Chapter 18 Notes
Astronomy Name: Date:
I. Olbers’s Paradox
A. Why is the sky dark at night?
i. If the Universe is ______, then every line of sight should end on the surface
of a ______at some point.
ii. The night sky should be as ______as the surface of stars!
iii. The fact that it is dark at night anyway, is called ______.
B. Solution to Olbers’s Paradox
i. If the universe had a ______, then we can only see light from galaxies
that have had ______to travel to us since the beginning of the universe
ii. The visible universe is ______!
II. Hubble’s Law
A. Distant galaxies are ______from us with a speed proportional to
______.
III. The Expanding Universe
A. On large scales, galaxies are moving ______with velocity proportional to distance.
B. It’s not galaxies moving ______space. Space is ______, carrying the galaxies along!
C. The galaxies themselves are ______expanding!
D. This does not mean we are at the ______of the universe! You have the same
impression from any other galaxy as well.
IV. The Necessity of a Big Bang
A. If galaxies are moving ______from each other with a speed proportional to
distance, there must have been a ______, when everything was concentrated in one single ______.
V. The Age of the Universe
A. Knowing the current rate of ______of the universe, we can estimate the time it
took for galaxies to move as far apart as they are today: ______billion years.
B. The most distant galaxies known are seen as they were only about ______years after the Big Bang.
VI. Looking Back Towards the Early Universe
A. The more ______the objects we observe, the ______into the past of the universe we are looking
VII. The Cosmic Background Radiation
A. The radiation from the very early phase of the universe should still be ______today.
B. Was, in fact, discovered in the mid 1960’s as Cosmic ______Background with a temperature of 2.73 K (______)
VIII. The Early History of the Universe
A. The Early universe was made up of electrons (______), positrons (______), and ______ray photons
i. Electrons, positrons, and gamma rays in equilibrium between pair production and ______
B. Protons and neutrons form a few ______nuclei; the rest of protons remain as ______nuclei
i. ______of mass in helium and ______in hydrogen
ii. almost no elements heavier than ______are produced
C. The first portion of the history of the universe was dominated by ______
IX. Recombination
A. Universe becomes transparent for ______
B. Transition to ______dominated era
X. The Cosmic Background Radiation
A. After recombination, photons can travel freely through ______
B. Their wavelength is stretched (______) by cosmic expansion
C. This is what we can observe today as the cosmic background ______
XI. Reionization
A. After less than one ______years, the first stars form.
B. Ultraviolet radiation from the first stars ______gas in the early universe
C. Reionization = the first stars form and the universe becomes ______again.
XII. The Cosmological Principle
A. Considering the largest scales in the universe, we make the following fundamental assumptions:
1) Homogeneity:
2) Istotropy:
3) Universality:
XIII. Deceleration of the Universe
A. Expansion of the universe should be slowed down by mutual ______
attraction of the galaxies
B. The fate of the universe depends on the ______density in the universe.
C. “Critical density” is the density above or below which the universe will behave ______
XIV. Model Universes
A. Three possibilities:
i. If the density of the universe is less than the “critical density”, ______
ii. If the density of the universe is equal to the “critical density”, ______
iii. If the density of the universe is greater than the “critical density”, ______
XV. Dark Matter
A. Combines mass of all ______matter (i.e. emitting any kind of ______)
in the universe adds up to much less than the ______density.
B. Gravitational lensing show that some clusters contain ______times as much mass as is directly visible.
C. Can dark matter be composed of normal matter
i. if so, then its mass would mostly come from ______and neutrons =
baryons
ii. Density of baryonic matter is only ______of critical density
iii. most dark matter must be ______!
XVI. 21st Century Cosmology
A. Inflation!
i. Inflation: period of sudden ______during the very early evolution of the universe
ii. Triggered by the sudden energy ______from the separation of the strong and electroweak force
XVII. Measuring the Deceleration of the Universe
A. by observing ______, astronomers can measure the Hubble relation at large distances
B. It was expected that this would measure the deceleration of the universe but in fact, measurements show that the universe is ______!
XVIII. The Fate of the Universe
A. Due to the effect of Dark Energy, the universe will keep expanding ______
B. We don’t know the nature of dark energy, and we don’t know how it will ______in the future
C. The universe may keep expanding in a regular manner, or be disrupted in a “______”
XIX. Large Scale Structure of the Universe
A. Distribution of bright ______in the Virgo region reveals the Virgo cluster and the presence of more distant, ______scale structure
B. A large survey of distant galaxies shows the largest structures in the universe: ______and walls of galaxy superclusters and ______basically empty space
XX. The Growth of Structures
A. Structures in the Universe could have developed in two fundamentally different ways:
i. Small structures ______could have developed first, then clustering into larger and larger structures
ii. The largest structures ______could have developed first then breaking up into smaller and smaller units
iii. The latter scenario seems to be favored by ______evidence
XI. Cosmology with the Cosmic Microwave Background
A. If the universe was perfectly ______, then the CMB should be perfectly homogenous over the sky
B. Instead it shows small-scale ______
XII. Fluctuations in the Cosmic Microwave Background
A. The size of CMB ______fluctuations allows us to study the geometry of space-time!
B. CMB fluctuations have a characteristic size of ______degree
C. This means that the universe must be ______!
XIII. The Contents of the Universe
A. Dark Energy ______, Dark Matter ______, Visible Matter ______
B. We only “see” about 4% of all the mass and energy in the universe!
C. The nature of about 96% of our Universe is yet mysterious and ______!