Units to be covered:77-82

1. Units to be covered:77-82 Also please send suggestions for the topics to be covered by my next lecture on Dec. 10. This lecture is going to cover exciting developments in Astrophysics and Cosmology not covered by your textbook. My mail is lazarian@astro.wisc.edu

2. Tasks for those who want to get extra points (not obligatory): • 1. If the universe were contracting instead of expanding, how would we know (what would the observations be)? • 2. The Andromeda galaxy and the Milky Way are rushing toward each other at a velocity of 130 km/s (or, 300,000 mph!). We will collide in about 60 billion years. Andromeda is about one and a half times larger in diameter and in mass than our Milky Way. Describe what might happen during and after the collision. 3. The Hubble Deep Field image shows (in true color) that some galaxies are blue. Why is this?

3. Additional tasks • 4. The volume of the nucleus of an atom, which contains 99.98% of the mass, is only one hundred millionth the size of the whole atom. Explain, using the electron cloud model of the atom, why we cannot walk through walls if the wall and we are mostly empty space. • 5. As we look around the sky at night, we can distinguish about 6,000 individual stars with our eyes. Many of these stars are close together in the sky and cultures all over the world have connected the dots to draw figures from them, much like seeing figures in passing clouds. We call these traditional groupings of stars "constellations." Are these stars physically related?

4. Additional tasks • 6. The extremely elliptical orbital path of an imaginary planet is shown in the picture above. According to Kepler's Laws, where would the star have to be located? 7. Kepler's Laws described HOW planets moved but not WHY. The explanation waited for Sir Isaac Newton, who showed that gravity is why the planets move the way they do. Can you think of an example from biology or chemistry of the "how" of a phenomenon being understood before the "why?"

5. As a star converts most of its hydrogen in its core into helium, the star gets a. less luminous and smaller b. hotter and fainter c. more luminous and bigger d. less luminous and red

6. A hydrogen burning shell is created near the helium core because a. helium diffuses into the shell b. hydrogen diffuses into the core c. core is hot and dense d. both a. and b.

7. If we observe a star cluster which has all the starsof main sequence present, this cluster is a. old b. young c. was born as a result of supernova explosion d. both a. and c.

8. Energy Source for Active Galactic Nuclei • Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths • A black hole can be both very small, and have an accretion disk that can emit enough radiation • Likely that at the centers of these galactic nuclei, there are supermassive black holes • Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus • There are still many questions to be answered…

9. Model of AGN Depends on our observational geometry. Superluminal motions are possible.

10. Quasars are • A. the remnant cores of exploding stars • B. the focused image of a distant galaxy by the gravitational lens effect of a closer galaxy • C. the central nuclei of very distant, very active galaxies

11. Missing Mass • In Unit 73, we calculated the mass of the Milky Way by measuring the orbital velocities of dwarf galaxies in orbit around our galaxy • We can also count the number of stars in the galaxy, and estimate the galactic mass. The two numbers do not agree! • Rotation curves do not show the expected decrease in stars’ orbital velocities with distance from the galactic center, so there must be much more mass present in our galaxy • Astronomers cannot find a large majority of this mass! • Astronomers call the missing mass dark matter

12. 99 percent of the stars in a galaxy are within 20 kpc of the center Gas extends far out into the disk, but is not very massive! Galaxies are now thought to be embedded in a dark matter halo that surrounds the entire galaxy Unfortunately, dark matter cannot be detected directly. Figure 78.03

13. Dark Matter in Clusters of Galaxies • Missing mass is also a problem in clusters of galaxies! • Not enough visible mass to hold the clusters together by gravitation, and to keep hot gas in their vicinity • Cluster mass must be 100 times greater than the visible mass! • Once again, dark matter seems to be the solution

14. Gravitational Lenses • Dark matter warps space just like ordinary matter does • The path of light rays bends in the presence of mass • A galaxy or other massive object can bend and distort the light from objects located behind it, producing multiple images • This is called gravitational lensing

15. Figure 78.06

16. Gravitational lensing of light by clusters of galaxies A. Indicates the existence of dark matter B. Proves that the Universe has large positive curvature C. Proves the expansion of the Universe D. Indicates the existence of dark energy in the Universe

