Lecture 24: The Hot Big Bang Model

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Lecture 24 The Hot Big Bang Model

• Objectives
• introduce the hot Big Bang, and to explain
• the formation of the light elements
• the cosmic microwave background (CMB)

• observed expansion of the Universe ? it had a
  beginning!
• so now we examine the most successful model of
  the Universe the Big Bang
• which extrapolates physics back in time as far
  as is possible
• essentially, what must happen as we run time
  backwards and the Universe becomes more
  compressed?
• it gets hotter!
• extrapolate back to initial very hot, dense
  state
• the Hot Big Bang model
• N.B. not an explosion at one instant,
  matter was created everywhere
• cannot extrapolate back to earlier than 10-43
  sec (the Planck time, requires theory combining
  Quantum Mechanics and General Relativity!)
• with particle accelerators, can reach to E
  100 GeV 10-12 sec
• which means we are extrapolating for t lt 10-12
  sec !

Additional reading Kaufmann (chap. 28), Zeilik
(chap. 25)
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Formation of light elements

• Early phase
• photons, particles and anti-particles at first in
  equilibrium
• photon energy drops, matter/anti-matter
  annihilation leaves small excess of matter (1 p
  or n per 109 photons)
• t lt 0.01s
• equal protons and neutrons
• as T? , equilibrium shifts 71 in favour of
  protons by T 1.3 x 109 K
• then stable, apart from n decays (1/2 life of 15
  mins)
• t gt 300s
• but could not happen until T lt 8 x 108 K
• then 2H ? He via p-p chain (lecture 15)
• almost every n ends up in He
• 71 pn ? 121 HHe ? He abundance should be 25
  (by mass), but too late to form heavier elements
  as T too low!

• hence in first 10 mins, H, He traces of D,
  3He, 7Li are formed, but nothing more !

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Cosmic Microwave Background (CMB)

• at first, matter and photons closely coupled (hot
  plasma)
• thermal equilibrium (cf. interior of star) ?
  black-body spectrum
• by t 300,000 yrs, T 4000K and so
• i.e. recombination, which means
• Universe becomes transparent for first time
• photons decouple from matter
• photons simply maintain black-body spectrum with
  T 1/(size of Universe)
• we see now as the CMB !
• predicted by Gamow in 1948!
• discovered (accidentally!) in 1965 (as antennae
  noise) by Penzias Wilson
• near-perfect black-body spectrum with T 2.725 K
• uniform to lt1 part in 104

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COBE detection (1990) of fluctuations (first
seeds of galaxies)

• blue 0K, red 4K
• very uniform!
• blue 2.721K, red 2.729K
• main dipole is due to our motion relative to
  CMB rest frame
• after subtracting the dipole, remaining
  fluctuations are 30x smaller
• red here is 0.0002K hotter than cold (blue)
  regions
• main emission feature is from Milky Way

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Wilkinson Microwave Anisotropy Probe (WMAP)
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Latest results from WMAP

• most detailed image yet of CMB (from NASA/WMAP
  Science Team)
• view of Universe at age of only 379,000 yrs
• with accuracy of 1 ? age of Universe is 13.7
  billion yrs (derived from scale of fluctuations
  seen in this image)

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Formation of structure in Universe

• Big Bang model rests upon
• observed expansion of Universe
• abundances of light elements
• CMB
• however, other assumptions (e.g. inflation)
• improving on it has been subject of intense
  research in last 20-30 years!

visualisation of Big Bang

• stems from seed fluctuations in CMB

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LANL n-body simulation
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LANL n-body simulation