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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.
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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
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
47 Tucana Old 12 Gyr Hyades 100 Myr Young-ish Cluster Old Cluster
(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
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 !
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)
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
(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
(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
(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
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
(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
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 (+ …)
(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%
(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)
3-D locations 200,000 galaxies out to 2 G-ly 10,000 quasars out to ~14 G-ly evolution now visible
(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)