17. The expansion of the Universe is not like the explosion of a bomb sending fragments in all directions Space itself is expanding! We can detect photons that appear to have moved at different speeds through space Rather, the speed of light is constant, and it is space that was moving relative to the photon If each galaxy is like a button attached to a rubber band, an ant walking along the band as it is stretched will appear to have a velocity slower than it really does. The buttons (galaxies) are fixed relative to space, but space itself is moving. An Expanding Universe

18. One More Analogy • The expansion of the universe and the increasing distance between galaxies is similar to the increase in distance between raisins in a rising loaf of raisin bread. • The raisins are fixed relative to the dough, but the dough expands, increasing the space between them. • Problem with these analogies – loaves and rubber bands have edges! • We have seen no ‘edge’ to the Universe; there are an equal number of galaxies in every direction! • Also, galaxies can move relative to space, as sometimes gravity can accelerate one galaxy toward another faster than space expands!

19. As light waves travel through space, they are stretched by expansion This increases the wave’s wavelength, making it appear more red! An objects redshift, z, is Here,  is the change in wavelength, and  is the original wavelength of the photon This is equivalent to: The Meaning of Redshift

20. The Age of the Universe • Thanks to the Hubble Law, we can estimate the age of the universe • At some point in the distant past, matter in the universe must have been densely packed. • From this point, the universe would have expanded at some high speed to become today’s universe • Assuming a constant expansion over time, we find that the age of the universe is around 14 billion years.

21. Static Universe and Big Bang Alexander Friedmann Fred Hoyle Died at the age 27

22. Light from the Big Bang • Every time we look at the night sky, we are looking back in time • Can we see light from the Big Bang? • Almost! G. Gamow Alpher, R. A., H. Bethe and G. Gamow. “The Origin of Chemical Elements,” Physical Review, 73 (1948), 803 A. Penzias and R. Wilson

23. Minutes after the Big Bang, the Universe was opaque High temperatures kept all matter ionized Photons could only travel a short distance before being absorbed After 400,000 years, the Universe cooled enough for electrons and ions to recombine, allowing light to pass Now the Universe was transparent! The Last Scattering Epoch

24. Light from the Early Universe • So what should light from 400,000 years after the Big Bang look like? • It should have a spectrum that corresponds to the temperature of the Universe at that time, 3000 K. • Expansion of space will stretch this light, however • The Universe has expanded by a factor of 1000 since this time, so the wavelength will have stretched by the same amount • Spectrum will correspond to a temperature of 3K. • This light from the early Universe has been found, and is called the Cosmic Microwave Background

25. Clumpiness in the CMB

26. A Timeline of the Universe

27. The Origin of Helium • Immediately after the Big Bang, only protons and electrons existed • Shortly after the BB, temperature and density was high enough for deuterium to form by fusion • After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei • The temperature continued to cool, and fusion stopped after a few minutes. • Big Bang theory predicts that around 24% of the matter in the early universe was helium, which matches what we see.

28. Modern technology allows us to test theories back to a time 10-33 seconds after the Universe Birth (UB). Physics as we know it ceases to function at 10-43 seconds after the UB, called the Plank Time Using particle colliders, scientists have uncovered a number of clues about what happened in the early universe, after the Plank time The early universe underwent a period of very rapid expansion By 10-33 seconds, the universe expanded from the size of a proton to the size of a basketball This expansion is called inflation The Epoch of Inflation

29. The fate of the universe is ultimately controlled by its total amount of energy Energy of expansion (positive) Gravitational energy that can slow the expansion (negative) Binding energy If the total energy is positive or zero, the expansion continues forever If the total energy is negative, the expansion will halt, and the universe will contract and eventually collapse. Expansion Forever? Or Collapse?

30. Dark energy may provide the solution to the mystery Dark energy remains constant everywhere, regardless of the universe’s expansion Provides an outward push to accelerate expansion Dark energy must make up around 70% of all of the energy in the universe Much work remains to be done on this frontier… Dark Energy