Download 85 nucleosynthesis26 - Boston University Physics

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
why is the density ~ critical…i.e. why is our universe so“flat?”
Density = Critical
geometry of
universe:
Density > Critical
Density < Critical
determined by
total density of
matter & energy
Inflation flattens
overall geometry like
blowing up a balloon,
overall density of
matter plus energy
becomes almost
critical
…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,
so neutrons weakly decay n → p e 
mn ~ mp = 1 GeV = 1000 MeV
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,
4He = α= 2 protons + 2 neutrons
but all neutrons already bound up in helium!
2
(read Steve Weinberg’s “The First Three Minutes”)
one of the reaction chains…
JET, next ITER
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
Big Bang theory prediction: 75% H, 25% He (12/16 vs 4/16 by mass)
Matches observations of primordial gases in star forming regions
Big Bang theory:
correctly predicts trace abundances of elements 2H = d, 3He, Li?
(will discuss horizontal axis when we discuss at dark matter)
But, can you observing the Big Bang for yourself?
the snow on the TV…
darkness of the night sky…evidence for Big Bang
first Kepler, then
Olbers’ Paradox
If universe were
1) infinite
2) unchanging
3) everywhere
the same
stars would cover
the night sky
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
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
…now, in summary
a brief history of the universe
as motivation for our study of particle physics
Do forces unify at high temperatures?
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Proton Decay Who knows?
Yes!
Fermilab & Cern (GUT?) (String Theory)
Planck Era
Before Planck
time (~10-43
sec)
No theory of
quantum
gravity
GUT Era
Lasts from
Planck time
(~10-43 sec) to
end of GUT
force (~10-38
sec)
Electroweak
Era
Lasts from end
of GUT force
(~10-38 sec) to
end of
electroweak
force (~10-10
sec)
Quark Era
Amounts of
matter and
antimatter
nearly equal
(Roughly 1
extra proton
for every 109
protonantiproton
pairs!)
Era of Nucleosynthesis
Begins when
matter
annihilates
remaining
antimatter at
~ 0.001 sec
Nuclei begin to
fuse
Era of Nuclei
Helium nuclei
form at age
~ 3 minutes
Universe has
become too
cool to blast
helium apart
Era of Atoms
Atoms form at
age ~ 380,000
years
Background
radiation
released
Era of
Galaxies
Galaxies form
at age ~ 1
billion years
RESERVE
PHOTONS DECOUPLING
from H & He atoms once formed
interactive
History of Universe
with black body spectra red shifted
for each era
interactive
Light from disconnected regions
coupled after inflation
interactive
The early
universe must
have been
extremely hot
and dense