PHYSICS 20B Lecture Notes - Lecture 18: Cosmological Principle, Cosmic Microwave Background, Robert H. Dicke
Lecture 18
The Cosmic Microwave Background (CMB)
Recap:
Composition of the Cosmos
● 5% Normal Matter
● 25% Dark Matter
● 70% Dark Energy
The Hot Big Bang Theory
● So supposebly, at time = 0, is when the Big Band started, and it was 13,8 Gyr ago
● When time = 0 and was slowly forming, the Universe started small and was very hot.
○ At ~1 sec, the temperature was T~10^10K.
○ The atoms at this time were “ionized”
■ This means that the electrons were roaming free without being bound to
nuclei
● 300,000 years after the Big Bang, the Universe started to cool down to a temperature of
T=3000 K. (3,000 @ 300,000)
○ Electrons at this point were combined to make neutral atoms and the Universe
became transparent to photons
● It is now about 14 billion years since the Big Bang and the Universe’s temperature now
is very cold, at a temperature of T=3k
Where’s the evidence of the Big Bang?
● One theory, is that if the Universe was expanding, then at some point, it was much
smaller in the past, and it would also have been much hotter in the past.
● Robert Dicke believed that the “glow” of the Big Bang could be observed
○ The Big Bang’s glow should be observable at T=3k, at the Blackbody curve, and
would be visible in all direction from empty space
HIstory of the Universe
● From 0-300,000 years, the Universe was extremely hot, this caused light to not be able
to propagate freely.
○ This means that the because the electrons were not bound to atoms, the light
scattered off of free electrons, much similar to a thick fog,
○ In the beginning, it was just a fog of light, light being not concrete
○ This means that the light that was produced then, could not reach us
● Recall that our telescopes are like time machines, because it takes a while for light to
reach us, by the time we see a visual, the actual object is actually much older, so what
we are viewing, is a younger version, the further away, the farther back in time we can
see
Document Summary
So supposebly, at time = 0, is when the big band started, and it was 13,8 gyr ago. When time = 0 and was slowly forming, the universe started small and was very hot. At ~1 sec, the temperature was t~10^10k. The atoms at this time were ionized . This means that the electrons were roaming free without being bound to nuclei. 300,000 years after the big bang, the universe started to cool down to a temperature of. Electrons at this point were combined to make neutral atoms and the universe became transparent to photons. It is now about 14 billion years since the big bang and the universe"s temperature now is very cold, at a temperature of t=3k. One theory, is that if the universe was expanding, then at some point, it was much smaller in the past, and it would also have been much hotter in the past.
