Evidence for the Big Bang

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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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1. Cosmic Ages
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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
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Young-ish Cluster
Old Cluster
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Star Cluster H-R Diagrams
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Older stars are less polluted
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Cosmic Expansion the Hubble Law
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(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
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View from galaxy A
B
A
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View from galaxy B
B
A
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Consequences 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 !

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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)
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Redshift
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
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3. The Youthful Distant Universe
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(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
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Galaxy mergers more common in the past
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(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

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(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
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Quasars more common in the past
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First evidence for evolving Universe
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4. The Cosmic Microwave Background
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(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
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(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
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5. Helium Deuterium Synthesis
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(12) Early Times (tlt10 min) Helium synthesis
star core ! Expect nuclear reactions
b) Calculations show p n ? He4 ( )
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(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
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The Cosmological Principleimplies Isotropy
Homogeneity
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(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)
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Deep Galaxy Quasar Surveys
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2 million galaxies approaching isotropy
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3-D locations
200,000 galaxies out to 2 G-ly
10,000 quasars out to 14 G-ly evolution now
visible
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Dark Matter
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(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)