Lecture 34 The early Universe I: Cooking the helium in the Universe - PowerPoint PPT Presentation

1 / 27
About This Presentation
Title:

Lecture 34 The early Universe I: Cooking the helium in the Universe

Description:

The early Universe I: Cooking the helium in the Universe. April ... Deuterium (1 proton, 1 neutron) Z=1, N=1, A=2. Helium (2 protons, 2 neutrons) Z=2, N=2, A=4 ... – PowerPoint PPT presentation

Number of Views:50
Avg rating:3.0/5.0
Slides: 28
Provided by: MatthiasS2
Category:

less

Transcript and Presenter's Notes

Title: Lecture 34 The early Universe I: Cooking the helium in the Universe


1
Lecture 34The early Universe I Cooking the
helium in the Universe
2
Penzias 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

3
Last scattering surface
transparent
opaque
4
More results from the CMB
  • The Earth is moving with respect to the CMB ?
    Doppler shift
  • The emission of the Galaxy
  • Fluctuations in the CMB

5
How 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

6
Measuring the Curvature of the Universe Using the
CMB
7
Measuring 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

8
What comes next ?
MAP
Planck
9
Performance of MAP/Planck
10
General 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

11
Georgy 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)

12
The structure of matter
13
The 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

14
The 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 ?

15
The 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-
16
The structure of matter
17
Nomenclature
or
  • Z number of protons
  • A number of nucleons (protons and neutrons)
  • N number of neutrons (A-Z)
  • X name of the element

18
Examples
  • Hydrogen (1 proton, 0 neutrons)
  • ? Z1, N0, A1
  • Deuterium (1 proton, 1 neutron)
  • ? Z1, N1, A2
  • Helium (2 protons, 2 neutrons)
  • ? Z2, N2, A4

19
More examples ...
  • Carbon (6 protons, 6 neutrons)
  • ? Z12, N6, A6
  • Iron (26 protons, 30 neutrons)
  • ? Z26, N30, A56
  • Uranium (92 protons, 146neutrons)
  • ? Z92, N146, A238

20
Isotopes
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)

21
Abundance of elements
  • Hydrogen and helium most abundant
  • gap around Li, Be, B

22
Thermal 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

23
Formation 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

24
Transforming 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

25
Mass 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

26
Mass 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

27
Primordial nucleosynthesis
  • Result
  • abundance of H,He and Li is consistent
  • but ?b 0.04
Write a Comment
User Comments (0)
About PowerShow.com