach us.
Fig 14

15. Galaxies will be farther apart in the future.
Galaxies were closer together in the past.
If the expansion has been going on for a long time, galaxies were once very close together.
Hubble time: time when separation was zero.
Fig 15

16. The Hubble time was 13.7 billion years ago.
At that time the Big Bang occurred  extremely dense point began expanding.
Galaxies are not flying away from each other.
Space itself is stretching or expanding.
Fig b 16

17. The redshift tells us how much the universe has expanded since a galaxy’s light was emitted.

18. Redshifts of galaxies are not due to Doppler shifts.
Instead, the light is “stretched out” as it travels through the expanding universe: cosmological redshift.
More travel = greater redshift.
Fig 18

19. If all matter is in a small volume, it means conditions were very hot.
Due to expansion, temperatures dropped.
The hot, dense gas should have a blackbody spectrum.
Fig 19

20. Predictions have been Confirmed
As the universe expands, the light will redshift and cool.
Prediction: a Planck spectrum uniformly redshifted by the expansion of the universe to a temperature of about 5–10 K.
Fig 20

21. This light was found in 1965 by Arno Penzias and Robert Wilson.
Cosmic microwave background radiation.
Form: a Planck spectrum with T = 2.73 K.
Peak at microwave wavelengths.
Fig 21

22. When the universe was hot and the gas was ionized, photons were trapped with matter.
At an age of several hundred thousand years, the temperature cooled enough that protons and electrons could form neutral H atoms: recombination.
Fig 22

23. At that time, light was no longer blocked from its travel by all of the matter.
The light could travel freely, and cooled by a factor of about 1,000 to about 2.7 K, as confirmed by satellite data.
Fig 23

24. At high densities, nuclear reactions occur 
Before recombination, everything would have been much hotter and more dense.
At high densities, nuclear reactions occur 
Big Bang nucleosynthesis.
Fig 24

25. Fusion in the early universe produced mainly hydrogen and helium.
Prediction: 24 percent of matter should be helium.
In fact, this is observed!
Fig 25

26. Preconceived notions cannot be built in to theories.
PROCESS OF SCIENCE
Preconceived notions cannot be built in to theories.
The data and the mathematics show the true story.
You must be able to listen to what the data tell you.
Process of Science figure 26

27. CONNECTIONS 19.1
When the redshift exceeds 1, that means that mathematically the recession velocity is greater than the speed of light.
The closer z gets to 1, the more relativistic effects need to be considered.
Need to interpret the meaning in terms of the scale factor instead of the speed:
A cosmological redshift of z = 2 means that the universe was 1/3 the size it is now. 27

28. Concept Quiz—Cosmological Principle
Suppose we observed that there were many more distant galaxies in the northern half of the sky than in the southern half. Which statement would be true about the universe?
It is homogeneous and isotropic.
It is homogeneous but not isotropic.
It is isotropic but not homogeneous.
It is neither homogenous nor isotropic.
Answer: D
Explanation: In this situation, the universe clearly is not isotropic, since we see a different view in different directions. It is also not homogeneous, since an observer in a distant galaxy to the north would see a different density of galaxies than would an observer to the south. 28 28

29. Concept Quiz—Supporting Evidence
Name one fact below that on its own does not support our idea that the Big Bang happened.
the existence of the cosmic microwave background radiation
that 24 percent of matter is helium
that the universe is expanding
Answer: C
Explanation: The mere existence of the expansion does not necessarily imply that the universe was once hot and dense. Perhaps the expansion is a recent phenomenon, or the combination of omegas means that the average separation between matter was never small. The other two observations directly support the idea of the Big Bang. 29