University of California, San Diego
Center for Astrophysics & Space Sciences

Gene Smith's Astronomy Tutorial

The Big Bang

The Big Bang

Hubble's Velocity-Distance relationship

v = H.d

established that the Universe is expanding. Edwin Hubble made a plot of recession velocity vs distance for galaxies that he had observed he found a straight line relationship. The slope of this line, known as the Hubble Parameter, H 20 (km/s)/million l.y., has units of 1/time. It is easy to see that, if the Universe is expanding at the present time, then at some point in the past, all matter was once together. Thus, 1/H, called the Hubble Time is an estimate of the Age of the Universe, about 15 billion years.

If we rewind the motion picture representing the history of the Universe, we can understand a great deal about its early state, just after the Big Bang. In its early stages the Universe was simpler than it has ever been. It was very hot and in a state of Thermal Equilibrium, that is its temperature determined all its other properties. Just after the Big Bang, temperatures were so high that particle pairs could be created purely out of the heat energy present. For example, a pair of thermal photons - which would be gamma-rays at these temperatures - might react to form an electron/positron pair:

+ e+ + e-

During its early phases the Universe was radiation dominated, that is the photons dominated the energy and pressure of the Universe. As the Universe expanded, it cooled, T 1/R, where R is some measure of the "scale of the Universe".

The Big Bang
Time since
Big Bang

  • < 10-43s
  •    Quantum era
    Universe consists of "soup" of leptons & quarks
  • ~ 10-43s
  • 1032K   Grand Unification Era
    Gravity separates from other Grand Unified Forces
  • ~ 10-35s
  • 1027K   End of Grand Unification
    Strong Force breaks symmetry w/ ElectroWeak Force.
  • ~ 10-35s - 10-33s
  •   Inflationary epoch
    Universe inflates by a factor of 1030 or more
    ("observable Universe" expands from size of an atomic nucleus
    to size of a cherry pit).
  • ~ 10-12s
  • 1015K   Particle Era
    Electromagnetic force and Weak Force break symmetry.
  • ~ 10-6s
  • 1013K   Quark --> Hadron transition.
    Protons and neutrons (plus antiprotons and anti neutrons) are formed from quarks - at this time the "matter" particles have an excess of ~ one in a billion over "antimatter" particles.
  • 0.01s
  • 1011K
    (100 billion K)
    4 x 109
    (4000 Volks/cm-3)
  • The Universe is expanding rapidly, scale is doubling every 0.02s.
  • As Universe expands it cools, T ~ 1/R.
  • Although the temperature is too low for Protons and neutrons to be created from the thermal energy of the early universe reactions such as:
    + n p+ + e-
    and vice-versa, maintain an equal number of protons and neutrons.
  • As the temperature decreases proton/neutron balance shifts in favor of less massive protons.
  • 1s
  • 1010K
    (10 billion K)
    4 x 105
    Weakly interacting neutrinos "decouple" from the rest of the Universe
  • 15s
  • 3 x 109K
    (3 billion K)
    4 x 104
  • Temperature is below threshold for creation of electron/positron pairs.
  • e+/ e- annihilate: e+ + e- +
  • The Universe is "reheated" about 35% by annihilation.
  • 3 min
  • 109K 400 Era of Nuclear Reactions
  • Nuclei can begin to hold together, e.g.
    p+ + n 2H +
  • At this time the baryons are divided into about 87% protons 13% neutrons.
  • 3 1/2 m
  • 108K  End of Nuclear Reactions
    neutrons have been "used-up" forming 4He
    Universe is now 90% H nuclei( p+) & 10% He nuclei
  • 106yr
  • 4000K  
    Era of Recombination
    nuclei & electrons "recombine to form atoms
    Universe becomes transparent
  • 109yr
  •     Era of Galaxy Formation

    The net result of the early nuclear reactions Big Bang Nucleosynthesis is to transform all of the neutrons, along with the necessary protons, into Helium nuclei plus traces of 2H (deuterium), 3He, 7Li, 6Li, 7Be.

    Nuclear Reactions in the Big Bang

    Why is nucleosynthesis in the Big Bang different from nucleosynthesis in stars?
    The answer lies in the particles present in the early Universe and the temperatures and densities present when nuclear reactions are occurring:

    Evidence for the Big Bang

    What a wonderful mythology! Is there any evidence beyond the hyperactive imaginations of Cosmologists that this really happened?

    The Cosmic Background Radiation

    The Cosmic Background Spectrum as measured by NASA's COBE Satellite.
    from Ned Wright's Cosmology Tutorial © Edward L. Wright (UCLA), used with permission.

    Tiny variations (< 10-5K) in the Cosmic Background Radiation temperature reflect small density fluctuations in the early Universe before matter and light parted company. After decoupling, the density fluctuations could grow under gravity to form the seeds for galaxies and clusters. The nature of these fluctuations agrees with current theories of the formation of structure in the Universe. Future space missions such as the Microwave Anisotropy Probe (MAP) have the potential of making detailed measurements of the structure of the early Universe at the time of recombination.

    Big Bang Links

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    Comments? Gene Smith
    Conducted by:
    Prof. H. E. (Gene) Smith
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    Last updated: 19 April 1999