Cosmology and the Origin of Life

Remember the ratio of photons to baryons in the Universe now is one billion to one.

Hence, at early times when the Univere was hot and the average energy per photon was large -> the universe has to be radiation dominated!

Remember, the threshold for particle creation only depends on the temperature. If the rest mass energy () of a particle is higher than the average energy of a photon that particle can't be created.

• Watch the animation

  In this animation:

  • shaded particles represent anti-matter
  • blue particles are the most massive and form only at the highest temperatures
  • red particles have intermediate mass
  • green particles are the least massive and thus can form at all the indicated temperatures

Epoch #1:

• time = 0.01 seconds
• temperature = 100 billion degrees (much hotter than the inside of any star)

• Constituents of the Universe at this time are:
  • Protons, neutrons, electrons and neutrinoes
  • a billion photons for each proton, neutron, and electron

• matter and energy exist in thermal equilibrium

At this time the Universe is still so dense that it is opaque to neutrinoes. So we can have

anti-neutrino+proton<---->neutron+anti-electron

neutrino + neutron <------> proton + electron

The above process keeps the # of protons and the # of neutrons in the early Universe constant. This is extremely important and will result in a testable prediction later.

Remember, a free neutron (one that is not bound in an atomic nucleus) will decay into a proton + electron (+ anti-neutrino) in about 900 seconds. AT t=0.01 seconds, none of the neutrons have decayed. Since all of matter has a neutron component, the Universe must find a way to prevent neutrons from decaying. Fortunately it did, but that's later.

During this epoch, the proton-to-neutron ratio is 1.0

Epoch #2:

• time = 0.11 seconds
• temperature = 30 billion degrees

Since neutrons are slightly heavier than protons (by the mass of an electron) then the n --> p cycle in the previous neutrino reaction is favored over the p --> n cycle

Now we have 62% protons and 38% neutrons where at 0.01 seconds we had 50% protons and 50% neutrons. The proton-to-neutron ratio is 62/38 or 1.6

Epoch #3:

• time =1.1 seconds
• temperature = 10 billion degrees (still hotter than centers of stars)

  Headline News:

  NEUTRINOES DECOUPLE --> they no longer interact with any other matter in the Universe but instead "freely stream" to fill the Universe. This predicts that in addition to the microwave background there should also be a neutrino background.

  A neutrino background would be impossible to directly detect but, if neutrinoes have mass, then we have already indirectly detected it via its gravitational effects (more on this later).

  electron+anti-electron --> photon+photon is now favored over the reverse reaction photon+photon -->electron+anti-electron

  Thus, after the neutrinoes decouple, the Universe gets an "extra" source of photons (heating) which the neutrinoes never see. Hence the neutrino background is colder (lower energy) than the photon background

  Census is now: 76% protons; 24% neutrons or roughly a 3:1 ratio of protons to neutrons

  We are now at the end of particle creation from the photon field

  The Universe consists of protons, neutrons and electrons plus a photon background and a neutrino background.

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