1. The Formation of everything!

The Universe had two phases:

• matter-dominated (today)

• radiation-dominated (the early universe)

Per unit volume:

- is there more matter or radiation (photons)?

Today matter wins by x 100

``Yesterday'' universe was smaller (higher density) and hotter, so radiation density gains twice:

(i) more photons per cubic cm, and

(ii) higher energy.

radiation wins eventually

Formation, in temporal order:

• During the radiation-dominated era:
  • fundamental particles (baryons, then leptons)

  • atomic nuclei (nucleosynthesis)

• During the matter-dominated era:
  • atoms (universe becomes transparent)

  • large scale structure

  • quasars, galaxies, stars, planets, life

2. The Genesis of Fundamental Particles

Recall that E = mc²

When the Universe was hot enough, photons had energy equivalent to mass of fundamental particles

pair-creation (photon particle + anti-particle)

pair-annihilation (particle + anti-particle photon)

. . . as long as kT > mc².

But, when the Universe expanded/cooled so that

kT < mc²

particles ``freeze out,'' ``decouple,'' or fall out of equilibrium with the radiation . . .

. . . and anhiliate, leaving a small residual of particles.

NB: Slightly more particle than anti-particles -- for unknown reasons! This requires a "fine-tuning" of the theory. Keep this in mind when we discuss "arguments" for = 1.

This occurs, then, in order of mass.

- revisit the particle zoo (below)

One exception: neutrinos

- so weakly interacting, they decouple early

In temporal order,

hadron and meson era (baryogenesis): 10^-15 sec

lepton era: 1 sec

Oh, and by the way:

Around us, everywhere in the Universe there are roughly 100 photons and 100 neutrinos (of each flavor) per cm³ left over from the Big Bang.

Universe continues to cool, but is still so hot that photons rip electrons off nuclei.

matter is ionized

Photons interact strongly with charged particles

Universe is opaque to radiation.

After 0.4 million years . . .

T < 10,000 K

(temperature of a typical star-forming region)

Below this temperature, electrons can combine with nuclei.

matter is neutral

universe is transparent to radiation

This corresponds to an epoch with redshifts ranging from 1100 to 1500.

This event (matter goes from being ionized neutral) produces a ``the surface of last scattering'' of photons off of matter. This is what we see as the cosmic microwave background radiation (CMBR)

(a) The rate of particle + anti-particle annihilation is the same as the rate of particle + anti-particle creation.

(b) Fundamental particles have different masses.

(c) The number of photons is in balance with the number of particles and anti-particles.

(d) The Universe eventually expands and cools to a critical temperature where a transition occurs.

(e) Neutral matter interacts less strongly with radiation.

(a) Stars in the early Universe were not massive enough to fuse heavier elements in late stages of nuclear burning.

(b) ⁴He is the first stable nucleus beyond H, and the deuterium bottleneck prevented earlier formation of heavier elements.

(c) The rate of particle + anti-particle annihilation is the same as the rate of particle + anti-particle creation.

(d) primordial nucleosynthesis took place during the matter-dominated era.

(e) The ``surface of last scattering'' prevented heavier nuclei from getting close enough to fuse.