3. Questions From Reading Activity?

4. IB Assessment Statements  Olbers’ Paradox E.4.1.Describe Newton’s model of the universe. E.4.2.Explain Olbers’paradox.

5. IB Assessment Statements  The Big Bang Model E.4.3.Suggest that the red-shift of light from galaxies indicates that the universe is expanding. E.4.4.Describe both space and time as originating with the Big Bang. E.4.5.Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

6. IB Assessment Statements  The Big Bang Model E.4.6.Explain how cosmic radiation in the microwave region is consistent with the Big Bang model. E.4.7.Suggest how the Big Bang model provides a resolution to Olbers’ Paradox.

7. IB Assessment Statements  The Development of the Universe E.4.8.Distinguish between the terms open, flat and closed when used to describe the development of the universe. E.4.9.Define the term critical density by reference to a flat model of the development of the universe. E.4.10.Discuss how the density of the universe determines the development of the universe. E.4.11.Discuss problems associated with determining the density of the universe.

8. IB Assessment Statements  The Development of the Universe E.4.12.State that the current scientific evidence suggests that the universe is open. E.4.13.Discuss an example of the international nature of recent astrophysics research. E.4.14.Evaluate arguments related to investing significant resources into researching the nature of the universe.

9. Olber’s Paradox  Cosmological Principle – uniformity of the universe  Near view - Appearance of hierarchy and structure  Planets in a solar system  Stars in a galaxy  Galaxy in a cluster of galaxies which is part of a larger super-cluster of galaxies

10. Olber’s Paradox  Cosmological Principle – uniformity of the universe  Beyond that, however, the universe looks uniform  Homogeneity principle – on a large scale, the universe looks uniform  Like comparing a serving spoon of vegetable soup to the whole pot  Isotropy principle – uniform in all directions

11. Olber’s Paradox  Cosmological Principle – uniformity of the universe  Implication that the universe has no edges and no center – infinite in extent  Newton proposed that it was infinite and static – it has been uniform and isotropic at all times  This led to Olbers’ Paradox

12. Olber’s Paradox  Why is the night sky dark?  Consider that the universe contains an infinite number of stars, basically evenly distributed

13. Olber’s Paradox  Why is the night sky dark?  Place an observer somewhere in the universe  The observer is at a distance, d, from a star which has some luminosity L  The apparent brightness (energy received per area per second) of that star is  Now consider that the one star is in a shell of some thickness, t, that encircles the observer

14. Olber’s Paradox  Why is the night sky dark?  Place an observer somewhere in the universe  The volume of that ring is equal to the surface area of a sphere times the thickness of the shell, 4πd 2 t, that contains a number density (i.e., number of stars per unit volume) n  The number of the stars in the ring would be density times volume or, 4πd 2 t

15. Olber’s Paradox  Why is the night sky dark?  Place an observer somewhere in the universe  The energy received by the observer per second per area from all the stars in the shell would be  This is a constant, if you consider an average number density of stars, that doesn’t depend on distance from the shell

16. Olber’s Paradox  Why is the night sky dark?  Place an observer somewhere in the universe  If there are an infinite number of shells containing stars that emit a constant amount of energy, the total energy received would be infinite which would make the night sky infinitely bright – This is Olbers’ Paradox

17. Olber’s Paradox  What happens to the energy?  Is it absorbed by intervening stars and other media?  In an infinite timeline, this does not hold up because eventually the media would heat up from the radiation to a point where it was emitting as much as it was receiving

18. Olber’s Paradox  What happens to the energy?  The only explanation is that the universe is finite and expanding  Stars are finite in number and have a finite lifetime  They have not been radiating forever and won’t continue to radiate forever – finite radiation  If the age of the universe is finite, light from stars that are extremely far away haven’t even reached us  If the universe is expanding, radiation from stars is redshifted, the “Doppler effect for light”, so it contains less energy

19. Expanding universe  When analyzing the absorption spectra of distant galaxies, the dark lines are longer as compared to the same chemicals on earth  This means that they have a longer than expected wavelength

20. Expanding universe  This can be explained by the star moving away from the earth which causes the radiation to be redshifted – i.e. shifted toward the red end – similar to the Doppler effect  Hubble suggested that the redshift was evidence that the galaxies were moving away from us and away from each other

21. Expanding universe  Suggests that the universe was originally much smaller and was much more compact  Expansion must have been caused by some type of explosion – Big Bang Theory

22. Cosmic Background Radiation  In 1964, two radio astronomers from Bell Laboratories set up an antenna to study radio signals from our galaxy  They kept picking up a microwave signal no matter where they pointed the antenna  Spectral analysis of the signal showed it to be a blackbody radiation corresponding to a temperature of 2.7 K

23. Cosmic Background Radiation  The theory is that the radiation is the remnant of a hot explosion that occurred at the beginning of time  As the universe expanded, the temperature dropped until it reached its current value of 2.7K

