1. Modern Astronomy Stars & Galaxies Lecture 9 Cosmology Geraint F. Lewis University of Sydney 2007

2. Outline  The Universe as we know it  Cosmological models  Observations of the Universe  The evolution of the Universe  The distribution of matter  The future of the Cosmos

3. The Universe as we know it www-ed.fnal.gov/projects/exhibits

4. Fundamental Forces Strong: 1 EM: 10 -2 Weak: 10 -5 Gravity: 10 -39

5. Forces: Unification As the energy of the interactions increases, these force look more like each other.  Quantum Electrodynamics: unifies EM & Weak  Quantum Chromodynamics: EM-Weak & Strong  Currently trying to add gravity Superstrings?????

6. Forces: Unification hyperphysics.phy-astr.gsu.edu/hbase/astro/unify.html

7. Cosmological Foundations

8. General Relativity In Einstein’s view, gravity is not really a force! Importantly, gravity is created not only by mass, but also by energy. Remember, mass is energy E=mc 2

9. Cosmology Imagine a universe filled with stationary stars. If finely balanced, it remains stationary. A single star out of place results in collapse.

10. Cosmological Constant This worried Einstein as he thought the Universe should be static. He added , the cosmological constant, which acts as anti- gravity. This acts against collapse, but has negative pressure and appears unphysical! Einstein’s Biggest Blunder!

11. Cosmology Friedmann & Lemaitre looked at cosmology within the framework of General Relativity. The concluded that, in general, universes must be expanding or contracting. Just how the universe changes with time depends upon the energies in the Universe. When this original work was done, the only energy considered was matter!

12. Evolution  Too little matter, and the Universe never slows down  Too much matter, and the Universe collapses  Just right, the expansion slows down forever, but never quite reaches zero But what does this scale factor mean? It is the change of separation between a pair of objects.

13. Evolution  The raisin cake picture encompasses this picture  As the cake expands, the distance between raisins increases  But for this picture to be accurate, there can be no edges to the cake!  Where is the centre of the Universe in this picture?

14. Geometry  The ratio of the circumference to the diameter of a circle is =3.141592….  But this is only true in the flat geometry of Euclid  What about non-flat geometry??

15. Open, flat, closed? Measure  on the surface of the Earth From pole to equator = 10000km Around the equator = 40000km = 40000/(2 £ 10000) = 2

16. Open, flat, closed? So, depending upon the density in the Universe  Open >: Infinitely large & expand forever  Flat = : Infinitely large & expand forever  Closed <: Finite & eventually collapse Critical density = 1 £ 10 -26 kg/m 3 = 6 hydrogen atoms / m 3 = 6 hydrogen atoms / m 3

17. Observing: The redshift

18. Hubble’s expansion Hubble found a relation between distance and velocity V = H o d V = H o d where d is the distance to the galaxy and v is its velocity. Hubble’s constant has been measured to be H o = 72 km/s/Mpc H o = 72 km/s/Mpc

19. Hubble’s expansion Imagine the Earth expands overnight. We might feel squashed in our beds, but when we awake in the morning, all the distances would have increased! What if we lived in the surface?

20. What is the redshift? The redshift is not a Doppler shift. As the Universe expands, the wavelength of the radiation is also stretched. Mathematically where z is the redshift and R is the distance between galaxies.

21. Which universe is ours? The cosmological models make specific predictions on how faint something should look at a particular redshift. This depends upon the content of the Universe. Observations of distant supernova show that we do not live in a universe containing only matter. In fact we live in a universe dominated by Something Else!!!!

22. Cosmological time dilation  Cosmological supernovae are like clocks, they brighten and fade in a fixed time.  The cosmological equations predict that we should see the distant universe run slowly, a time dilation effect.  This has now been observed, providing further evidence that the cosmological models are a good description of the cosmos.

