PHYSICS 20B Lecture Notes - Lecture 26: Allan Sandage, Walter Baade, Parsec
12 Jun 2018
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PHYSICS 20B - Lecture 26 - Cosmic Age Crisis
*Images from Pawlowski’s lecture
1929: Hubble Observes Galaxies Expanding Away from Us
●Eventually gave rise to the idea that the Universe itself is expanding
●This was consistent with the expectations of Einstein’s theory of gravity (1915) but
Einstein did not embrace the idea until he saw proof
Expanding Universe leads to the idea of a “Big Bang”
●If the Universe is expanding, we expect that at some time in the past the size was nearly
“zero”. This implies that the Universe has a finite age (not infinitely old)
●Solves Olbert’s paradox (“Why is the night sky dark?”)
How Old is the Universe According to Hubble?
●“Hubble time”: 1/H
[0]
○Roughly an upper limit on the age of the universe
●Edwin Hubble’s Value: 00H0≃5Mpc
kms−1
○ 2 billion years! 1/H
[0]=
˜
1950’s: Solution to the Age Problem
●Walter Baade used a new 200 inch telescope on Mt. Palomer in Southern California to
discover that there were two types of Cepheid variable stars
●Hubble had compared dimmer stars in our own galaxy with stars that were actually
brighter in other galaxies
●This meant that Hubble’s original measurement of his own constant was WRONG
●Under-estimated distances of the galaxies by a factor of ~2
○dv =H0
● only half as big, so age of universe (= ) doubledH0/H10
●Allan Sandage (worked with Hubble, Walter Baade was his advisor) realized that Hubble
had also misidentified star forming regions and star clusters for the brightest stars in
galaxies
●Correcting this error resulted in a further reduction of H0
●In 1958, Sandage published the first realistic value (by today’s standards):
○5 km/s/M pcH0= 7
○13 billion years 1/H
[0]=
˜
●Modern value for Earth’s age: 4.5 billion years
Document Summary
Physics 20b - lecture 26 - cosmic age crisis. 1929: hubble observes galaxies expanding away from us. Eventually gave rise to the idea that the universe itself is expanding. This was consistent with the expectations of einstein"s theory of gravity (1915) but. Einstein did not embrace the idea until he saw proof. Expanding universe leads to the idea of a big bang . If the universe is expanding, we expect that at some time in the past the size was nearly. This implies that the universe has a finite age (not infinitely old) Solves olbert"s paradox ( why is the night sky dark? ) Roughly an upper limit on the age of the universe. Walter baade used a new 200 inch telescope on mt. Palomer in southern california to discover that there were two types of cepheid variable stars. Hubble had compared dimmer stars in our own galaxy with stars that were actually brighter in other galaxies.
