Title: Evidence for the Big Bang
1Evidence 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
21. Cosmic Ages
3Star 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
4Young-ish Cluster
Old Cluster
5Star Cluster H-R Diagrams
6Older stars are less polluted
7Cosmic Expansion the Hubble Law
8(5) The Hubble Law
- 1910 1915 Slipher most galaxies show Doppler
redshifts - 1930 Hubble found V H d H Hubbles
constant - H 75 5 km/s/Mpc (for V in km/s d in Mpc)
Nearby Galaxies Cepheid distances
Hubble
Many Galaxies various methods
9View from galaxy A
B
A
10View from galaxy B
B
A
11Consequences of the Hubble Law
The fact that V d has some fascinating
consequences
- Everyone sees the same expansion !
- 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 !
12Age 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)
13Redshift
Cake-mix (space) expands Carries raisins
(galaxies) along Raisin speeds obey Hubble law
Light stretched as it crosses expanding space.
?obs / ?em Sizenow / Sizethen Astronomers
use z ?? / ?em so Sizenow / Sizethen 1
z
143. 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
16Galaxy mergers more common in the past
17(10b) Star Formation History
- Galaxy building blocks appear blue/distorted/inte
racting - ? interactions common (Universe much smaller)
- ? mergers trigger high star formation rate
- ? Universes youth more dramatic relatively
quiet today
18(10c) Universe at 1 Gyr
- Spectacular sight proto-galaxies merging,
everything closer, - huge HII regions, OB stars common, SN common
Cosmic history like fireworks display in
reverse Grand finale occurs first
Artists view of galaxy youth
19Quasars more common in the past
20First evidence for evolving Universe
214. The Cosmic Microwave Background
22(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 _at_
- 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
Rotate
Flyby
245. Helium Deuterium Synthesis
25(12) Early Times (tlt10 min) Helium synthesis
star core ! Expect nuclear reactions
b) Calculations show p n ? He4 ( )
26(12b) Cosmic Nucleosynthesis details
Cosmic thermal history 1010 yr (3K) now, 3K
109 yr (30K) first galaxies 108 yr (300K)
first stars 4105 yr (3000K) fog clears 5104 yr
(104K) ?rad ?matter 1 3 min (109K) fusion
allowed range 4
27The Cosmological Principleimplies Isotropy
Homogeneity
28(6) The Universe is Isotropic
Nearby, the universe is highly anisotropic On
large (gt100 M-ly) scales, it is highly isotropic
Two pictures of 31,000 distant radio
galaxies (typically, 2-5 G-ly)
29Deep Galaxy Quasar Surveys
302 million galaxies approaching isotropy
313-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 matter halo Mhalo 10 Mstars
- nature unknown, probably WIMPs (like heavy
neutrinos)