An Expanding Universe

1. An Expanding Universe Astronomy 112 Chapter 18 & 21

2. But… let’s talk ‘science’ • I have a theory… • Water causes cancer • Only water causes cancer • Not cigarettes… • Not UV radiation… • Not nothing else… • Who/how it affects cannot be predicted • Now or ever

3. But… let’s talk ‘science’ • I have a theory… • Water causes cancer • Only water causes cancer • Who/how it affects cannot be predicted • Evidence: • Every single cancer victim has been exposed to water • Rebutting evidence to the contrary: • Not every lung cancer victim smokes • Not every smoker gets lung cancer

4. Cosmology • Study of the universe, including • Structure – how matter is arranged • History – how stars, galaxies, and structure change • Origins – conditions at early times • Fate – ultimate future of the universe

5. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions

6. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions

7. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions

8. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions

9. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions

10. Cosmological Principle • We assume two properties of the universe • Homogeneous – generally same in all places • Isotropic – same in all directions • These assumptions are confirmed by observations. • But are true on large scales

11. Edwin Hubble • Redshift

12. Edwin Hubble • Redshift

13. Edwin Hubble • Hubble’s Law

14. Edwin Hubble • Hubble’s Law

15. Edwin Hubble • Hubble’s Law

16. Edwin Hubble • Hubble’s Law • …so it appears that everything in the universe emerged from a dense (hot) initial state and exploded outward… • Or everything was perfectly placed to provide that impression

17. Hubble Constant H0 • Hubble constant is a fundamental number • It tells us the age of the universe • To measure/determine: • We need to have distances to galaxies • We need to have things inside galaxies with known luminosity • Type I SN are especially good • Determine SN luminosities with distance ladder

18. Hubble Constant H0 • Distance Ladder

19. Hubble Constant H0 • Ideas • Space is doing the expanding • Distant galaxies have a large look-back time • Hubble maps space into time • Galaxies will be farther apart in the future • Galaxies were closer together in the past • If expansion has been going on for long time, galaxies were once very close together • Hubble time is the time when separation was zero

20. Redshift and Scale Factor • Redshift tells us how much the universe has expanded since a galaxy’s light was emitted • Scale factor RU • z=1 gives RU = 0.5… the universe was half it’s current size when light was emitted from galaxy

21. Redshift and Scale Factor

22. The Big Bang • H0 ≈ 70.8±4.0 (km/s)/MPc • Hubble time was 13.8 billion years ago • This moment is called the Big Bang • The Big Bang happened everywhere • Does not affect atoms, stars, or anything else • Everything in universe was once in very small (!) volume

23. The Big Bang • Small volume • Conditions hot, above a few thousand K • As expansion proceeds, temperature (pressure) drops • Hydrogen goes from hot/ionized to cool/neutral • Called recombination • Recombination emits light

24. The Big Bang • Cosmic Microwave Background (CMB) • also Cosmic Background Radiation (CBR) • Light from recombination • Planck spectrum – blackbody radiation • Emitted long ago • Comes from far away • Highly redshifted

25. The Big Bang • Cosmic Microwave Background (CMB)

26. The Big Bang • Predictions • Before recombination, much hotter & denser • At high densities, nuclear reactions occur • Big Bang nucleosynthesis (primordial nucleosynthesis) • Predicts 24% of matter is helium • Observed! • Dark matter cannot be protons or neutrons

28. Applause

29. The future… • Universe is expanding now • Future expansion might be faster or slower • Expansion could halt and reverse • Answer depends upon how much matter is in the universe • Ordinary matter (gravity) slows expansion • It is not known if there is enough to halt expansion

30. The Cosmological Constant • Will expansion halt? • If Ωmass = 1, density is at critical mass density • If Ωmass > 1, expansion will stop and the universe will contract • If Ωmass < 1, expansion will continue forever • Ordinary stars & galaxies Ω = 0.02 • Dark matter within/between galaxies Ω = 0.3

31. The Cosmological Constant • Will expansion halt?

32. The Cosmological Constant • But expansion is speeding up! 

33. The Cosmological Constant • But expansion is speeding up! 

34. The Cosmological Constant • But expansion is speeding up!  • There is energy (described by the cosmological constant) that is pushing space outward • Called ΩΛ • We measure ΩΛ from • Supernovae • galaxy clusters • ΩΛ ≈ 0.7

35. The Cosmological Constant • But expansion is speeding up!  • There is energy (described by the cosmological constant) that is pushing space outward • Called ΩΛ • We measure ΩΛ from • Supernovae • galaxy clusters • ΩΛ ≈ 0.7

36. The Cosmological Constant

37. Challenges • Big Bang doesn’t explain everything • Flatness problem – the Ωmass + ΩΛ is very close to 1 • Horizon problem – the CMB is exactly the same in all directions • Inflation • Rapid expansion of universe at extremely early times • Solves flatness and horizon problems Applause