ASTR 115: Introduction to Astronomy. Stephen Kane

Transcription

1 ASTR 115: Introduction to Astronomy Stephen Kane

2 ASTR 115: Final Exam The Final Exam (Dec 18th): - Number 2 pencil, 882-E Scantron form, calculator - Two and a half hour exam - Covers entire course split into thirds Format: Multiple choice section and essay/math section. Multiple choice: 45 questions covering thirds of the course. 15 from Chapters 1-5, 15 from Chapters 6-10, 15 from Chapters Final week of class: Life in the Universe & The Ultimate Study Guide!

3 Chapter 17 The Birth of the Universe

4 17.1 The Big Bang Our goals for learning: What were conditions like in the early universe? What is the history of the universe according to the Big Bang theory?

5 What were conditions like in the early universe?

6 The early universe must have been extremely hot and dense.

7 Photons converted into particle antiparticle pairs and vice versa. E = mc2 The early universe was full of particles and radiation because of its high temperature.

8 Four known forces Do forces unify at high temperatures? in universe: Strong Force Electromagnetism Weak Force Gravity Atoms formed at age ~380,000 years. Yes! (Electroweak) Maybe Who knows? (GUT) (String Theory)

9 17.2 Evidence for the Big Bang Our goals for learning: How do we observe the radiation left over from the Big Bang? How do the abundances of elements support the Big Bang theory?

10 Primary Evidence for the Big Bang 1. We have detected the leftover radiation from the Big Bang. 2. The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe.

11 How do we observe the radiation left over from the Big Bang?

12 The cosmic microwave background the radiation left over from the Big Bang was detected by Penzias and Wilson in 1965.

13 Background radiation from the Big Bang has been freely streaming across the universe since atoms formed at temperature ~3000 K: visible/ir.

14 Background has perfect thermal radiation spectrum at temperature 2.73 K. Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.

15 17.3 The Big Bang and Inflation Our goals for learning: What aspects of the universe were originally unexplained by the Big Bang model? How does inflation explain these features of the universe? How can we test the idea of inflation?

16 What aspects of the universe were originally unexplained by the Big Bang model?

17 Mysteries Needing Explanation 1. Where does structure come from? 2. Why is the overall distribution of matter so uniform? 3. Why is the density of the universe so close to the critical density?

18 Mysteries Needing Explanation 1. Where does structure come from? 2. Why is the overall distribution of matter so uniform? 3. Why is the density of the universe so close to the critical density? An early episode of rapid inflation can solve all three mysteries!

19 How does inflation explain these features of the universe?

20 Inflation can make structure by stretching tiny quantum ripples to enormous sizes. These ripples in density then become the seeds for all structure in the universe.

21 Density = Critical Density > Critical Density < Critical The overall geometry of the universe is closely related to total density of matter and energy.

22 How can we test the idea of inflation?

23 Patterns of structure observed by WMAP show us the seeds of the universe.

24 Observed patterns of structure in the universe agree (so far) with the seeds that inflation would produce.

25 Chapter 18 Dark Matter, Dark Energy, and the Fate of the Universe

26 18.1 Unseen Influences in the Cosmos Our goals for learning: What do we mean by dark matter and dark energy?

27 What do we mean by dark matter and dark energy?

28 Unseen Influences Dark matter: An undetected form of mass that emits little or no light but whose existence we infer from its gravitational influence Dark energy: An unknown form of energy that seems to be the source of a repulsive force causing the expansion of the universe to accelerate

29 Contents of Universe Normal matter: ~ 4.6% Normal matter inside stars: ~ 0.7% Normal matter outside stars: ~ 3.9% Dark matter: Dark energy: ~ 23% ~ 72%

30 18.2 Evidence for Dark Matter Our goals for learning: What is the evidence for dark matter in galaxies? What is the evidence for dark matter in clusters of galaxies? Does dark matter really exist? What might dark matter be made of?

31 What is the evidence for dark matter in galaxies?

32 Rotation curve A plot of orbital speed versus orbital radius Rotation curve of merry-go-round rises with radius.

33 We measure the mass of the solar system using the orbits of planets. Or for circles: Orbital period Average distance Orbital velocity Orbital radius

34 Rotation curve A plot of orbital speed versus orbital radius Solar system s rotation curve declines because Sun has almost all the mass.

35 The rotation curve of the Milky Way stays flat with distance. The mass in the Milky Way is spread out over a larger region than the stars. Most of the Milky Way s mass seems to be dark matter!

36 Mass within Sun s orbit: MSun Total mass: ~1012MSun

37 The visible portion of a galaxy lies deep in the heart of a large halo of dark matter. Calculating a galaxy's massto-light ratio also reveals missing mass.

38 We can measure orbital velocities in other spiral galaxies using the Doppler shift of the 21-cm line of atomic H.

39 Spiral galaxies all tend to have orbital velocities that remain constant at large radii, indicating large amounts (> 90%) of dark matter.

40 The broadening of spectral lines in elliptical galaxies tells us how fast the stars are orbiting. These galaxies also have dark matter.

41 Does dark matter really exist?

42 Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.

43 Our Options 1. Dark matter really exists, and we are observing the effects of its gravitational attraction. 2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter. Because gravity is so well tested, most astronomers prefer option #1.

44 18.4 The Fate of the Universe Our goals for learning: Will the universe continue expanding forever? Is the expansion of the universe accelerating?

45 Will the universe continue expanding forever?

46 Does the universe have enough kinetic energy to escape its own gravitational pull? Fate of universe depends on the amount of dark matter. Lots of dark matter Critical density of matter Not enough dark matter

47 Amount of matter is ~25% of the critical density, suggesting fate is eternal expansion. Not enough dark matter

48 Is the expansion of the universe accelerating?

49 old older oldest Estimated age depends on both dark matter and dark energy.

50 The brightness of distant white dwarf supernovae tells us how much the universe has expanded since they exploded.

51 An accelerating universe is the best fit to supernova data.

52 The expansion appears to be speeding up! Dark energy? Not enough dark matter