Thermal History of the Universe

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:

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 . Why is this a big deal?

A Framework for describing the Thermal Evolution of the Universe:

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) (and refer to the applet linked above)


If Ephoton < mpc2 then that photon has insufficient energy to spontaneously convert itself to mp + its anti-particle

Some Important Rules

So what really is a particle? Good Question:

Schematic of a proton:

Now it gets really weird: The Universe at times less than 0.0000000000000000000001 seconds.

The uncertainty principle is important as the universe is in a quantum state initially (?)

What this all means is that if quantum mechanics holds at very early times then large energy fluctuations (on which our universe is one) had no choice but to happen.

So, are these quantum fluctuations supposed to evolve and ask questions?

At very early times: