Cosmology and the Origin of Life

Moving towards the Standard Cosmological Model (the Big Bang)

Two main observations:

Universal Expansion --> means the universe used to be a small place where all the matter was together --> as the universe expands (ages) its density goes down

The Microwave Background:

• Accidentally discovered in 1965
• 400 MWB photons per cubic centimeter no matter where you look --> This is a manifestation of homogeneity in the Universe.
• Spectrum like that of a Blackbody radiator
  • Photon energy and wavelength depend upon Temperature
• Currently the characteristic temperature of the Universe is 3K where K refers to the Kelvin temperature scale
• The origin of the microwave background (MWB) will become clear later- for now considered it as the energy from the very early universe now redshifted to very long wavelengths.
• As the universe ages (expands) it cools (just like an expanding gas cools) and the temperature drops

Now we know something real important. If the Universe is cooling then it was hotter in the past. Hence the Universe used to be a real hot and dense place. The average energy per photon depends on the temperature of the Universe and therefore, at early times, photons have a large amount of energy.

The observed ratio of photons (400 per cubic centimeter) to hydrogen in the Universe is about 1 billion to 1. That is, for every H-atom there are one billion microwave background photons. This will be extremely important later.

Now remember the Principle of Equivalence . Why is this a big deal?

A Framework for describing the Thermal Evolution of the Universe:

• Principle of Equivalence means that mass and energy are the same thing. You can convert from one form to another, if you know how.
  photon + photon = particle + antiparticle

  photon + photon <-----> particle +antiparticle

Average photon energy depends on the Temperature of the Universe

This temperature is steadily in decline and therefore the average energy per photon declines as the Universe expands and cools

At any given time in the Universe, the distribution of photon energies looks like this (this is the blackbody curve):

Repeat:

If E_photon < m_pc² then that photon has insufficient energy to spontaneously convert itself to m_p + its anti-particle

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