1. Lecture 36: The Expanding Universe

2. Review the ‘zoo’ of observed galaxies and the Hubble Sequence the ‘zoo’ of observed galaxies and the Hubble Sequence the distance ladder the distance ladder redshift redshift Hubble’s Law and the expansion of the Universe Hubble’s Law and the expansion of the Universe

3. Hubble Sequence

4. Trends along the Hubble Sequence red blue old stars young stars gas poorgas rich no star formation lots of star formation Elliptical Lenticular Spiral Irregular

5. The distance ladder planets planets radar ranging radar ranging nearby stars nearby stars parallax parallax Milky Way Milky Way main sequence fitting main sequence fitting nearby galaxies nearby galaxies Cepheid variable stars Cepheid variable stars distant galaxies distant galaxies white dwarf supernovae white dwarf supernovae Tully-Fisher relation Tully-Fisher relation

6. Tully-Fisher Relation for spiral galaxies for spiral galaxies relationship between rotation velocity and luminosity relationship between rotation velocity and luminosity

7. measuring recession velocity

8. reminder: Doppler formula redshift = z = ( observed - rest )/ rest

9. Hubble’s Law distance recession velocity

10. Hubble’s Law (modern version)

11. Hubble’s Law Formula v = H 0 r recession velocity = constant times distance units of H 0 : km/s/Mpc best estimates: H 0 is between 65 km/s/Mpc and 79 km/s/Mpc

12. Example: Using Hubble’s Law The K line of singly ionized calcium has a wavelength of 393.3 nm when measured in the laboratory. In the giant elliptical galaxy NGC4889, this line is observed to be at 401.8 nm. The K line of singly ionized calcium has a wavelength of 393.3 nm when measured in the laboratory. In the giant elliptical galaxy NGC4889, this line is observed to be at 401.8 nm. what is the redshift of this galaxy? what is the redshift of this galaxy? what is its recession velocity? what is its recession velocity? how far away is it? how far away is it?

13. The expanding Universe Hubble’s Law implies that most galaxies are moving away from us Hubble’s Law implies that most galaxies are moving away from us and, the farther away they are, the faster they are moving away from us and, the farther away they are, the faster they are moving away from us

14. The center of the Universe? does this mean that we are at the center of the Universe? does this mean that we are at the center of the Universe? no – an observer in any other galaxy would see the same thing no – an observer in any other galaxy would see the same thing

18. Cosmological Redshift

19. The Cosmological Principle on large scales, the Universe is homogeneous (uniform) and isotropic (same in all directions) on large scales, the Universe is homogeneous (uniform) and isotropic (same in all directions)

20. Is everything expanding? the expansion of space-time acts like a pressure the expansion of space-time acts like a pressure where the force of gravity is stronger than the expansion pressure, a gravitationally bound object is formed where the force of gravity is stronger than the expansion pressure, a gravitationally bound object is formed for example, the stars in our Galaxy are bound together by gravity, so it does not expand. for example, the stars in our Galaxy are bound together by gravity, so it does not expand.

21. The Age of the Universe beginning of time now time r v = r/t = H 0 r space t = 1/H 0

22. The Cosmological Horizon the distance that light can travel in the age of the Universe is called our cosmological horizon the distance that light can travel in the age of the Universe is called our cosmological horizon we cannot receive any information from beyond our horizon we cannot receive any information from beyond our horizon r horizon = c/H 0 = 4300 Mpc (assuming H 0 = 70 km/s/Mpc) r horizon = c/H 0 = 4300 Mpc (assuming H 0 = 70 km/s/Mpc)

23. Lookback time time it takes for light to travel from an object to us time it takes for light to travel from an object to us directly related to redshift of object – more sensible measure of distance directly related to redshift of object – more sensible measure of distance

24. Coma Cluster

25. Hercules Cluster (200 Mpc)

26. Two Million Galaxies

27. Large Scale Structure

30. Galaxy Formation and Evolution How do galaxies form, and how do they change over time? How do galaxies form, and how do they change over time? Why do we see so many different kinds of galaxies? Are their differences a result of ‘nature’ or ‘nurture’? Why do we see so many different kinds of galaxies? Are their differences a result of ‘nature’ or ‘nurture’? How do the properties of galaxies depend on their environment? How do the properties of galaxies depend on their environment?

31. let’s start from the beginning… the Universe started out very dense and very smooth (the `Big Bang’). the Universe started out very dense and very smooth (the `Big Bang’). there were small lumps caused by quantum fluctuations there were small lumps caused by quantum fluctuations as the Universe expanded, these lumps grew larger and denser because of the force of gravity as the Universe expanded, these lumps grew larger and denser because of the force of gravity

32. expanding collapsing