PHYSICS 20B Lecture Notes - Lecture 16: Walter Baade
Lecture 16:
The Expanding Universe and the Cosmic Age Crisis
Earth Age Problem
● So we know in the previous lecture, that Hubble observed that galaxies are expanding
away from us, which leads to the idea of the Universe itself expanding as well.
● If the Universe is expanding, that means there was a size when it was near zero
● Hubble calculated a rough upper limit of the age of the universe using his Hubble Time
formula
○ This formula took into consideration that the speed at which the Universe is
expanding, was constant.
● Hubble found that the Universe is around 2 billion years old, however, there is evidence
through Geology, that the Earth itself is around 3 billion years old, so how can the Earth
be older than the Universe
Solution to the Age Problem
● Walter Baade found out that Hubble’s original measurement of his own constant was
wrong
● Hubble was comparing dimmer stars in our galaxy with stars that were actually brighter
in other galaxies
○ The stars were actually brighter than Hubble thought
● Walter Baade discovered that there was TWO Cepheid variable stars
● Ho doubled, and the age of universe doubled
● Allan Sange also found out that Hubble has misidentified star forming regions and star
clusters were the brightest stars in galaxies
● So the updated age of the universe is 13 billion years
More precise age of the Universe
● An important factor in calculating the Universe’s age, is to find more about its speed
● We know that the more mass there is, the more it slows down. This is something that
also applies to the Universe. (So maybe Hubble was wrong assuming the speed was
constant)
● Thus, the bigger the density, the more we expect the Universe to be slowing down
Precontext
● If the planet is light, thus a small gravitational force/pull, and if a ball is thrown thrown
from a planet, it can escape gravity and continue to travel upwards forever.
● If the planet has a certain density, then the ball will eventually come back down
Document Summary
The expanding universe and the cosmic age crisis. So we know in the previous lecture, that hubble observed that galaxies are expanding away from us, which leads to the idea of the universe itself expanding as well. If the universe is expanding, that means there was a size when it was near zero. Hubble calculated a rough upper limit of the age of the universe using his hubble time formula. This formula took into consideration that the speed at which the universe is expanding, was constant. Hubble found that the universe is around 2 billion years old, however, there is evidence through geology, that the earth itself is around 3 billion years old, so how can the earth be older than the universe. Walter baade found out that hubble"s original measurement of his own constant was wrong. Hubble was comparing dimmer stars in our galaxy with stars that were actually brighter in other galaxies.
