PHYSICS 20B Lecture Notes - Lecture 18: Big Bang, Robert H. Dicke, Recessional Velocity
20 May 2018
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PHYSICS 20B - Lecture 18 - The Cosmic Microwave
*Images from Professor Cooper
Composition of the Cosmos
●Normal matter: 5%
●Dark Matter: 25%
●Dark Energy: 70%
The Big Bang Theory
●Universe was smaller and hotter in the past
○At ~1 sec after the big bang, the universe was very hot, T~ k. So hot that01 10
atoms were “ionized”.
○Electrons were roaming free without being bound to nuclei
○300,000 years after the Big Bang, the universe had cooled down to T=3000 K
■Now, electrons 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 is
very cold, T=3k
●Evidence for the big bang?
○Just because the universe is expanding, it doesn’t mean that there really was a
“big bang”
○What are the observational predictions of a model where the universe was very
much smaller in the past?
●If the universe was much smaller in the past
, it would have been much hotter in the past
●Theorists like Robert Dicke in the 1960’s predicted that this “glow” from the Big Bang
could still be observed
●The glow from the big bang should be observable as a T=3k Blackbody curve, seen in all
directions from empty space
History of the Universe
●For the first 300,000 years the universe was so hot that light could not propagate freely
○Electrons were not bound to atoms (atoms were ionized) and since light tends to
scatter off of free electrons, the universe was like a thick fog
Document Summary
Physics 20b - lecture 18 - the cosmic microwave. Universe was smaller and hotter in the past. At ~1 sec after the big bang, the universe was very hot, t~ 01 10: so hot that atoms were ionized . Electrons were roaming free without being bound to nuclei. 300,000 years after the big bang, the universe had cooled down to t=3000 k. Now, electrons 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 is very cold, t=3k. Just because the universe is expanding, it doesn"t mean that there really was a. If the universe was much smaller in the past , it would have been much hotter in the past. Theorists like robert dicke in the 1960"s predicted that this glow from the big bang could still be observed.
