3. Universe Scale We can’t measure size of universe (especially if infinite), so compare distances at different times in history: Distances between non-moving galaxies stretch in same way light wavelengths do: PRESENT: PAST: density and temperature change: R U = 1, z = 0: present R U 1: past

4. Thought Question: If the temperature of the universe drops by a factor of 10 (to 1/10 th its value), how does the density of matter change? (Enter the ratio )

5. Mass and energy can be converted, but only in certain ways: Light can make a matter-antimatter pair of particles if there is enough energy: Pairs of particles can annihilate and produce energy (light)

7. Thought Question: Electrons have less mass than protons, which have less mass than neutrons. Which of the following particles/antiparticles can be made by thermal radiation photons via pair production at the lowest temperature? A.protons and anti-protons B.electrons and anti-electrons C.neutrons and anti-neutrons

8. Building the Universe JUST MATTER: TIME PASSES AND TEMPERATURE DROPS NEUTRONS ELECTRONS PROTONS 15 sec old ANNIHILATION

9. For some reason, there was more matter than antimatter! Why??? BEFORE AFTER

10. Building the Universe JUST MATTER: NUCLEI AND ELECTRONS TIME PASSES AND TEMPERATURE DROPS NEUTRONS ELECTRONS PROTONS 15 sec old ANNIHILATION 3 min old T = 10 9 K NUCLEAR REACTIONS H NUCLEI He NUCLEI

11. EVIDENCE: Abundances of the Elements 1 H: 75% of mass 4 He: 25% of mass 2 H (Deuterium), 3 He, 7 Li in trace amounts; nearly Ø heavier elements Nuclear reactions had to occur in early universe: high temperature and density required  All other chemicals produced in stars!! In the oldest stars:

12. Building the Universe JUST MATTER: NUCLEI AND ELECTRONS TIME PASSES AND TEMPERATURE DROPS NEUTRONS ELECTRONS PROTONS 15 sec old ANNIHILATION 3 min old T = 10 9 K NUCLEAR REACTIONS H NUCLEI He NUCLEI ATOMS 380,000 years old T = 3000 K ATOMS FORM

13. Thought Question: If the temperature of the universe is about 3 K today, what was the scale factor R U of the universe when it had T = 3000 K?

15. Building the Universe AS TIME PASSES, TEMPERATURE DECREASES 100 million years old FIRST STARS FORM 1 billion years old z  10 - 20 FIRST GALAXIES FORM stars make heavy atoms (like iron, carbon, oxygen) 380,000 years old z  1100 T = 3000 K ATOMS FORM universe becomes transparent; cosmic background radiation launches

16. EVIDENCE: Cosmic Microwave Background Radiation very small fluctuations in microwave background point out where galaxies started forming: 2  10 -4 K variations

17. Gravity pulls mass into denser regions – universe grows lumpier with time

18. The Fate of the Universe? “BIG CHILL”: gravity unable to stop expansion Universe lasts forever, stars die out… so cold, so dark…. “BIG CRUNCH” (or “gnaB giB”): gravity strong enough to stop expansion and cause Universe to contract Universe ends in a hot, violent “crunch”

19. Can the Expansion Be Stopped? critical density: average density of mass/energy needed to slow Universe to a stop at infinite age  harder to stop if: expansion is fast (H 0 large) gravity is weak (G small)

20. What is the Universe Made Of? Nuclear Reactions in Early Universe  density of “normal” matter (p +, n, e - ) determines how much 4 He, 2 H, 3 He, 7 Li is produced in Big Bang BARYONS:

21. What is the Universe Made of? ORBIT SPEEDS ACTUAL GALAXY IMAGE OF GALAXY GRAVITATIONAL LENSING US Count Mass (including Dark Matter): Galaxies in Clusters  use orbits to get mass  see how much light gets bent

22. Gravitational Lensing light bent by the mass of many galaxies

23. The Cosmic Mass Pie: “Dark matter” seems to be about 5x more abundant than light-releasing matter!

24. REDSHIFT tells us how much Universe has stretched since supernova went off DISTANCE (from its apparent brightness) tells us how far back in time a supernova happened  fainter supernovas are farther away: SUPERNOVA How Fast is the Universe Decelerating?

25. Coasting Universe (Steady Stretching): PAST AVERAGE GALAXY SEPARATION US OTHER GALAXIES WITH SUPERNOVAS: NOW FUTURE TIME more redshift, more stretching BEGINNING OF UNIVERSE

26. Coasting Car (Steady Speed): DISTANCE FROM START TIME BEGINNING OF MOTION PAST NOW FUTURE

27. Thought Question PAST US NOW FUTURE TIME Which of these universes was expanding (“stretching”) fastest in the past? A. B. C. AVERAGE GALAXY SEPARATION (COMPARED TO TODAY)

28. Thought Question PAST DISTANCE FROM START NOW FUTURE TIME The three graphs below represent cars that are now at the same distance from the start and moving at the same speed. Which of the graphs represents the motion of a decelerating car? A) B) C)

29. Decelerating Universe: PAST US OTHER GALAXIES WITH SUPERNOVAS: NOW FUTURE TIME universe CRUNCHES in future universe stretched more quickly in past BEGINNING OF UNIVERSE AVERAGE GALAXY SEPARATION (COMPARED TO TODAY)

30. Critical Universe PAST US OTHER GALAXIES WITH SUPERNOVAS: NOW FUTURE TIME universe slows, but never stops expanding universe decelerates slowly in past BEGINNING OF UNIVERSE AVERAGE GALAXY SEPARATION (COMPARED TO TODAY)

31. Universe is Accelerating! PAST US OTHER GALAXIES WITH SUPERNOVAS: NOW FUTURE TIME stretching getting faster!! universe stretched MUCH slower in past WHAT IS DOING THIS!?!?!? AVERAGE GALAXY SEPARATION (COMPARED TO TODAY)

32. The Cosmic Energy Pie: “Dark energy” appears to be several times stronger than normal matter and dark matter combined, and is causing accelerated expansion

33. Some Questions… What was the Universe like very shortly after the Big Bang? What is dark matter made of? What is “dark energy”?