Evidence for the Big Bang

1. Evidence for the Big Bang • Ages of the oldest cosmic objects  converge on ~15 Gyr • Cosmic Expansion  began ~ 15 Gyr ago • Distant universe looks different  Extreme youth ~15Gyr • Cosmic Microwave Background  Seeing early fireball • 24% Helium abundance everywhere  made in v. hot BB

2. 1. Cosmic Ages

3. AGB RGB HB MSTO MS Star Cluster H-R Diagrams • Cluster stars have same age but range of mass • Main Sequence Turn Off  age of cluster • Oldest (globular) clusters are 10-15 Gyr old Globular Cluster M55

4. 47 Tucana Old 12 Gyr Hyades 100 Myr Young-ish Cluster Old Cluster

5. Star Cluster H-R Diagrams

6. Older stars are less polluted

7. Cosmic Expansion & the Hubble Law

8. (5) The Hubble Law • 1910 – 1915 Slipher: most galaxies show Doppler redshifts • 1930 Hubble: found V = H × dH = Hubble’s constant • H = 75 ± 5 km/s/Mpc (for V in km/s & d in Mpc) Nearby Galaxies: Cepheid distances Hubble Many Galaxies: various methods

9. View from galaxy A B A

10. View from galaxy B B A

11. v 2v us 2d d Consequences of the Hubble Law The fact that V ~ d has some fascinating consequences • Everyone sees the sameexpansion ! • = Cosmological Principle (deeply egalitarian) • There is no central location ! • or…. everywhere feels central to the expansion • (we will discuss the question of edges later) • Future Universe  emptier & lonely • Past Universe more crowded v. different • Everything together at particular time  Big Bang !

12. km s Mpc 1 Ho 1 Mpc 70 km  = s • Ho = 70 1 3.08 × 1013 × 106 km 70 1 km Age = = 4.40 × 1017 sec Age of the Universe When did the Big Bang happen ? Easy: use V = Ho× d • Assume expansion “velocities” have been constant : •  time to reach d moving at V is tstart = d / V • Hubble law gives : d = V / Ho • tstart = V / Ho / V = 1/Ho •  Age of Universe ≈inverse of the Hubble constant ! • (how long does it take to travel 1Mpc moving at 70 km/s) = 1.39 × 1010 years = 13.9 Gyr(13.7 with change in V) (Universe is ~ 3× older than the Earth/Solar system)

13. Redshift Cake-mix (=space) expands Carries raisins (galaxies) along Raisin speeds obey Hubble law Light stretched as it crosses expanding space. λobs / λem = Sizenow / Sizethen Astronomers usez = Δλ / λem so: Sizenow / Sizethen = 1 + z

14. 3. The Youthful Distant Universe

15. (10) Galaxy Construction • Look very far  long ago e.g. HDF • see to 1.5 Gyr ABB (adolescents, not infants) Irregular (e.g. LMC) • Many smaller galaxies, resemble irregulars • Not yet mature – no spirals/ellipticals • Small pre-galactic clumps merge to make bigger galaxies hierarchical assembly : small→ bigger → big → huge

16. Galaxy mergers more common in the past

17. (10b) Star Formation History • Galaxy building blocks appear blue/distorted/interacting •  interactions common (Universe much smaller) •  mergers trigger high star formation rate •  Universe’s youth more dramatic; relatively quiet today

18. (10c) Universe at 1 Gyr • Spectacular sight: proto-galaxies merging, everything closer, • huge HII regions, O&B stars common, SN common Cosmic history like fireworks display in reverse: Grand finale occurs first Artists view of galaxy youth

19. Quasars more common in the past

20. First evidence for evolving Universe

21. 4. The Cosmic Microwave Background

22. Big Bang Near Far Now red-shift Then Far Near Big Bang Then red-shift Now (7) Cosmic Microwave Background • Look very far away  very long ago  see Big Bang !! • Direction ? Everywhere = the whole sky !! • Spectrum ? Microwaves = red-shifted flash !! What we see Universe at Big Bang Universe today

23. (7e) CMB Image • Exceedingly uniform, • with two contaminants : 1) “dipole” : MW moving @ 540 km/s towards Virgo 2) MW plane contamination • Remove these to reveal : • Highly uniform  no stars or galaxies : diffuse hot gas • Very slight patchiness: ~10-5 variations = sound waves; grow into galaxies Flyby Rotate

24. 5. Helium & Deuterium Synthesis

25. expansion hotter denser cooler less denser a) t ~ 1 min z ~ 108 T ~ 109K ρ ~ 1 gm/cm3 t ~ 380,000 yrs z ~ 1000 T ~ 3000 K ρ ~ fewatoms/cm3 simple physics 10-3 % H2 10-4 % He3 10-7 % Li7 76 % H1 24 % He4 insensitive to details depend on ρbaryons • 24% He4 seen everywhere • (even old stars) •  strong evidence for hot big bang • Use to estimate ρbaryons •  Ω bary = 0.04 (small !!) (12) Early Times (t<10 min): Helium synthesis ~ star core!Expect nuclear reactions b) Calculations show: p + n  He4 (+ …)

26. (12b) Cosmic Nucleosynthesis: details Cosmic thermal history : 1010yr (3K):now, 3K 109 yr(30K):first galaxies 108 yr (300K):first stars 4×105yr (3000K):fog clears 5×104yr(104K):ρrad= ρmatter 1 – 3min(109K):fusion allowed range ~4%

27. The Cosmological Principleimplies Isotropy & Homogeneity

28. (6) The Universe is Isotropic Nearby, the universe is highly anisotropic On large (>100 M-ly) scales, it is highly isotropic Two pictures of 31,000 distant radio galaxies (typically, 2-5 G-ly)

29. Deep Galaxy & Quasar Surveys

30. 2 million galaxies; approaching isotropy

31. 3-D locations 200,000 galaxies out to 2 G-ly 10,000 quasars out to ~14 G-ly evolution now visible

32. Dark Matter

33. (7) Galaxy Rotation & Dark Matter • Use Doppler shifts to measure rotation curves • similar to MW  rapid rise, flat out beyond galaxy edge •  dark matterhalo : Mhalo ~ 10 × Mstars • nature unknown, probably WIMPs (like heavy neutrinos)