PHYSICS 20B Lecture Notes - Lecture 19: Stellar Evolution, Cosmic Microwave Background, Cosmological Principle
Lecture 19
Big Bang Nucleosynthesis
Observing the Cosmos
● The speed of light is finite, and so is the time and physical scales of the Universe, this
means that distance and time are connected, but we can only observe each point at
~one time
○ So each point is like a ring, we can only view one ring at a time, similar to onion
layers, and ogres
The 4 Pillars of the Big Bang Theory
1. Expansion of the Universe
2. Cosmic Microwave Background
3. Primordial Nucleosynthesis
4. Evolution of galaxies and large scale structure over the last ~14 billion yrs.
2. Cosmic Microwave Background
● This pillar supports the Cosmological Principle by actually giving us a visual of how the
Universe is seen in such aspects
○ Recall that the Cosmological Principle is that the Universe is “homogeneous” and
“isotropic” on large scales
○ Which basically means the Universe looks the same whoever and wherever you
are
History of the Universe
● At 300,000 years after the Big Bang, the light was no longer a fog due to the Universe
becoming transparent. This is the era we see from cosmic microwave background
radiation.
● <1 sec after the Big Bang, the Universe’s temperature was around >1 Billion degrees
○ At this temperature, protons and neutrons are in equilibrium
○ The timescale for the interactions depend on the energy of the neutrinos and the
number density of particles.
● At t~ 0.7s, the timescale of these interactions go beyond the age of the Universe. Thus,
a freeze-out
○ At this point of time, the number of neutrons to protons will be 0.21
■ However, neutrons are not stable, thus they begin to decay while the
Universe continues on its expansion and cooling
Document Summary
The speed of light is finite, and so is the time and physical scales of the universe, this means that distance and time are connected, but we can only observe each point at. So each point is like a ring, we can only view one ring at a time, similar to onion layers, and ogres. The 4 pillars of the big bang theory: expansion of the universe, cosmic microwave background, primordial nucleosynthesis, evolution of galaxies and large scale structure over the last ~14 billion yrs, cosmic microwave background. This pillar supports the cosmological principle by actually giving us a visual of how the. Recall that the cosmological principle is that the universe is homogeneous and. Which basically means the universe looks the same whoever and wherever you are. At 300,000 years after the big bang, the light was no longer a fog due to the universe becoming transparent. This is the era we see from cosmic microwave background radiation.