24. Big Bang Theory  The answer to Olbers’ Paradox combined with the discovery of cosmic background radiation and an abundance of helium in the universe led to the Big Bang Theory  At the beginning of time, about 14 billion years ago, the universe consisted of a single solid mass

25. Big Bang Theory  The mass exploded with matter flying outwards in all directions, creating space as it went along  The aftermath of the explosion continues as the universe continues to expand

26. Big Bang Theory  The explosion created tremendous heat with the residual radiation still observed  The theory predicts that about 25% of the mass in the universe would be helium  Measurements in nearby galaxies have shown that the values are never less than 25%

27. Development of the Universe – What’s Next?  The universe is expanding right now, but what happens next

28. Development of the Universe – What’s Next?  Consider two galaxies that are some distance apart, x 0  At some future time, t, the separation can be represented by,  R(t) is called the scale factor of the universe or sometimes just the radius of the universe  R(t) describes what the eventual size of the universe will be

29. Development of the Universe – What’s Next?  Three possibilities:  It will continue to expand forever at an increasing rate, open universe  It will continue to expand forever, but at a rate that approaches zero, flat universe  Expansion will eventually stop, followed by a collapse, closed universe

30. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  Consider a mass that is expanding outward  It has kinetic energy due to its expansion

31. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  But it has gravitational attraction that opposes the expansion  If kinetic energy is greater than gravitational attraction, the universe will expand forever  If kinetic energy is equal to gravitational attraction, the universe will expand forever, but at a rate that approaches zero  If kinetic energy is less than the gravitational attraction, the universe will eventually stop expanding and then start to collapse in on itself

32. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  The equation for total energy of the expanding universe is  H is the Hubble constant  G is the gravitational constant

33. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  The key variable here is density, ρ  Critical density causes E=0 and is estimated to be,

34. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  ρ < ρ c, the universe is open, the universe will expand forever

35. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  ρ = ρ c, the universe is flat, the universe will expand forever, but at a rate that approaches zero

36. Development of the Universe – What’s Next?  What will happen next is dependent on the density of the universe relative to its critical density  ρ > ρ c, the universe is closed, the universe will eventually stop expanding and then start to collapse in on itself

37. Development of the Universe – What’s Next?  Problems in determining the mass density of the universe  Dark matter – matter that we can’t see because it is too cold to radiate (brown dwarfs)  Two hypotheses:  WIMPS – weakly interacting massive particles  Neutrino masses are not yet determined so contribution is unknown  MACHOS – massive compact halo objects

38. Development of the Universe – What’s Next?  Dark energy  Previous discussion on Big Bang Theory is the classic perspective that has been outdated since 1998 when it was discovered that distant supernovas are moving away from us at speeds much greater than expected  Theory that the universe is filled with an all- permeating vacuum energy called Dark Energy

39. Development of the Universe – What’s Next?  Dark energy  Creates a repulsive force that opposes the force of gravity  Appears that dark energy started to dominate gravity about 5 billion years ago

40. Development of the Universe – What’s Next?  Dark energy  Belief is that the density of the universe is equal to critical density, but instead of the rate of expansion approaching zero, dark energy is causing the expansion to accelerate????

41. Development of the Universe – What’s Next?  Dark energy  Universe is now thought to be made of 73% dark energy and 27% matter  Of the 27% matter, 85% is thought to be dark matter and 15% (only 4% of the universe) to be ordinary matter SO WHAT HAPPENS NEXT???

42.  IB Exams  College  Job  Family  Retirement  Ashes to ashes, dust to dust

43. IB Assessment Statements  Olbers’ Paradox E.4.1.Describe Newton’s model of the universe. E.4.2.Explain Olbers’paradox.

44. IB Assessment Statements  The Big Bang Model E.4.3.Suggest that the red-shift of light from galaxies indicates that the universe is expanding. E.4.4.Describe both space and time as originating with the Big Bang. E.4.5.Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

45. IB Assessment Statements  The Big Bang Model E.4.6.Explain how cosmic radiation in the microwave region is consistent with the Big Bang model. E.4.7.Suggest how the Big Bang model provides a resolution to Olbers’ Paradox.

46. IB Assessment Statements  The Development of the Universe E.4.8.Distinguish between the terms open, flat and closed when used to describe the development of the universe. E.4.9.Define the term critical density by reference to a flat model of the development of the universe. E.4.10.Discuss how the density of the universe determines the development of the universe. E.4.11.Discuss problems associated with determining the density of the universe.

47. IB Assessment Statements  The Development of the Universe E.4.12.State that the current scientific evidence suggests that the universe is open. E.4.13.Discuss an example of the international nature of recent astrophysics research. E.4.14.Evaluate arguments related to investing significant resources into researching the nature of the universe.

49. #1-15 Homework