1 Announcements Cosmos Assignment 5, due Monday 4/26, Angel Quiz Monday, April 26 Quiz 3 & Review, chapters Wednesday, April 28, Midterm 3: chapters Links to power point slides are now on the course homepage Answers to assignments are in process of being linked to the course homepage Up to date Grades will be posted Monday

2 What have we learned? Universe is Expanding Space between clusters of galaxies is getting larger

3 What did we learn last class? Galaxies (and stars) form by Gravitational Instability  Regions with slightly more concentrated mass have more gravity  Pull in more matter, which further strengthens gravity  Mass concentration grows

4 What did we learn last class? Dark Matter is needed to produce large enough gravity to make very small initial density fluctuations grow into galaxies by now Other evidence for Dark Matter –Orbits in galaxies (rotation curve) –Motions in clusters of galaxies –Hot gas in clusters of galaxies (balance of pressure vs. gravity)

5 Question 1: Because the Universe is expanding its temperature is A.Increasing B.Staying constant C.Decreasing Why?

6 Question 1: Because the Universe is expanding its temperature is A.Increasing B.Staying constant C.Decreasing Why?

7 Why is the temperature of the Universe decreasing as it expands? Expanding against pull of gravity requires work and uses energy Converts kinetic energy to gravitational potential energy Experiment: Blow on your hand with open mouth and with pursed lips. Why is work required in this case?

8 Early Universe was HOT & DENSE Expansion -> cooling Expansion -> things farther apart (lower density)  Going back in time, Universe was smaller, denser and hotter than today

9 Big Bang - history of the Universe In the beginning, universe was so hot, photons and particles had so much energy, collisions destroyed nuclei as fast as they formed

10 Big Bang - history of the Universe As the universe expanded it cooled At a temperature ~ 50X center of the Sun nuclear forces could hold protons and neutrons together (time about 1 second)

11 Big Bang - history of the Universe Primordial Nucleosynthesis (T ~ 10 9 K, t ~ 3 min) –Proton + Neutron -> Deuterium ( 2 H) –Deuterium + Proton + Neutron -> Helium

12 Big Bang - history of the Universe

13 Big Bang - history of the Universe The larger the density of matter at the time of primordial nucleosynthesis  The more collisions  The faster the fusion reactions  The more Helium made and the less Deuterium left

14 Big Bang - history of the Universe (See Fig 23.11)

15 Big Bang - history of the Universe As the Universe expanded more, its density become too small for nuclear fusion reactions (time ~ 10 min)

16 Big Bang - history of the Universe Universe continued to expand and cool Era of Radiation (book calls it era of nuclei) Photons, Nuclei and Electrons (too hot for atoms) Opaque photons scatter off electrons (random walk as in Sun)

17 Big Bang - history of the Universe Universe becomes Transparent Electrons combine with nuclei of H & He to make atoms No more electrons to scatter photons T ~ 3000 K, time ~ 400,000 years Pressure of photons no longer prevents gravity from pulling matter together Proto-galactic clumps of matter begin to grow

18 Big Bang - history of the Universe Era of Atoms and Galaxies galaxies and stars form time ~ 1 billion years redshift ~ 10

19 Big Bang - history of the Universe

20 Big Bang - history of the Universe Fig. 23.2

21 Big Bang - history of the Universe

22 Alternative Theory: Steady State Universe is, on average, unchanging Universe is expanding, but new hydrogen is being created to keep average density constant (Rate too small to notice) All other elements are made in stars Impetus for development of theory of heavy element production in stars

23 Tests 1.Relic Elements 2.Relic Radiation

24 Relic Elements Theory Observations Universe is 75% H 25% He Deuterium abundance constrains density of ordinary matter

25 Relic Radiation If universe was once hot and opaque, should see radiation from that time –Should come at us from all directions –Should have a thermal spectrum –Should be cold now because of expansion

26 Relic Radiation: Cosmic Microwave Background Radiation Accidental first detection

27 Spectrum is Thermal, T=2.7 K

28 CMB Radiation Radiation is nearly the same from all directions, Doppler Shift due to motion of the Milky Way  T/T ~ After subtracting emission from MW, see Primoridial fluctuations from when universe became transparent,  T/T ~ 10 -5

29 Expansion speed & Fate of universe

30 Fate of Universe & Mass Density

31 Fig 22.18

32 Type Ia Supernovae Type Ia SN are standard candles (i.e. have identical luminosities) Observed flux decreases with increasing distance So apparent brightness measures distance.

33

34 Supernovae appear fainter in an accelerating universe Accelerating universe was expanding slower in the past than non-accelerating universe Universe takes longer to get from early scale to today in accelerating universe Light has more time to travel from SN at early scale to us in accelerating universe SN appears fainter

35 Fig 22.18

36 Einstein’s Equations govern evolution of Scale of Universe Mass x Acceleration = Gravitational Force The extent to which expansion is speeding up Due to the contents of the universe: Usually it is attractive, leading to deceleration

37 Density + Pressure Density + Pressure Einstein’s Theory of General Relativity: Gravitational force is proportional to Accelerating universe implies contents have negative Pressure, to produce negative Gravity, called DARK ENERGY

38 Another View Expansion Rate (or the speed of the rocket) Kinetic Energy = - Potential Energy Decreases as rocket gets further from Earth

39 Energy density due to matter decreases as universe expands Number of particles in a fixed volume goes down as the universe expands (think of number of raisins in a fixed volume as the bread rises) Decrease in energy density leads to decrease in Expansion Rate After a while, Acceleration dominates over Deceleration

40

41 What do we know about Dark Energy? Constitutes 2/3 of energy in universe Is smoothly distributed and invisible Doesn’t clump into galaxies like Matter, visible or dark Has negative pressure produces Acceleration

42 Fate of Universe and Dark Energy

43 Fate of Universe years - Era of Stars & Galaxies ends when no new stars years - Degenerate Era Only BH, NS, WD, planets ends when protons decay years - Black Hole Era ends when black holes evaporate Dark Era low energy photons, electrons, neutrinos

44 Problems with the Big Bang Model 1. How can two pieces on opposites sides of the universe have the same temperature at the time the universe became transparent? They are too far apart to have communicated with each other within the age of the universe, since light from them has just now reached us half way between.

45 Problems with the Big Bang Model 2. Why is the space-time geometry of the universe so nearly flat, equivalent to the sum of the Ordinary Matter, Dark Matter and Dark Energy = Critical Density?

46 Inflation At very beginning of Big Bang, the Universe underwent a tremendous expansion (inflation).

47 Inflation Before Inflation the two parts of the universe were close enough together to communicate with each other Fig 23.14

48 Inflation Expansion smoothes out fluctuations and makes things appear flatter (e.g. blowing up a balloon).