Nucleosynthesis
Steinn Sigurdsson Astro 120-17 - Feb 2016
in the beginning...
universe was really, really hot
all different particles were hanging around
about one second after Big Bang temperature was
low enough that neutrons and protons “froze out”
protons and neutrons collectively called “baryons”
Saturday, February 9, 2013
compounded
NB: neutrons and protons are not fundamental
particles, they are compound particles made from
“quarks”
atomic nuclei are made from protons and neutrons they are fairly high mass, so require quite a lot of
energy
neutrons are more massive and are unstable - they
decay to proton + electron + neutrino on a time
scale of a few minutes
Saturday, February 9, 2013
Era of nucleosynthesis
Nuclei begin to fuse. The
Universe is essentially a big
nuclear furnace.
Era of nuclei
Helium nuclei form at age ~ 3
minutes.
Universe became too cool to
blast helium apart.
Protons and neutrons combined to make long-lasting helium nuclei when
universe was ~ 3 minutes old.
How do the abundances of elements support the Big Bang theory?
Insert TCP 6e Figure 23.11
Big Bang theory prediction: 75% H, 25% He (by mass). This prediction matches
observations of primordial gases.
beyond helium
some unburned deuterium and 3He is left
behind when temperatures get low enough
for fusion to fizzle
tiny bit of helium fuses to the next element,
7Li (3p + 4n) - but note that to get there
from 4He (2p+2n) takes multiple interactions.
some beryllium and boron also gets made
Saturday, February 9, 2013
instability
BUT - fusing helium is unstable
in particular 4He + 4He fusion gives an
unstable beryllium isotope which decays very
rapidly back to 4He
to get to the next step you need carbon,
with most stable isotope 12C (6p+6n)
Saturday, February 9, 2013
carbon, nitrogen, oxygen
carbon is important because it catalyzes
additional fusion reactions - it will fuse with
hydrogen to make nitrogen, then oxygen and
so on up to iron, without the carbon being
burned up in the process
it can do this at relatively low temperature
mere millions of degrees K
Saturday, February 9, 2013
triple fusion
3 4He -> 1 12C
this “triple alpha” reaction happens very
rapdily, but ONLY at very high temperatures
by the time 4He is made from deuterium, the
universe is TOO COOL for any triple alpha
fusion to take place
within a few minutes remaining free neutrons
decay to protons
Saturday, February 9, 2013
prediction
Big Bang theory makes a strong prediction
Primordial nucleosynthesis makes hydrogen and
helium in 3:1 mass ratio with tiny trace of
incompletely burned deuterium, 3He and 7Li
and maybe some beryllium and boron
and NOTHING else
abundances calculable (Be and B tricky because of
later “spallation” reactions with cosmic rays etc)
Saturday, February 9, 2013
Astronomer’s Periodic Table
H
Text
He
Metals
Saturday, February 9, 2013
metal free universe
cosmology predicts that the early universe is
essentially metal free
where “metal” means any element not
hydrogen or helium
all metals seen in the current universe made
later (mostly in stars)
Saturday, February 9, 2013
oops
oh, one slight problem
if you take the universe to be at the critical density
(ie a flat universe)
and the mass is all protons and neutrons to begin with
then even more deuterium should be burned up
ie as little as there is, we observe too much deuterium
for a flat “baryonic” universe
Saturday, February 9, 2013
Abundances of
other light
elements
indicate that
the Universe’s
v i s i bl e m a s s
density is ~ 4%
the critical
density.
Recall that the
addition of
dar k matter
means the
total mass
density is ~
25%
the
critical density.
“Seeds” Inferred from CMB
• Overall geometry is flat.
– Total mass + dark and normal energy
has critical density.
• Ordinary matter is ~ 4.4% of total.
• Total matter is ~ 27% of total.
– Dark matter is ~ 23% of total.
– Dark energy is ~ 73% of total.
• Age is 13.7 billion years.
structure formation
cold, neutral, pressure free universe
structure forms as density perturbations
grow under their internal gravity
as H/He gas collapses under gravity density
increases and temperature increases
Saturday, February 9, 2013
booooring
Nothing much happens from redshift z=34,000 until
about redshift z=20 at which point the universe is about
650 million years old and the cosmic radiation
background has a temperature of about 60 K
main radiation source is “21 cm” radio emission from
neutral hydrogen
would now be seen at wavelengths > 5 m due to redshift
nothing much to see, we think, from this epoch
Saturday, February 9, 2013
Era of atoms
Atoms form at age ~ 380,000
years.
Background radiation
released.
S
E
G
A
RK
DA
Era of galaxies
Galaxies form at age ~ 1
billion years.
Saturday, February 9, 2013