Lecture 32: Cosmology IV: The Early Universe
Astronomy 101/103
Terry Herter, Cornell University
Course Home Page

Lecture
Topics
  • Inflation
  • The Early universe
  • Pair Production
  • Matter and radiation dominated eras
  • Evolution from the Big Bang to now

Epochs of the
Universe
  • From the Big Bang until now, the universe can be viewed as proceeding through different "epochs" (time periods).
  • Distinguishing characteristics:
    • Each succeeding epoch is cooler and thinner.
    • Different "forces" and/or "particles" may dominate!
  • Pair Prduction is important in the early Universe
    • Spontaneous production of particle/anti-particle pairs from high energy photons

Epochs

To the Beginnning?
  • How far can we extrapolate into the past to determine what happened?
  • Until ~10-43 seconds after the Big Bang.
    • Planck epoch: time from 0 to 10-43 sec
  • Beyond this the physics is unknown.
    • Need to know how to combine gravity and quantum mechanics (quantum gravity).

The Radiation
Era

Background:

  • Immediately after the Planck Epoch, the universe was filled with an array of subatomic particles created by pair production.
  • For T > 1028 K, the strong, weak and E-M forces were indistinguishable.
    • These forces were "unified".
    • GUT (Grand Unified Theory)
  • The two forces were gravity and GUT forces.

Epochs:

  • GUT Epoch: ended when T became less than 1028 K and the strong force became distinguishable from the "electroweak" force.
  • Hadron Epoch: characterized by "Heavy" elementary particles in equilibrium with the radiation field.

Pair Production
  • Particle-antiparticle annihilation occurs when matter and anti-matter destroy each other in a burst of gamma-rays.
  • The reverse is called:
    • 2 gamma ray photons -> particle + anti-particle
  • Pair production happens spontaneously and depends upon the temperature.
  • Higher T => more energetic photons
    • => more massive particles produced
  • In the early universe temperatures were high enough for pair production to take place.
  • We then had a "sea" of photons, particles and anti-particles.
  • The "threshold" temperatures are:

    Temperature (K)

    Particle Pairs

    ~ 1013

    proton, anti-proton

    ~ 6 x 109

    electron, positron

    < 109

    no pair production

  • Above these thresholds (temperatures), particles and anti-particles will be in equilibrium.

  • As the universe expands and the "plasma" cools, we expect particles and anti-particles to annihilate one another leaving just photons.
  • This didn't happen! We are here.
  • We're not quite sure why.

The Radiation
Era

More Epochs of the Radiation Era:

  • Lepton Epoch: starts (electrons, muon, neutrinos, etc. in equilibrium) when T < 1013 K (after 0.1 second!).
  • Nuclear Epoch: began when T ~ 109 K, after the universe was about 100 seconds old.
  • The universe became transparent to neutrinos, and protons and neutrons fuse together to form heavier nuclei.
    • deuterium, helium and some lithium

Big Bang
Nucleosynthesis
  • Light Element Creation
    • Helium, Helium-3, Deuterium, and Lithium were among the light elements created in the early universe.
    • The amount of each produced is coupled to the present day baryon density as shown in the plot below.
    • The prediction of these light elemental abundances is a key feature of the Big Bang

Epochs of the
Matter Era
  • Atomic Epoch: occurred 103 - 106 years after the Big Bang; atoms first formed.
    • The CMB results from the "decoupling" of the atoms and radiation at the beginning of the atomic epoch.
  • Galactic Epoch: took place 106 - 109 years after the Big Bang; clumping and inhomogeneous regions began giving rise to galaxies.
  • Stellar Epoch: stars form.


The Radiation
Era

 


The Radiation
Era

 



The Matter
Era

Matter Era



Epochs
Course Home Page