hw for next time: onion, “the nucleus and forces of nature” chapters 3 & 4
DESCRIPTION
Today: “Nucleosynthesis… another phase change in early universe… and why is the Universe so Flat?”. HW for next time: Onion, “the nucleus and forces of nature” chapters 3 & 4 reminder: use the Glossary Session 7: 2/6/07 ? min. - PowerPoint PPT PresentationTRANSCRIPT
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Today: “Nucleosynthesis… another phase change in early universe…
and why is the Universe so Flat?”
HW for next time:
Onion, “the nucleus and forces of nature”
chapters 3 & 4
reminder: use the Glossary
Session 7: 2/6/07 ? min
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geometry of universe:
determined by total density of matter & energy
Density = Critical
Density > Critical
Density < Critical
why is the density ~ critical…i.e. why is our universe so“flat?”
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Inflation flattens overall geometry like blowing up a balloon,
overall density of matter plus energy becomes almost critical
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…during the first 3 minutes, Big Bang nucleosynthesis created
deuterium (d), 3He, 4He, traces of Li…but nothing else
at t ~1 minute, universe cooled to temperature T ~ 1MeV, nuclear energies
E = kT, 1 MeV ~ 1010 °K
1/40 eV ~10,000 °K
too cool for n↔p equilibrium since mn > mp by 1.3 MeV,
mn ~ mp = 1 GeV = 1000 MeV
so neutrons weakly decay n → p e beta decay
between 1 and 3 minutes,
temperature high enough to fuse n + p → d, then d + t → He t=tritium=3H
not too high to dissociate weakly-bound d 2.2 MeV binding energy
nor to destroy very stable 4He (=α) 24 MeV binding energy
at t = 1 min, Big Bang predicts protons:neutrons = 7:1
after 3 minutes, universe too cool to fuse a neutron and proton to deuterium,
but all neutrons already bound up in helium! 4He2 = α= 2 protons + 2 neutrons
(read Steve Weinberg’s “The First Three Minutes”)
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In hot primordial plasma, protons and neutrons combine making ultra-stable, long-lasting helium
but universe only hot enough up to ~ 3 minutes of age
one of the reaction chains… JET, next ITER
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Big Bang theory prediction: 75% H, 25% He (12/16 vs 4/16 by mass) Matches observations of primordial gases in star forming regions
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Big Bang theory: correctly predicts trace abundances of elements 2H = d, 3He, Li?
(will discuss horizontal axis when we discuss at dark matter)
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But, can you observing the Big Bang for yourself? the snow on the TV…darkness of the night sky…evidence for Big Bang
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first Kepler, thenOlbers’ Paradox
If universe were
1) infinite
2) unchanging
3) everywhere the same
stars would cover the night sky
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Night sky is dark because the universe changes with time
As we look out in space, we can look back to a time when there were no stars
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Night sky is dark because the universe changes with time
As we look out in space, we can look back to a time when there were no stars
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…now, in summary
a brief history of the universe
as motivation for our study of particle physics
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Four known forces in universe:
Strong Force Electromagnetism
Weak Force
Gravity
Do forces unify at high temperatures?
Yes!Fermilab & Cern
Proton Decay (GUT?)
Who knows?(String Theory)
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Planck Era
Before Planck time (~10-43 sec)
No theory of quantum gravity
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GUT Era
Lasts from Planck time (~10-43 sec) to end of GUT force (~10-38 sec)
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Electroweak Era
Lasts from end of GUT force (~10-38 sec) to end of electroweak force (~10-10 sec)
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Quark Era
Amounts of matter and antimatter nearly equal
(Roughly 1 extra proton for every 109 proton-antiproton pairs!)
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Era of Nucleo-synthesis
Begins when matter annihilates remaining antimatter at ~ 0.001 sec
Nuclei begin to fuse
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Era of Nuclei
Helium nuclei form at age ~ 3 minutes
Universe has become too cool to blast helium apart
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Era of Atoms
Atoms form at age ~ 380,000 years
Background radiation released
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Era of Galaxies
Galaxies form at age ~ 1 billion years
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RESERVE
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PHOTONS DECOUPLING
from H & He atoms once formed
interactive
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History of Universewith black body spectra red shifted
for each era
interactive
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Light from disconnected regionscoupled after inflation
interactive
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The early universe must have been extremely hot and dense