Title: Lecture 34 The early Universe I: Cooking the helium in the Universe
1Lecture 34The early Universe I Cooking the
helium in the Universe
2Penzias and Wilson 1965
- Working at Bell labs
- Used a satellite dish to measure radio emission
of the Milky Way - They found some extra noise in the receiver, but
couldnt explain it? discovery of the background
radiation - Most significant cosmological observation since
Hubble - Nobel prize for physics 1978
3Last scattering surface
transparent
opaque
4More results from the CMB
- The Earth is moving with respect to the CMB ?
Doppler shift - The emission of the Galaxy
- Fluctuations in the CMB
5How can we measure the geometry of the universe
- We need a yard stick on the CMB
- For different curvatures, a yard stick of given
length appears under different angles
6Measuring the Curvature of the Universe Using the
CMB
7Measuring the Curvature of the Universe Using the
CMB
- Recall with supernovae, one measures q0 ½?0
?? - CMB fluctuations measure curvature? ?0 ??
- two equations for two variables? problem solved
8What comes next ?
MAP
Planck
9Performance of MAP/Planck
10General acceptance of the big bang model
- Until mid 60ies big bang model very
controversial, many alternative models - After mid 60ies little doubt on validity of the
big bang model - Four pillars on which the big bang theory is
resting - Hubbles law ?
- Cosmic microwave background radiation
- The origin of the elements
- Structure formation in the universe
- Until mid 60ies big bang model very
controversial, many alternative models - After mid 60ies little doubt on validity of the
big bang model - Four pillars on which the big bang theory is
resting - Hubbles law ?
- Cosmic microwave background radiation ?
- The origin of the elements
- Structure formation in the universe
11Georgy Gamov (1904-1968)
- If the universe is expanding, then there has
been a big bang - Therefore, the early universe must have been
very dense and hot - Optimum environment to breed the elements by
nuclear fusion (Alpher, Bethe Gamow, 1948) - success predicted that helium abundance is 25
- failure could not reproduce elements more
massive than lithium and beryllium (? formed in
stars)
12The structure of matter
13The structure of matter
- Atoms are mostly empty space
- Atoms consist of protons (), neutrons (o) and
electrons (-) - protons and neutrons form the atomic nucleus
- of protons deter-mines the element
- electrons in the outskirts determine chemistry
14The structure of matter
- Neutrons and protons are very similar, but
- Protons are electrically charged, neutrons are
not - Neutrons have a slightly higher mass
- Electrons are much less massive than nucleons ?
most of the mass of an atom is in its nucleus - If charges of the same sign repel, and the
nucleus is made of protons, why dont the protons
fly apart ?
15The four forces of nature
- gravity
- electromagnetism
- strong nuclear force
- keeps atomic nuclei together
- weak nuclear force
- decay of free neutrons into protons
n ? p
n ? p e-
? n ? p e-
16The structure of matter
17Nomenclature
or
- Z number of protons
- A number of nucleons (protons and neutrons)
- N number of neutrons (A-Z)
- X name of the element
18Examples
- Hydrogen (1 proton, 0 neutrons)
- ? Z1, N0, A1
- Deuterium (1 proton, 1 neutron)
- ? Z1, N1, A2
- Helium (2 protons, 2 neutrons)
- ? Z2, N2, A4
19More examples ...
- Carbon (6 protons, 6 neutrons)
- ? Z12, N6, A6
- Iron (26 protons, 30 neutrons)
- ? Z26, N30, A56
- Uranium (92 protons, 146neutrons)
- ? Z92, N146, A238
20Isotopes
Two atoms that have the same number of protons,
but a different number of neutrons are called
Isotopes. Isotopes have the same chemical but
different nuclear properties.
- Uranium-238
- (Z92, N146, A238)
- Uranium-235
- (Z92, N143, A235)
21Abundance of elements
- Hydrogen and helium most abundant
- gap around Li, Be, B
22Thermal history of the universe
- When the universe was younger than 300 000 yrs,
it was so hot that neutral atoms separated into
nuclei and electrons. It was too hot to bind
atomic nuclei and electrons to atoms by the
electromagnetic force - When the universe was younger than 1 sec, it
was so hot that atom nuclei separated into
neutrons and protons. It was too hot to bind
protons and neutrons to atomic nuclei by the
strong nuclear force
23Formation of helium in the big bang
- Hydrogen 1 nucleon (proton)
- Helium 4 nucleons (2 protons, 2 neutrons)
- In order to from helium from hydrogen one has to
- bring 2 protons and 2 neutrons close together, so
the strong nuclear force can act and hold them
together - close together Coulomb repulsion has to be
overcome ? high velocities ? high temperatures - but 4 body collisions are highly unlikely
24Transforming hydrogen into helium
- Hot big bang neutrons and protons
- Use a multi step procedure
- p n ? 2H
- p 2H ? 3He
- n 2H ? 3H
- 3He 3He ? 4He 2 p
- some side reactions
- 3He 3H ? 7Li
- 3He 3He ? 7Be
25Mass gap/stability gap at A5 and 8
- There is no stable atomic nucleus with 5 or with
8 nucleons - Reaction chain stops at 7Li
- So how to form the more massive elements?
- There exist a meta-stable nucleus (8B). If this
nucleus is hit by another 4He during its
lifetime, 12C and other elements can be formed
26Mass gap/stability gap at A5 and 8
- Reaction chain
- 4He 4He ? 8B
- 8B 4He ? 12C
- so-called 3-body reaction
- in order to have 3-body reactions, high particle
densities are required - densities are not high enough in the big-bang
- but they are in the center of evolved stars
- Conclusion big bang synthesizes elements up to
7Li. Higher elements are formed in stars
27Primordial nucleosynthesis
- Result
- abundance of H,He and Li is consistent
- but ?b 0.04