23. Putting it all together  Dark Matter Interacts mainly via gravity Not stars, gas, rocks etc Not large black holes Possibly an elementary particle  Dark Energy Not dark matter! Looks like Einstein’s  Must be exotic: strings, defects Is beginning to dominate Accelerates the expansion The Universe is flat The Universe is 13.7 billion yrs old

24. Accelerated expansion

25. Running backwards  Running the Universe backwards, we see there was a point where the scale factor was zero.  This marks the Big Bang, or start of the Universe.  Currently, science cannot answer what caused this event.  But science can describe the universe in detail from 10 -43 seconds after the event to the present day!

26. At the beginning  The energy from inflation became particles and radiation  10 -6 secs, protons and neutrons formed  1 sec, protons & neutrons join to make deuterium and helium  3 mins, cooking ceased with the universe 75% hydrogen and 25% helium  300,000 yrs, the Universe is cool enough for electrons to join atoms There should be a background of radiation left over from this event!! The smaller the scale factor, the hotter the universe was. The very early universe was very hot!

27. Background Radiation In 1941, Herzberg saw that molecules in interstellar space were too energetic. He concluded that they must be bathed in a radiation of temperature 2.3K. “a rotational temperature of 2.3k follows, which has of course only a very restricted meaning” Penzias & Wilson confirmed this in 1965 (and won the Nobel in 1978)

28. Cosmic microwave background The universe is bathed in cool radiation left over from its energetic start. Some of the snow on tv is this CMB Two important predictions  CMB was hotter in the past  CMB should not be completely smooth

29. Cosmic microwave background Relativistic cosmology predicts that the temperature of the CMB should scale with the size of the universe. Astronomers have looked at distant molecules in the early universe and have mentioned temperature of the CMB, finding it to be hotter in the past, in agreement with the cosmological model. www.eso.org/outreach/press-rel/pr-2000/pr-27-00.html www.eso.org/outreach/press-rel/pr-2000/pr-27-00.html

30. The CMB  While the CMB has a mean temperature of 2.7K, there should be small (10 -5 ) variations superimposed on it.  These variations reflect the distribution of matter in the very early Universe.

31. Anisotropy  Inflation predicts a very specific pattern on the CMB (red line)  Data from COBE and now WMAP have revealed this pattern matches the theory!

32. In the beginning…  … the Universe was very smooth  Inflation results in very small ripples in the matter at the level of 1 part in 10000 (as seen in the CMB)  Gravity causes material to flow into denser regions, forming the large scale structure of galaxy groups, clusters and superclusters  Again need a Universe in a computer

33. The growth of large scale structure http://virgo.dur.ac.uk http://virgo.dur.ac.uk & http://www.nbody.net

34. Large Scale Structure Surveys of the Universe reveal galaxies to lie on the foamy structure predicted from inflation! Sloan galaxy survey (www.sdss.org)

35. Present: 14£10 9 yrs  Stars are metal rich  More metals made in supernova  New stars have less and less hydrogen as they become polluted with metals  Mass is getting locked up in white dwarfs, neutron stars and black holes!

36. Future: 16£10 9 yrs  The Milky Way and Andromeda collide and form a single large elliptical

37. Future: 17£10 9 yrs  The Sun dies

38. Future: 7£10 11 yrs  With the accelerated expansion, galaxies beyond the Local Group fade from view.  The sky outside the Local Group will be black!

39. Future: 10 13 yrs  Nuclear fuel is exhausted  Small red stars finally burn out  All star light begins to fade  All potential raw material for star formation is locked away in stellar remnants

40. Future: 10 16 yrs  Stellar interactions finally eject all planets

41. Future: 10 28 yrs  Galaxies finally dissolve, with 90% of stars ejected into intergalactic space  The remaining stars spiral into the central black hole

42. Future: 10 32 yrs  Protons decay and matter dissolves into radiation and electrons.

43. Future: 10 67 ! 10 100 yrs  Stellar and supermassive black holes evaporate via Hawking radiation  Nothing left by a sea of ever cooling radiation  The universe is now a cold, dark, lifeless place, a little bit like Canberra.

44. See you next week!